Exercise Science curriculum

A single session already changes some things on the day you do it: blood pressure dips and insulin sensitivity rises. Gains in cardiorespiratory fitness and muscular strength take a few weeks or months of repeated sessions, so they are adaptations that belong to the training history, not responses to any one workout.

Exercise pushes you away from your resting balance: the lungs must move more air, the heart must pump more blood, and stress hormones such as cortisol rise. Endurance training brings adaptations that shrink how far a given workload disturbs that balance, so the same session becomes a smaller shock to the system.

Physical activity is any movement made by skeletal muscle that raises energy use above a basal level, from raking the yard to carrying groceries upstairs. Exercise is the subset that is planned, structured, repetitive and aimed at improving health or fitness, so the purpose and plan decide which word fits, not how hard it felt.

HHS describes physical fitness as the ability to carry out daily tasks with vigor and without undue fatigue, then splits it into parts: cardiorespiratory fitness, musculoskeletal fitness (strength, endurance and power working together), flexibility, balance and speed of movement. Most research linking fitness to health has measured the cardiorespiratory part, and only sometimes the musculoskeletal part.

Health-related fitness is about disease risk and daily function: greater fitness is linked with lower all-cause and cardiovascular mortality. Performance qualities such as agility, speed and power matter for sport and jobs, which is why some military and fire-service tests now mimic job tasks instead of a run, push-ups and a flexibility measure.

A MET is a ratio: an activity's rate of energy use divided by the rate at rest, so 4 METs is four times resting. Joules and kilocalories measure an amount of energy, and a kilocalorie is about 4.2 kJ; a watt is a rate, one joule per second. Multiply a rate by time to get a cost: 4 METs for 30 minutes is 120 MET-minutes.

Designs differ in what they hold constant: when each person trains one leg one way and the other leg another way, both limbs share the same hormones and nutrients, so a difference between legs reflects the training. Starting fitness matters as much. In a large meta-regression, people with lower initial fitness gained more in mitochondria, capillaries and VO2max, so results from beginners overstate what trained athletes can expect.

A whole muscle is an organ wrapped in epimysium. Inside, fibers are bundled into fascicles, each wrapped in perimysium, and every fiber, one long multinucleated cell, has its own endomysium. Each fiber is packed with myofibrils built from repeating sarcomeres, and the sheaths also carry blood vessels and nerves inward.

A sarcomere runs from one Z-disc to the next. The dark A-band holds the myosin filaments, the light I-bands hold actin only, and the H-zone is the middle stretch of the A-band that actin does not reach. As a sarcomere shortens, the A-band keeps its length while the I-bands shrink and the H-zone narrows.

Thick filaments are myosin; thin filaments are actin wound with two strands of tropomyosin carrying troponin complexes. In the relaxed state, as mapped in detail in heart muscle, tropomyosin blocks myosin's interaction with actin. When calcium binds troponin C, tropomyosin shifts, the thin filament switches on, and myosin heads can form cross-bridges.

The sarcoplasmic reticulum is the fiber's internal calcium store. T-tubules are infoldings of the cell membrane that conduct the electrical signal inward, where voltage-sensing proteins in the tubule open release channels in the neighboring store. Afterward, calcium is pumped back into the store and the fiber relaxes.

The endomysium, perimysium and epimysium do not stop where the fleshy belly ends. They run on together to form a ropelike tendon or a broad, flat aponeurosis that attaches to bone, so force made by the fibers reaches the skeleton through continuous connective tissue. The same coverings help fragile fibers withstand the forces of contraction.

Satellite cells sit between a muscle fiber's membrane and its basement membrane and stay quiescent until damage wakes them; they then drive regeneration while the pool of satellite cells is rebuilt. Fusing with fibers to add nuclei during growth was long thought necessary, but growth has been reported without new nuclei, so that rule is now disputed.

Force scales with how many sarcomeres sit side by side, and shortening speed with how many sit end to end. Pennation lets more fiber area fit along a limited aponeurosis, raising force capacity, while longer fascicles add speed; elite sprinters tend to have long fascicles. How much pennation angle matters for performance is still contested.

A fiber contracts only after a nerve impulse arrives. The resulting action potential runs along the membrane and down the T-tubules, where coupled DHPR and ryanodine receptors open, calcium pours out of the sarcoplasmic reticulum, binds troponin C and lets cross-bridges cycle. When stimulation stops, calcium returns to the store and the fiber relaxes.

Neither filament gets shorter when a sarcomere contracts. Myosin and actin keep nearly constant lengths and slide past each other, so overlap increases and the Z-discs are drawn together. The evidence was that the A-band, which marks the myosin filaments, held its length while almost all the change happened in the I-bands.

In rigor, a myosin head is locked to actin. Binding ATP releases it; the head then splits ATP into ADP and phosphate, keeping both products, and in that state it can rebind actin. Once attached, the head swings from roughly 90 degrees to 45 degrees, sliding the filaments past each other, and the cycle repeats while calcium and ATP last.

ATP is needed to let go, not only to pull: a myosin head stays bound to actin until a fresh ATP releases it. After death, ATP runs out while calcium stays high, so cross-bridges stay attached and muscle stiffens in rigor mortis. In living muscle, fatigue normally stops contraction before ATP falls that far.

Force depends on how many cross-bridges can reach actin, and that depends on filament overlap. In classic frog-muscle work, active force peaks on a short plateau near 2.0 to 2.25 micrometers of sarcomere length. Stretched beyond it, force falls roughly in proportion to the lost overlap, reaching zero near 3.5 micrometers, and it also declines when the sarcomere shortens below about 2.0.

The faster a muscle shortens, the less force it can produce, and the relationship is a curve rather than a straight line; its shape reflects how quickly cross-bridges attach and detach. Lengthening under load sits on the other side of the curve: eccentric actions generate more force than other contraction types at the same angular velocity.

Concentric actions shorten the muscle, eccentric actions let it lengthen under load, and isometric actions hold its length. Eccentric work needs less motor-unit activation and costs roughly a quarter of the energy of the same exercise done concentrically, yet unaccustomed eccentric work is known to cause muscle damage and delayed-onset soreness; a repeated bout brings much less.

Fiber types are named for the myosin heavy chain they mainly express. Type I fibers are slow, highly oxidative and fatigue resistant; type IIX are the fastest and most forceful but glycolytic and fatigable; type IIA sit between, rich in both oxidative and glycolytic enzymes. An untrained man's vastus lateralis is roughly 40 to 50 percent type I.

Strength and power training commonly push type IIX fibers toward IIA. Shifts between the slow and fast families are harder to pin down: cross-sectional studies cannot tell whether endurance athletes' high type I share came from training or from birth, and some sprint-training studies even report type I fibers moving toward IIA.

Agonist, antagonist and synergist name a muscle's job in one movement, not a fixed property of the muscle. In elbow flexion the biceps is the agonist and the triceps the antagonist; reverse the movement and the labels swap. Synergists and stabilizers assist the agonist, and strength training usually lowers antagonist coactivation, which helps raise force.

A lever's class depends on what sits in the middle: the fulcrum (first class), the load (second) or the applied force (third). A muscle pulling close to the joint moves the far end of the limb farther than the muscle shortens, at a cost in force. A 1909 school text held that the body has no true second-class levers, yet a 2015 study treats the calf muscle at the ankle as one.

Torque, the turning effect at a joint, depends on how much force a muscle makes and on its moment arm, the distance from the joint's center of rotation to the muscle's line of pull. As the joint angle changes, both the moment arm and the muscle's length change, so the same muscle makes different torque across a movement; for the same force, a longer moment arm means more torque.

A strength curve maps how much force you can produce across a lift's range of motion. With a constant load, the weakest position, the sticking region, caps the whole set. Bands or chains add resistance as you move into stronger positions so the load follows the curve better; one meta-analysis found this beat constant resistance for maximal strength, though earlier studies disagreed.

In open-chain exercise the far end of the limb moves freely; in closed-chain exercise it is fixed, as when your foot stays planted. Open-chain work was long assumed to strain a reconstructed ACL more, but a meta-analysis of the few rehabilitation trials available found no significant difference between the two for pain, knee-extensor strength or laxity.

A stretch-shortening cycle is an eccentric stretch followed straight away by a concentric push, as in a countermovement jump. The prestretch boosts the push through stored elastic energy, the stretch reflex and extra neural excitation, and adults jump about 18 to 30 percent higher with a countermovement than without one.

When you push on the ground, it pushes back on you with an equal and opposite force, the ground reaction force. What decides take-off speed is net impulse, the force beyond body weight summed over the push, because impulse equals the change in momentum; take-off velocity then fixes how high you jump.

Bone is rebuilt throughout life: osteoclasts resorb old bone and osteoblasts lay down new, and the balance between them sets bone mass. Osteocytes, which make up 85 to 90 percent of adult bone cells, are the main load sensors steering the process. When load is removed, as in long bed rest or paralysis, resorption wins and bone is lost.

Bone adapts to the forces placed on it, the idea Wolff proposed in 1892 and Frost later framed as a strain-sensing mechanostat. Impact with the ground is a strong signal: in a systematic review, swimmers had bone density similar to or lower than inactive controls and lower than athletes in high-impact sports.

Joints can be grouped by how much they move. Immovable joints, such as skull sutures, are held by a thin layer of fibrous tissue; slightly movable joints, such as those between vertebrae, are joined by cartilage; freely movable synovial joints, the most common kind in adults, have a fluid-filled cavity between cartilage-capped bone ends.

In a synovial joint the bone ends are capped with smooth articular cartilage and separated by a joint cavity. A dense fibrous capsule encloses it: the outer layer includes the ligaments that hold the bones together, and the inner synovial membrane secretes the fluid that lubricates the joint and, in adults, is thought to be the main route by which nutrients reach the cartilage.

Tendons adapt to training, but they need a strong signal. A meta-analysis of exercise studies found that stiffness gains depended on loading intensity rather than contraction type, and tended, though not significantly, to be larger after 12 weeks or more. Most of the change came from the tendon material itself, not from a thicker cross-section.

Articular cartilage is avascular, so its cells depend on nutrients diffusing in, from synovial fluid or from vessels in the bone beneath. Which route matters more has been debated. In mature joints a calcified layer is thought to block much of the bone route, and one rabbit study that blocked each route in turn found synovial fluid to be the dominant source.

Bone answers repeated loading along a continuum: normal remodeling, accelerated remodeling, stress reaction, stress fracture and finally complete fracture. Repair starts by resorbing bone before new bone forms, so when loading outpaces recovery, microdamage can build up and join into a fracture.

A motor unit is a single motor neuron together with every muscle fiber its axon reaches. Slow type I fibers usually sit in units with few fibers per neuron, while units with many fibers per neuron often connect to fast type II fibers. The nerve-muscle junction has a high safety margin, so each impulse makes every fiber in the unit fire.

As force demand rises, motor units join in order of size: small motor neurons driving slow type I fibers first, then larger ones driving IIA, and IIX last. Light efforts therefore run on fatigue-resistant units. Effort, not load alone, reaches the big units: light loads of about 30 percent of 1RM taken to failure recruit both type I and type II fibers.

Besides recruiting more motor units, the nervous system raises force by firing each one faster. After a single twitch, calcium returns to its store and the fiber fully relaxes; stimulate again sooner and twitches summate, peaking in tetanus, where they overlap and no relaxation occurs. High firing rates also speed the rise of force.

Maximal strength rises early in training before muscle size measurably changes, which points to the nervous system: more motor units activated, higher firing rates and fuller voluntary activation. Exactly where in the nervous system the early change happens is still unresolved, partly because it is hard to measure during realistic lifting.

Muscle spindles are small encapsulated receptors lying parallel to ordinary muscle fibers and reporting changes in muscle length. Their group Ia afferents connect directly onto the motor neurons of the same muscle, the circuit behind the stretch reflex. People born without proprioception cannot keep their balance with eyes closed and have no tendon reflexes.

Golgi tendon organs sit at the junction of muscle and tendon, each supplied by a single group Ib afferent, and they respond to muscle load and rapid changes in force. Their input mostly inhibits the same muscle's motor neurons through two- or three-synapse pathways, but its central pathways and role in motor control remain uncertain, so the 'safety brake' story is a simplification.

Proprioception is the ability to combine signals from mechanoreceptors to know where your body is and how it is moving. For balance, the brain blends it with vision and the vestibular sense and can shift its reliance toward whichever source is most reliable. Training such as wobble-board work appears to improve it, though genetics also play a part.

The classic Fitts and Posner model divides skill learning into three stages: a cognitive stage, when performance is variable and you explore movement solutions; an associative stage of refinement; and an autonomous stage, when control shifts toward automaticity. One recent review proposes that instruction and outcome feedback matter most early, with practice taking over later.

Blocked practice repeats one task variant many times before moving on; random practice mixes variants from trial to trial. Blocked practice usually looks better during the session, but random practice tends to win on later retention and transfer, the contextual interference effect. For complex skills the advantage shows up less consistently.

Muscle holds only about 8 mmol of ATP per kilogram, and even all-out work barely dents it because three systems, phosphagen, glycolytic and mitochondrial, remake it as fast as it is used. Stored ATP is not a fuel tank; fatigue sets in first, which keeps ATP from falling low enough to cause rigor.

Creatine phosphate hands its phosphate to ADP in a single step that needs no oxygen, so this system regenerates ATP fastest. Muscle stores only about 26 mmol of it per kilogram, which is why it dominates maximal efforts of about 5 to 6 seconds; refilling it after exhausting work takes from under 5 to more than 15 minutes.

Glycolysis splits glucose or glycogen to pyruvate with no oxygen involved, netting 2 ATP per glucose, or 3 when the glucose comes from glycogen. Its rate peaks after about 10 to 15 seconds and it supplies about half the energy of a 30-second sprint. Turning pyruvate into lactate regenerates NAD+ and keeps it running.

Mitochondrial respiration burns fuel completely with oxygen, and the citric acid cycle is a common pathway for carbohydrate, fat and amino acids. Its ATP turnover rate is low, so it cannot meet demand at the very start of exercise, but it responds quickly; in a maximal effort of about 75 seconds it already supplies roughly half the energy.

No energy system switches on or off. All three contribute from the start, in proportions set by intensity and duration: a 10-second sprint has been estimated at about 53 percent phosphagen, 44 percent glycolysis and 3 percent aerobic, while a 30-second sprint shifts to about 23, 49 and 28 percent.

Glycolysis to lactate nets 2 ATP from a glucose molecule, or 3 when the glucose comes from glycogen. Taking the same carbohydrate through the mitochondria yields far more; one review counts 37 ATP per glycogen-derived glucose unit, a total that depends on which shuttle carries electrons in. The low-yield path wins on speed.

Lactate is produced whenever exercise is intense enough, even with plenty of oxygen, and it is a fuel rather than a dead end: working skeletal muscle and the heart oxidize it, and the liver turns it back into glucose. One review argues its production slows acidosis rather than causing it. Delayed soreness follows unaccustomed eccentric work, and lactic acid is only one of several proposed explanations for it.

At the start of exercise the aerobic system cannot yet meet demand, so stored phosphagens and glycolysis cover the gap, the oxygen deficit. Afterward oxygen use stays raised while ATP and creatine phosphate are rebuilt, myoglobin reloads and hormone effects fade. In one study of young men with obesity, sessions of about 300 kcal were followed by only about 54 to 66 kcal of EPOC over the next 30 minutes.

As intensity rises, reliance on carbohydrate over fat climbs steeply; fat oxidation peaks at around 65 percent of VO2max and falls beyond it. Endurance training shifts the mix toward fat at a given workload, sparing limited carbohydrate stores, so the point where carbohydrate takes over moves to a higher intensity.

A 75-kg person at 15 percent body fat stores over 100,000 kcal of fat, yet fat oxidation falls as you push harder and cannot supply ATP at the rate hard work demands. Proposed reasons include rising catecholamines restraining fat release, falling blood flow through fat tissue, and a shortage of free carnitine to carry fatty acids into the mitochondria.

Muscle and liver glycogen together hold under about 3,000 kcal, little enough that they can limit moderate-to-hard exercise lasting more than about 45 minutes. Muscle glycogen is used faster the harder you go, and low liver glycogen is now also generally accepted as a major contributor to fatigue in endurance exercise.

Working muscle draws glucose from the blood, and the liver matches it, at first by breaking down its glycogen and then increasingly by making new glucose as that glycogen runs down. If liver output falls behind on a long effort, blood glucose drops. In a glucose-infusion study, endurance improved without any change in total carbohydrate burned, possibly by keeping fuel flowing to the brain.

Protein is a minor fuel during prolonged endurance exercise: about 10 percent of ATP turnover in one estimate, up to 20 percent in others. Branched-chain amino acids such as leucine are the ones preferentially burned. The true share stays uncertain because the methods for measuring it are limited.

The respiratory exchange ratio compares the carbon dioxide you breathe out with the oxygen you take up. Burning carbohydrate produces more CO2 than burning fat, so a high ratio signals mostly carbohydrate and a low one more fat. It reflects fuel use only at a steady state; in very hard work it can climb past 1.0, one of several criteria labs use to judge a maximal test.

The lactate threshold is the workload at which blood lactate starts to climb above resting values. It marks the upper edge of the moderate intensity domain, and studies of endurance athletes find it closely tied to the first ventilatory threshold. Labs define and detect it in several ways, so values from different methods are not always interchangeable.

In the moderate domain, below the lactate threshold, oxygen uptake and blood lactate settle quickly. In the heavy domain they settle only after a delay and an extra slow component of oxygen cost. Above critical power, in the severe domain, no steady state exists and muscle homeostasis is progressively lost until you stop.

Cardiac output, the blood pumped per minute, is heart rate times stroke volume, so it can only rise through one or both. During dynamic exercise it climbs roughly in line with oxygen uptake. Elite endurance athletes can reach about 35 to 40 liters a minute, almost double a non-athlete's, largely through a bigger stroke volume.

Stroke volume depends on how full the ventricle gets before it beats, how strongly it contracts and the resistance it pumps against. In dynamic exercise the muscle pump and venous constriction raise filling, sympathetic drive raises contractility, and dilating muscle vessels lower resistance. In untrained people stroke volume often plateaus at about 40 to 50 percent of VO2max, a classic finding not every person follows.

Greater filling stretches the ventricular muscle, and stretched heart muscle contracts more forcefully on the next beat. The effect comes from length-dependent activation of the sarcomeres, involving titin and the thin-filament switch, and it works beat to beat. During exercise, extra venous return from the muscle pump recruits it to raise stroke volume.

At rest the heart is under strong vagal, parasympathetic control with little sympathetic drive. When exercise starts, vagal tone is withdrawn first, giving a near-instant rise; sympathetic drive builds more slowly and dominates at high workloads. Heart rate keeps climbing with workload until it reaches its maximum.

Several published formulas predict maximal heart rate from age alone, each a fixed number minus some multiple of age. Group averages can land close, but individuals scatter widely: in a large sample of endurance athletes the root-mean-square error was about 9 to 10 beats a minute. Any training zone built on a predicted maximum inherits that error.

Classic echocardiography work described two patterns: endurance training enlarging the left-ventricular cavity, called eccentric hypertrophy, and strength training thickening its wall, called concentric hypertrophy. Later work questions how cleanly they separate: an MRI study found similar cavity and wall changes after static and dynamic training, and a study of mixed-sport athletes found neither shape.

Regular exercise lowers resting heart rate, by about 6 beats a minute in older adults in one meta-analysis. A larger stroke volume lets the heart deliver the same output with fewer beats. Why the rate itself falls is still debated: one review argues against more vagal tone and for a lower intrinsic pacemaker rate.

During prolonged steady exercise, often from about the 10th minute, stroke volume gradually falls and heart rate rises while cardiac output stays stable. The classic explanation is that rising body temperature sends more blood to the skin and sweating shrinks plasma volume, both reducing ventricular filling; dehydration makes it worse.

During exercise, sympathetic drive constricts vessels in the gut organs and in resting muscle while active muscle dilates, preserving most of the cardiac output for the working muscle. As workload rises, active muscle takes a growing fraction of output and kidney blood flow becomes a progressively smaller share.

Contracting muscle releases candidate vasodilators, including potassium, adenosine, hydrogen ions, carbon dioxide and phosphate, that widen nearby vessels so flow rises where the demand is; vessel diameter sets most of the resistance to flow. A rise in pulse pressure also raises shear stress on vessel walls, prompting nitric oxide release, which dilates further.

In dynamic exercise mean arterial pressure rises only slightly, even though muscle vessels open wide, because cardiac output climbs to match the fall in resistance; pulse pressure widens. Static, isometric effort behaves differently: pressure rises progressively for as long as the contraction is held.

Blood in the veins is under little pressure, so venous return depends on skeletal muscle action: contracting muscles squeeze the veins, and valves keep blood moving toward the heart. In dynamic exercise this pump raises filling and stroke volume. Stop abruptly and the pump stops, stroke volume falls, and pressure would drop if reflexes did not step in.

The Fick equation says oxygen uptake equals cardiac output times the arteriovenous oxygen difference: how much more oxygen the blood carries going out to the muscles than coming back. Extraction widens as you work harder and may improve with training, but meta-analyses of 5 to 52 weeks of endurance training in untrained or moderately trained people traced VO2max gains to maximal cardiac output, not extraction.

Endurance training adds capillaries around muscle fibers, bringing blood closer to where oxygen is used; elite endurance athletes have about 50 percent higher capillary density than untrained people. In a large meta-regression, capillaries per fiber rose by roughly 10 to 15 percent across training types, mostly in the first four weeks and only in untrained to moderately trained people.

Plasma volume can expand within 24 hours of one hard interval session, probably because shifts in albumin draw fluid into the blood. Blood-volume expansion has been thought to drive early VO2max gains, but in one 10-week study of 12 untrained people, removing the added blood by phlebotomy left the gains intact, so its role is contested.

Minute ventilation has two parts: how deep each breath is (tidal volume) and how often you breathe (frequency). They seem to be steered by different inputs. Tidal volume tracks metabolic signals such as carbon dioxide output, while frequency answers fast to effort and central command, so a sudden jump in workload shows first in breathing rate.

Oxygen and carbon dioxide cross between alveolar air and lung capillary blood by diffusion, through walls of thin squamous cells that allow rapid exchange. No pump moves the gases; each flows down its own partial-pressure gradient, which is why the lower arterial oxygen pressure at altitude limits oxygen transfer.

Most oxygen in the blood rides on hemoglobin inside red cells. The dissociation curve shows how much is bound at each oxygen pressure. Acidity, carbon dioxide and heat, all higher in working muscle, shift the curve right, lowering hemoglobin's affinity so it releases more oxygen; higher affinity in the lung favors loading.

At moderate intensity, ventilation follows carbon dioxide output closely. At the first ventilatory threshold, bicarbonate buffering of lactic acid adds extra carbon dioxide and breathing rises out of step with oxygen uptake; at the second, acidosis can no longer be offset by breathing. The first threshold sits close to the lactate threshold, though the two are measured differently.

VO2max is the highest rate of oxygen uptake you can reach, the combined capacity of the lungs, heart and blood to deliver oxygen and of muscle to extract and use it. The gold-standard sign of a true maximum is a plateau, where uptake stops rising as work keeps increasing; in one population sample only 42 percent showed one, so labs add other end criteria, and the choice changes the result.

VO2max rests on a chain of steps, from lung diffusion and hemoglobin through cardiac output and muscle blood vessels to mitochondria, but in healthy people delivery usually sets the ceiling. Meta-analyses traced training gains to maximal cardiac output rather than extraction, and VO2max falls about 6 to 8 percent per 1,000 meters of altitude.

Running economy is the steady-state oxygen cost of moving at a given submaximal speed. Runners with good economy use less oxygen at the same speed, and economy can differ by as much as 30 percent among trained runners with similar VO2max, so it is a useful predictor of performance, especially among athletes whose VO2max values are alike.

Norepinephrine from sympathetic nerves and epinephrine from the adrenal medulla drive the fight-or-flight response to exercise. They raise heart rate and contractility and help regulate liver glucose output and fat metabolism; norepinephrine rises within 15 minutes, before epinephrine changes. Their release climbs steeply with intensity, which at high intensity restrains fat release.

A single endurance session raises cortisol once intensity passes about 60 percent of VO2max, and above that the rise grows with intensity; below it, cortisol climbs only after long efforts, such as 90 minutes at 40 percent. In highly trained endurance runners resting cortisol runs mildly high, yet the rise after a session has been reported as smaller than the workload would predict.

Resistance exercise triggers short rises in testosterone, growth hormone and IGF-1. Whether those spikes drive muscle growth is doubtful: in one 16-week study of young men, the size of the post-exercise hormone rise did not predict fiber growth, while signals inside the muscle did. A major review likewise rates acute hormone changes as minor next to chronic effects.

Muscle contraction moves GLUT4 glucose transporters to the cell surface through calcium, metabolic-stress and mechanical signals, independently of insulin. That is why contraction raises glucose uptake about as well in insulin-resistant muscle as in insulin-sensitive muscle, one mechanism behind exercise's effect on blood glucose.

After one bout, the exercised muscles respond to insulin more strongly for many hours, and HHS lists better insulin sensitivity among the benefits seen on the day of activity. The effect is local to the muscle that worked. One human study found that a prior bout rearranges GLUT4 inside the fiber and boosts insulin-stimulated GLUT4 movement to the surface; the full mechanism is still being worked out.

Contracting muscle releases signaling molecules called myokines, among them IL-6, IL-15, irisin and BAIBA, that act on the muscle itself, nearby tissue and distant organs. IL-6 released this way, for instance, promotes fat breakdown in adipose tissue and glucose production in the liver. Much of the wider systemic story is still hypothesis.

A meta-analysis of studies in naturally cycling women found that performance might be trivially lower in the early follicular phase than in other phases. Because the effect was trivial, studies varied widely and most of the evidence was rated low quality, the authors concluded that general guidelines cannot be set and individual responses matter more.

Metabolism turns chemical energy into work and heat, and muscles are by far the largest tissue group in the body. Resting heat production is about 1 kcal per kilogram per hour, rising to about 5 in moderately hard work and higher still in exercise. That heat warms the muscles and then the core unless it is lost to the environment.

The body sheds heat by radiation, conduction, convection and evaporation of sweat. Conduction usually plays a minor role unless you touch hot surfaces or are in water. Convection and radiation depend on the temperature gap between skin and surroundings, so in hot conditions evaporation carries the load: each liter of sweat that evaporates removes about 580 kcal.

Sweat cools only as it evaporates, and the rate depends on the vapor-pressure gap between wet skin and the air. In humid air you keep sweating and losing water, but evaporation is choked off and body temperature climbs; at about 70 percent skin wettedness, sweat starts to drip off instead of evaporating. High humidity can bring heat stress even at a comfortable air temperature.

Sweat production is one of the individual differences NIOSH says may be large. Endurance athletes average 1.5 to 2.0 liters an hour, heat-acclimatized people can peak near 3 liters, and few people know how much they actually sweat. Because sweat is more dilute than plasma, little sodium is lost in the first hour or two.

Unreplaced sweat losses shrink plasma volume, which raises heart rate and cardiovascular strain; dehydration also cuts sweating, so body temperature rises. Deficits of 2 to 3 percent of body weight are common in hot work, and beyond 3 percent, rectal temperature climbs with the deficit, impairing prolonged moderate exercise in the heat.

Exercise-associated hyponatremia develops when someone drinks too much plain water during long, heavy sweating, diluting blood sodium below 135 mmol per liter; severe cases can cause brain and lung swelling. Risk rises with events over 4 hours, low body mass, intake above 1.5 liters an hour and female sex, and mild cases are often symptom-free.

Weighing before and after exercise is the simplest field measure: a 0.45-kg drop is roughly 450 mL of body water lost. Urine color charts give a rough reading of concentration. Thirst is a weak guide during heavy sweating, since it is usually not strong enough to make people drink what they have lost.

Repeated days of working in the heat expand plasma volume, so blood volume holds up even while you sweat. Sweating starts sooner, flows more freely and loses less salt, and the same work is done at a lower core temperature and heart rate. Most of the change builds over roughly 7 to 14 days of gradually lengthened exposure.

Heat cramps have no settled cause but are linked to heavy salt loss in sweat replaced with plain water. Heat exhaustion brings headache, nausea, weakness and heavy sweating at a mildly raised core temperature, and it often comes before heat stroke. Heat stroke is a medical emergency in which temperature control fails and mental function is disrupted; in the exertional form the skin may still be sweating.

Air temperature alone misses the two things that most change heat strain: humidity, which limits sweat evaporation, and radiant heat from sun or hot surfaces. The wet bulb globe temperature blends a wet-bulb reading (humidity and air movement), a black-globe reading (radiation) and, outdoors in sun, air temperature. The wet-bulb term carries most of the weight.

In the cold, sympathetic drive narrows skin blood vessels to hold heat in the core, and muscle adds heat by shivering. Wind chill describes how fast wind strips heat from exposed skin, so it changes risk to people but cannot cool an object below air temperature. After rescue, core temperature can keep falling for a time, called afterdrop; its exact mechanism is still debated.

As you climb, the pressure of oxygen in the air falls with barometric pressure, so less oxygen loads onto hemoglobin and blood saturation drops. Above about 2,500 meters, faster breathing and a higher cardiac output can no longer fully compensate, so maximal oxygen uptake and endurance fall. The loss varies a good deal between people, trained or not.

The first response to altitude is faster breathing, driven by the hypoxic ventilatory response, plus a fluid shift that concentrates the blood. After the first week or two, erythropoietin sustains a rise in hemoglobin and arterial oxygen content. Saturation and submaximal endurance partly recover over two to three weeks, but maximal oxygen uptake does not recover in the same way.

Athletes who live and train at altitude often cannot train at the intensity they reach at sea level, so they can come back partly detrained. Living high while descending for hard sessions aims to keep the altitude stimulus for acclimatization without giving up training quality. Results across studies conflict, partly because hypoxic dose and training plans differ.

Overload means asking more of a body system than it is used to, in amount or intensity. The system responds by adapting: aerobic work, for example, makes the heart, lungs and working muscles more efficient and more capable. The size of the gap between new and usual activity also sets injury risk, which is why overload is added in small steps.

A load that once overloaded you becomes ordinary as you adapt, so it has to keep advancing for change to continue. You can progress by raising intensity, duration, frequency or total amount. Small, steady increases let the body catch up with each new stress while keeping injury risk down, and the right rate of increase differs from person to person.

The benefits of training stay largely with the systems and movements doing the work. Walking mainly trains the legs and the cardiovascular system, while push-ups mostly train the chest, shoulders and upper arms. So a test of one quality, or a sport, rewards training that resembles it, and a gain in one exercise does not transfer automatically to another.

When endurance training stops, the early fall in maximal oxygen uptake, within about four weeks, comes mainly from lost blood and plasma volume. Muscle oxygen extraction declines later, after several weeks off. Cutting volume while keeping some intensity can hold maximal oxygen uptake for about a month, so reducing training is not the same as stopping it.

People given the same program change by very different amounts, and a group average hides that spread. In one six-week study, non-responders, whose maximal oxygen uptake barely moved, gained more cycling efficiency than responders and matched them on changes in metabolic markers. So a non-responder on one measure may still be responding on others.

Frequency, intensity, time and type together set the dose of aerobic activity. For health outcomes, the total weekly amount matters more than any one dial, so more days can offset shorter sessions, and harder effort can offset fewer minutes. For strength work the matching dials are load, frequency and sets and repetitions.

In untrained people, even light loads can raise maximal strength, because early gains come largely from learning and coordinating the lift. With experience, further strength gains depend more on training close to your one-repetition maximum and on how closely the training matches the goal. The easy gains are spent first, so each further gain asks for more precise work.

In untrained lifters, the first strength gains come mainly from motor learning and better recruitment of motor units, and visible changes in muscle structure may follow later. Losses also run on separate clocks: when endurance training stops, blood and plasma volume fall within weeks, while muscle oxygen extraction declines only after a longer break.

Your one-repetition maximum (1RM) is the most weight you can lift once, and training loads are often written as a percentage of it, with more than about 60 percent usually called high load. A percentage does not fix how many repetitions you can do: in one small study, lifters at the same 70 percent load differed in reps to failure according to their muscular endurance.

The classic continuum assigns heavy loads and few reps to strength, moderate loads to hypertrophy and light loads with many reps to local endurance. Evidence supports the strength end: heavier loads win for maximal strength. The hypertrophy zone is weaker than it looks, because similar growth appears across a wide range of loads, from about 30 percent of 1RM up, when sets are hard.

Volume load multiplies sets by repetitions by load into one workload number. More weekly volume tends to bring more growth in a graded way, though even four or fewer weekly sets per muscle produced substantial gains. Volume load misleads across loads: heavy sets allow fewer reps, yet their strength advantage shows up whether or not volume load is matched.

Proximity to failure is how many reps you have left in a set before you cannot complete another. As a set gets harder, more muscle fibers are pushed into heavy work, but a meta-analysis found no evidence that going all the way to failure builds more muscle than stopping short. Failure also adds fatigue that can cut the work you complete later, and lifters misjudge reps in reserve most when far from failure.

Resting longer between sets lets you lift closer to your earlier load and reps, so you finish more total work. A 2024 meta-analysis found a small growth benefit to resting more than about 60 seconds, possibly through that preserved volume, and no clear difference once rest exceeded 90 seconds. The estimates overlapped widely, so it is a modest effect rather than a rule.

Fatigue accumulates across a session, so the exercise you do first is the one you perform best. In one study of untrained adolescents, repetitions fell from the first exercise to every later one, which is why the main lift usually goes early. For muscle growth over weeks, a meta-analysis found that order made no clear difference.

Mechanical tension on muscle fibers is treated as the key stimulus for growth, which is why hard sets with many fibers working are central. Metabolic stress and muscle damage are also proposed contributors, but their role is less settled; for example, the idea that lactate itself drives growth has plausible pathways yet little firm evidence.

Bigger muscles can produce more force, but strength is not just size. Light-load training to failure can grow muscle about as much as heavy training, yet heavy loads still give larger gains in maximal strength, partly through better voluntary activation of the muscle. In beginners, early strength gains come largely from learning and coordinating the movement.

Rep speed changes the load you can use: slower tempos force a lighter weight but stretch time under tension. Repetitions lasting anywhere from about half a second to eight seconds produced similar growth when sets went to failure, while very slow reps of about ten seconds did worse. One review leaned toward a slower lowering phase with a faster lift, but called the evidence inconclusive.

Steady moderate work and interval work reach overlapping fitness gains by different routes. Intervals are described as a strong stimulus for the heart and circulation, while moderate continuous training mainly drives changes in the muscle that improve oxygen extraction and efficiency. Intervals can match or beat continuous training on some measures despite less total volume.

High-intensity intervals are repeated near-maximal efforts, around 90 percent of maximal oxygen uptake or more, separated by rest or easy work. Even a few weeks of sprint intervals can raise muscle oxidative capacity and markers of mitochondrial growth to levels comparable with moderate continuous training. Reported gains span maximal oxygen uptake, aerobic endurance and anaerobic capacity, in healthy and clinical groups alike.

Polarized training puts most sessions at low intensity, roughly three-quarters or more, a small share at high intensity and very little in between. One nine-week trial in well-trained athletes found larger gains with this pattern than with threshold or high-volume plans. A later 16-week trial found the biggest gains when a pyramidal block came before a polarized one, so the best mix likely depends on the training phase.

Intensity can be set from percentages of maximal heart rate or heart-rate reserve, from percentages of maximal oxygen uptake, from measured thresholds, or from perceived exertion. A fixed percentage lands people at different places relative to their own thresholds, so threshold-based prescription is preferred where testing allows; this guidance comes from cardiac rehabilitation work.

Endurance training builds more mitochondria in muscle, and the protein PGC-1 alpha is often called the master regulator of that process; its expression rises with exercise intensity. The hub picture is not settled, though: in one study, mice bred without muscle PGC-1 alpha still built new mitochondria in response to wheel running.

Blood lactate climbs steeply once muscles produce lactate faster than the body clears it, and the intensity at that bend predicts endurance performance well. Threshold training works at or near that point to push it higher. Its value for already well-trained athletes is debated, with some data suggesting much time near threshold adds little.

Training strength and endurance together can blunt strength gains, an effect linked to competing signals inside muscle. High-volume, frequent, moderate continuous endurance work is thought to interfere most, while short high-intensity intervals seem to interfere less. Separating the two kinds of session has given larger adaptations in several studies, though the mechanisms are still open questions.

Power is how fast force does work: for a joint it is torque times angular velocity. Because both factors count, two athletes can post the same sprint time with very different force and velocity properties. Heavy lifting builds the force side, while other exercise types target speed or how force is applied, so load alone trains only part of power.

A force-velocity profile plots force against velocity across several loads to estimate maximal force, maximal velocity and peak power, with the aim of steering training toward an athlete's weaker end. The estimate depends on method: in one study a two-point loaded method was valid, while a shortcut using a bodyweight jump plus an isometric test showed only poor to fair validity for most variables.

Rate of force development is how quickly you raise force at the start of a contraction. It appears to relate more closely than maximal strength to most sport and daily tasks, such as countering a sudden loss of balance. Its earliest phase, the first 50 to 75 milliseconds, depends mainly on how fast you activate muscle; explosive and heavy training can both raise it, though it is hard to measure reliably.

Plyometric movements pair a lengthening phase, a brief transition called the amortization phase, and a shortening phase; a quick handover is the stretch-shortening cycle. Reactive strength, often measured by the reactive strength index, is the ability to produce a powerful push right after that stretch. Drop jumps typically show a fast cycle, under 250 milliseconds of ground contact, while a countermovement jump is slower.

Classic post-activation potentiation boosts twitch force after a strong contraction, through changes inside fast-twitch fibers, but it fades within a few minutes, with a half-life of about half a minute. The performance boost athletes seek often peaks 6 to 10 minutes later, may come from temperature, fluid and activation changes, and is now called post-activation performance enhancement. After a thorough warm-up, the extra benefit is uncertain.

A sprint starts with acceleration, where you must apply large horizontal force against the ground, and moves toward maximum velocity. Horizontal force per step falls steadily as running speed rises, which is why the early steps demand the most horizontal force and later steps demand speed. One theoretical model links the two phases through hip torque.

Change-of-direction speed is a preplanned cut or turn: you know the route in advance. Agility adds a reaction, changing speed or direction in response to a stimulus, so it also depends on perception and decision-making. In one study of junior soccer players, the two kinds of test shared only 25 to 40 percent of their variance, so they measure largely different things.

How far a joint moves depends partly on how stiff the muscle and tendon are and partly on how much stretch you can tolerate before stopping. The balance differs between people: in one study, calf range in men tracked both stiffness and tolerance, while in women it tracked tolerance only. Another study found stretch tolerance, not stiffness, explained hamstring range in men.

Static stretching eases a muscle into a stretch and then holds still. Dynamic stretching moves a joint through its range under control, ballistic stretching bounces at the end of the range, and PNF methods add a contraction of the stretched muscle before stretching again. In a meta-analysis, a single bout of any of these produced similar small, short-term gains in range of motion.

Long static holds right before maximal or explosive work have been tied to short-lived losses of force, which pushed warm-ups toward dynamic stretching. Shorter holds, under about 60 seconds per muscle group, are now seen as acceptable in a warm-up, and one small study found 15- or 60-second stretches did not change jump height or maximal force. Hold duration appears to matter more than the act of stretching itself.

A 2025 meta-analysis found that weeks of static stretching moderately raised the stretch you can tolerate and slightly lowered muscle stiffness, but did not lengthen muscle fascicles. A single session lowered stiffness without changing tolerance. So much of the lasting range gain reflects how much stretch your nervous system accepts, not longer muscle tissue.

A sprint-based warm-up raised thigh muscle temperature by about 3 degrees C in one study, and keeping muscles warm afterward has been linked to better sprint and power output. Warm-ups can also carry injury-prevention exercises: across fifteen studies in youth sport, structured warm-up programs cut injury rates by about a third, with compliance shaping the effect.

Range of motion can be measured passively, with the limb moved for you, or actively, with your own muscles moving it. A meta-analysis found strength training improved range of motion about as much as stretching, for both active and passive measures, though the studies varied widely. So loaded movement through a full range is a second way to gain range.

The supercompensation cycle describes a training session as a stress that first causes fatigue and a drop in performance. Over the following hours and days, recovery returns you to baseline, and the body keeps responding past it, lifting performance above where it started. The idea, first described by Yakovlev, remains one of the pillars of training theory.

The fitness-fatigue model treats each training dose as creating two opposing effects, a fitness gain and a fatigue cost, with performance reflecting the balance between them. It is used to plan a sequence of training doses that peaks performance at a chosen time, the logic behind a pre-event taper. The model is mathematically neat, but its ability to describe and predict real performance is limited, since it simplifies the physiology.

Periodization plans how volume and intensity change over time. Linear plans start with high volume at low intensity and shift toward low volume at high intensity, while undulating plans vary both within the same week. A 2026 meta-analysis of 29 studies found them broadly comparable for athletic capacity and body composition, with differences only in some subgroups, so neither is clearly better overall.

A taper cuts training, mainly volume, for about one to four weeks so fatigue clears before a key event; for peaking strength, a review suggests about half the volume over two weeks or less while keeping or slightly lowering intensity. A full stop is different: in one trial, a week off midway through a nine-week program blunted strength gains without changing muscle growth.

Central fatigue is a drop in the drive the nervous system sends to muscle, seen as reduced voluntary activation. Peripheral fatigue is a loss of force inside the muscle itself. Researchers separate them with electrical stimulation: voluntary activation and twitches added during a task track central drive, while the twitch of the resting muscle tracks weakening inside it.

Delayed-onset muscle soreness usually starts several hours after exercise, peaks one to two days later and fades within a week. It is most associated with repeated eccentric work, where muscles lengthen under load. Many causes have been proposed, from lactic acid to muscle and connective-tissue damage and inflammation, and no single theory fully explains it; one recent hypothesis points to injury of sensory nerve endings.

Functional overreaching is a short dip after hard training that rebounds into better performance after several days of recovery. Overtraining syndrome is a long-term performance decline that needs weeks or months to resolve. What separates them is the time to recover, not how bad the drop looks, and there is still no definitive diagnosis or reliable biomarker for the syndrome.

Coaches track external load, such as volume and GPS distance, and internal load, such as session ratings of perceived exertion, alongside fatigue markers like wellness questionnaires and sleep. A review linked injury and illness risk to periods of intensified and accumulated load. Signal and noise matter: in one test of heart rate variability, the smallest detectable change exceeded 100 percent for one measure.

Too little sleep slows reaction time, raises errors and pushes blood sugar higher by changing how the body responds to insulin. In a review of 25 athlete studies, sleep extension and naps were the most effective ways to improve both sleep and performance, while sleep hygiene advice and evening device bans showed no effect in the few studies that tested them.

Reliability asks whether a test gives the same answer when repeated; validity asks whether it measures what you intend. The two are separate: blood lactate thresholds repeated well across five tests and also predicted race performance, while thirteen published equations for predicting maximal oxygen uptake in endurance athletes showed only moderate accuracy, erring in opposite directions for young and older athletes.

Measuring maximal oxygen uptake directly needs gas analysis and an all-out effort, so most settings estimate it from equations instead. Estimates trade accuracy for convenience: in one study of 99 active adults, a widely used metabolic equation overestimated maximal oxygen uptake from both maximal and submaximal tests, though the submaximal estimates came closer.

A field test such as the 6-minute walk estimates aerobic capacity without gas analysis, and it is used where that lab testing is not available, as in cardiac rehabilitation. Any prediction model works best in people like those it was built on: models applied to endurance athletes underestimated younger, fitter athletes and overestimated older, less trained ones.

Strength can be tested as a one-repetition maximum, as isometric force against a fixed resistance, or on an isokinetic machine that fixes movement speed; in one early study of young men the three correlated closely. Handgrip strength is a quick general marker: in over 490,000 UK Biobank adults, the weakest quarter had higher risk of developing combined heart and metabolic disease.

The classic model splits short efforts into anaerobic power, mostly from phosphocreatine and spent within seconds, and anaerobic capacity, limited by lactate build-up and exhausted within about 45 seconds. The 30-second Wingate cycle test targets that span. Its peak and mean power repeat reliably, but its fatigue index does not, so that number deserves less trust.

The Borg 6 to 20 scale lets you rate effort without equipment. In over 6,300 lab tests, ratings of about 12 to 14 matched moderate and 15 to 17 vigorous intensity, with age, sex, fitness and test design shifting the link. In a small indoor-cycling study, Borg and OMNI ratings agreed with each other but tracked heart rate poorly.

Screening sorts people by whether a medical opinion should come before starting activity. Federal guidance says people without diagnosed chronic conditions or symptoms such as chest pain or dizziness most likely do not need one before gradually becoming active. Older screening models set a low bar for physician clearance, which critics argue adds cost and keeps people from starting.

Movement screens score how well you perform a set of basic patterns, and a score of 14 or less out of 21 on the Functional Movement Screen is often used as a cutoff. A meta-analysis found a pooled association between low scores and later injury but with high heterogeneity, and concluded the link is unclear; studies in active women split on whether the screen predicts injury.

The two-compartment model splits the body into fat mass and fat-free mass: measure one, and the other follows by subtraction. Density-based methods convert body density to fat using assumed densities for each compartment, and those assumptions may not hold in elite athletes. Three- and four-compartment models add bone mineral and body water to reduce that error.

Underwater weighing and air-displacement devices such as the BOD POD both estimate body volume, and from it body density. Underwater weighing needs you to blow out all your air, so leftover lung volume is an error source. Air displacement correlates well with underwater weighing and repeats reliably, but clothing, hair, moisture and temperature can sway it, and it may miss small changes.

DXA passes two x-ray energies through the body, and because fat, lean tissue and bone absorb them differently, it reports all three, including for single limbs. Many treat it as the criterion standard, but it still needs controlled conditions: recent food, exercise and hydration all shift the result.

Skinfold calipers measure folds of skin and fat at set sites, and prediction equations turn those numbers into body fat percentage. The equation alone can swing the answer: one soccer player's data gave anywhere from 4 to 8 percent fat depending on the equation. Reporting the sum of the raw skinfolds avoids that conversion error, and in novice hands calipers agree between measurers less well than ultrasound.

Bioelectrical impedance sends a small current through the body and estimates body water from the resistance it meets; fat-free tissue holds more water and conducts better than fat. Fat is then inferred from equations, so hydration and temperature shift the reading. Until recently, equations not built for athletes made results in athletes inaccurate.

A tape measure around the waist captures where fat sits, not just how much there is. Fat carried around the waist rather than the hips is linked to higher risk of heart disease and type 2 diabetes, so waist size adds information about where fat is carried, and it costs almost nothing to measure.

Every method estimates rather than measures fat directly, and each is sensitive to conditions such as hydration, food and recent exercise. For tracking change, a method that varies little day to day and is used the same way each time can beat a more sophisticated one; one review favors raw skinfold sums for exactly that reason.

The US guidelines for adults set a weekly range of moderate aerobic activity, or a shorter vigorous range, or a mix of the two, and add muscle-strengthening work for all major muscle groups on at least two days. Older adults are also asked to include balance training. Setting the target per week, with activity spread across it, leaves room to fit it into different schedules.

As a rule of thumb, one minute of vigorous activity counts the same as two minutes of moderate activity. Intensity can be judged in absolute terms, the energy an activity costs, or in relative terms, the effort it takes you; on a 0 to 10 effort scale, moderate is about 5 or 6. Because the same walk is easy for one person and hard for another, older adults are asked to judge effort relative to their own fitness.

The health return from activity is steepest at the bottom. The biggest gain in lower death risk comes from moving from no activity to even a small weekly amount, there is no threshold before benefits begin, and each added minute buys relatively more for people who are not yet active than for those already meeting the guidelines.

Sedentary behavior means waking time spent sitting, reclining or lying at a very low energy cost. People who sit a lot have higher risk of death and of heart disease, type 2 diabetes and some cancers. That sitting risk disappears only at activity levels well beyond what most people reach, so cutting sitting time and adding activity both help.

Self-efficacy, your confidence that you can carry out a behavior, is widely treated as a route by which activity programs work. Techniques such as action planning, time management, self-monitoring and planning social support have raised it. Yet in a meta-analysis of programs for adults with obesity, changes in self-efficacy did not track changes in activity, so the link is not settled.

Behavior change techniques are named, testable ingredients of an intervention, such as action planning, coping planning for obstacles, and self-monitoring. In a factorial trial of 473 adults using an online program, self-monitoring raised activity and cut sitting, coping planning raised activity, and combining all three worked best for activity.

Where people live shapes how much they move: a review of 28 studies found walkable neighborhoods, good parks and playgrounds, and new or better walking and cycling infrastructure increased activity and active travel. The gains may favor better-off groups. Time is another structural barrier, which is part of the appeal of shorter interval-based exercise.

Older doubts about whether young people should lift have given way to evidence that resistance training improves performance and health markers in healthy children and adolescents, when it is properly prescribed and supervised. Children mostly build strength through play and rarely need formal programs, but such programs are considered safe under those conditions.

Before the adolescent growth spurt, children gain strength from training mainly through better nervous-system activation of muscle. Low levels of circulating androgens before then limit how much muscle structure can change. Circulating androgens rise as youth move through adolescence, easing that limit, so the same program can work through different routes at different ages.

With aging and inactivity, the ability to produce force quickly, measured as power and rate of force development, falls faster than maximal strength. That matters for countering a sudden loss of balance, which needs force fast. The US guidelines ask older adults to combine balance training with aerobic and muscle-strengthening activity, and such multicomponent programs have the best record for reducing falls.

Sarcopenia is the age-related loss of muscle mass and function that raises the risk of falls, fractures and disability. Resistance training is a leading countermeasure under study, and protein may add to it: in a meta-analysis of older adults with sarcopenia, whey protein during resistance training gave a small extra gain in muscle mass and handgrip strength, while effects on physical performance were inconsistent.

Under US guidelines, moderate activity is considered safe for generally healthy pregnant women and lowers the risk of excess weight gain and gestational diabetes. Lying on the back after the first trimester can restrict blood flow to the uterus, and contact sports raise the risk of falls or abdominal trauma. After delivery, activity improves fitness and eases symptoms of postpartum depression.

Activity helps in metabolic and cardiovascular conditions largely by acting on their risk factors. In type 2 diabetes it helps lower blood pressure, body weight, blood lipids and hemoglobin A1c, and in hypertension it slows cardiovascular disease progression. Some effects, such as lower blood pressure and better insulin sensitivity, appear on the day of a single session.

Para-sport groups athletes into classes with similar activity limitations, and each sport is meant to base its classes on how an impairment affects performance in that sport. Physiology also differs: in athletes with spinal cord injury, fixed percentages of peak heart rate or oxygen uptake landed at different intensities in tetraplegia than in paraplegia, so such percentages misjudge intensity.

On average, men gain slightly more muscle in absolute terms from the same resistance program, but relative gains from each person's starting point are similar between sexes. A 2025 meta-analysis of 29 studies found that pattern, with the male advantage limited to upper-body size. Growth of type II fibers was also similar between the sexes.

Energy availability is the dietary energy left for normal body functions after subtracting what exercise burns, usually scaled to lean body mass. It differs from energy balance, which compares intake with all expenditure and tends to drift back toward equilibrium as metabolism adapts. In short lab studies, low availability disrupted hormones and bone markers, though the exact cutoff is debated in women and unknown in men.

Rather than eating the same carbohydrate every day, athletes can match it to the work planned. Starting some sessions with low carbohydrate, the train-low idea, boosts signaling and gene changes linked to endurance adaptation in most studies, but those changes improved performance in only about a third of studies that tested it. Fuel for the work required tries to keep hard sessions fueled while training low on easier ones.

For muscle gain with resistance training, how much protein you eat across the day matters more than precise timing around a workout. A meta-analysis found no timing effect once other factors were controlled, with total protein intake the strongest predictor of growth. More of a dose above about 20 grams in one meal is burned for energy, but some still goes to building tissue, so 20 grams is not a hard per-meal ceiling.

Creatine supplements raise muscle stores of phosphocreatine, which regenerates ATP at the start of intense effort and refills it between efforts, so benefits show up in short, high-intensity, repeated work rather than endurance. Responses vary between people. Across more than 1,000 studies, the only consistently reported side effect has been weight gain, mostly from water retention.

Caffeine works mainly in the brain, blocking adenosine receptors, which raises motor unit firing and lowers perceived pain and effort. It most consistently helps aerobic endurance, in both trained and untrained people. Responses vary widely between people, partly through genes for caffeine metabolism and possibly habitual intake, and side effects include poor sleep and anxiety.

Sodium bicarbonate briefly raises blood pH and is thought to help clear hydrogen ions from working muscle, though the mechanism is unclear; trials conflict, and it can cause nausea, vomiting and stomach pain. Beta-alanine raises muscle carnosine, an internal buffer, and can cause harmless tingling; reviews disagree on whether it helps, and benefits look smaller in efforts longer than about four minutes.

Beets are rich in dietary nitrate, which the body converts to nitric oxide; that widens blood vessels and lowers the oxygen cost of submaximal exercise. Benefits are more likely in recreationally active people than in elite athletes, and several studies in highly trained endurance athletes found no effect. One small study suggested a dose-response that leveled off at higher doses.

Supplements can contain substances not on the label, through cross-contamination in poor manufacturing or deliberate spiking; anabolic steroids and stimulants are common finds, and an athlete can fail a drug test this way. FDA notes that bodybuilding products are among the most often adulterated. Third-party certification checks identity and content, which adds independent assurance rather than a guarantee.

Overuse injuries build up from doing too much of one kind of activity, so variety and gradual increases are the main defenses. Acute injury risk tracks the activity itself: contact and collision sports carry more risk than swimming or walking. Structured warm-up programs have cut both overall and overuse lower-limb injuries in trials of young female players.

Injury risk rises with the gap between what you ask of a tissue and what it is used to, and with total activity: someone running many miles a week has higher risk than someone running few. Capacity is the other side, since less fit people are more likely to be injured doing the same activity. Building capacity gradually shifts that balance.

Injury and illness risk rises during periods when training intensifies or load accumulates. The acute-to-chronic workload ratio compares recent load with longer-term load to flag spikes; a 2026 meta-analysis found a small-to-moderate link between high ratios and injury, but with very high heterogeneity across load measures and calculation methods, so its practical value remains debated.

Achilles tendinopathy is among the most frequent foot and ankle overuse injuries. Patellofemoral pain at the front of the knee often appears without structural damage and is linked to inward knee collapse and weak hip abductors. Metatarsal stress fractures are common in repetitive weight-bearing training, where high intensity and poorly adapted training raise risk. Each is a problem of how load is applied and absorbed.

Older terms such as tendinitis implied inflammation, but tendinopathy tissue shows a failed healing response with degenerative change: more tendon cells, disorganized and fragmented collagen and new blood vessel growth. It is described as moving from a reactive stage through disrepair to degeneration. In one trial in men, only high-load tendon exercise made the tendon stiffer and thicker, though all groups improved clinically.

Neuromuscular training programs cut lower-limb injury risk by about 42 percent across 16 trials in youth athletes, with two or three sessions a week giving the largest reductions. Specific warm-up programs such as the 11+ have reduced knee and overall leg injuries in young female soccer players.

Tissue repair begins with acute inflammation, which clears debris and switches on repair; well-regulated inflammation is part of healing, while disrupted inflammation can leave chronic injuries. Muscle repair runs through overlapping phases from damage and inflammation to regeneration, remodeling and functional recovery. Function can lag: after knee ligament reconstruction, many patients still had strength deficits at nine months.

Time since injury does not guarantee readiness. In one study of 62 people tested after knee ligament reconstruction with strength, hop and landing tests plus questionnaires, only about 3 percent passed every criterion at six months and 11 percent at nine. Criteria often compare the injured limb with the healthy one, such as a limb symmetry index above 90 percent.