Physiology

VO2max is a rate, not a store: millilitres of oxygen consumed per minute at the point where uptake stops rising despite harder work. It reflects the whole delivery-and-use chain rather than lung size.

Source: Exercise physiology reference

Experiments that raise delivery raise VO2max, while adding muscle oxidative capacity alone generally does not. In a substantial minority of highly trained athletes, and more often in women, arterial saturation does fall at maximal effort and the lung becomes a contributing limitation.

Source: Exercise physiology reference

Because the value is mode-specific, a cycling result cannot be read against a published running norm or against a training partner tested on a treadmill. Compare an athlete only with their own earlier test in the same mode.

Source: Exercise physiology reference

Family studies find both the starting value and the size of the response to training to be substantially heritable. Quite how large that inherited share is remains debated, but it is clearly not small.

Source: Exercise physiology reference

A trained athlete may race for hours at a high percentage of VO2max where an untrained one fades far below that. This is a large part of why performance keeps improving long after VO2max has plateaued.

Source: Exercise physiology reference

The plateau criterion comes from early work, but a large share of tests never produce one, so labs fall back on respiratory exchange ratio, heart rate and perceived effort. Those substitutes are conventions rather than proof.

Source: Exercise physiology reference

Plasma volume expands within days, which fills the heart more completely and raises stroke volume. Mitochondrial and capillary changes come later and matter more for endurance than for the maximum itself.

Source: Exercise physiology reference

Scaling to mass matters when mass is part of the load being moved. A heavy rider with a large absolute VO2max can be fast on the flat and still be dropped on a climb.

Source: Exercise physiology reference

Endurance training produces eccentric hypertrophy: the chamber enlarges with a proportional increase in wall thickness and total mass, as distinct from a thickened wall around a small chamber. Better filling is what raises stroke volume.

Source: Exercise physiology reference

Blood volume and stroke volume, which the trained athlete has the most of, are the first things to go. The recreational athlete simply has less of that gain to lose.

Source: Exercise physiology reference

Glycolysis runs all the time, and lactate forms whenever its rate outpaces the entry of pyruvate into the mitochondria. There is measurable lactate in your blood while you sit still.

Source: Exercise physiology reference

Blood and muscle lactate return to resting values within about an hour of stopping, long before soreness peaks. Delayed soreness tracks eccentric work, which is why downhill running hurts and cycling rarely does.

Source: Exercise physiology reference

Lactic acid as such does not pile up; what rises is lactate, and forming it actually consumes a proton. Acidosis arises from the overall rate of ATP turnover, and how much it contributes to fatigue is itself disputed.

Source: Exercise physiology reference

Lactate is a usable fuel that moves between tissues, not a dead end. That is exactly why a blood reading reflects a balance rather than a total produced.

Source: Exercise physiology reference

Lactate has been shown to act on receptors and to influence gene expression, which is why it is sometimes called a signalling metabolite. Treat the detail as an active research area rather than settled physiology.

Source: Exercise physiology reference

Concentration is a net figure. Two athletes producing identical amounts can show very different blood values if one clears it faster, and clearance is one of the things training improves.

Source: Exercise physiology reference

A fixed 4 mmol/L value, the first rise above baseline and curve-fitting methods are answering different questions. Longer stages also give lactate more time to diffuse, which shifts the curve.

Source: Exercise physiology reference

Lactate comes from carbohydrate, so a glycogen-depleted athlete simply has less substrate to make it from. It is a well-known way to produce a flattering test result that says nothing about fitness.

Source: Exercise physiology reference

The threshold shifts because production and clearance rebalance, not because production stops. Those peripheral adaptations are precisely what a large base of aerobic training builds.

Source: Exercise physiology reference

Muscle oxygenation measurements show oxygen present well above the so-called anaerobic threshold. The lactate rise reflects the rate of glycolysis relative to mitochondrial uptake, not an absence of oxygen.

Source: Exercise physiology reference

Naming them separately is a teaching convenience, not a description of three switches. Even in the first seconds of a sprint the aerobic system is contributing, and on an easy jog there is still glycolytic flux.

Source: Exercise physiology reference

Phosphocreatine donates its phosphate to rebuild ATP almost instantly, which is why it powers the start, the jump and the surge. The store is tiny, so maximal work spends it within seconds.

Source: Exercise physiology reference

Fat can only be oxidised inside the mitochondria and needs oxygen, so it cannot feed the fast oxygen-independent pathway. That is a large part of why hard efforts depend so heavily on carbohydrate.

Source: Exercise physiology reference

Its defining feature is fuel flexibility, at the price of a slower maximum rate of ATP supply. Protein normally contributes only a few per cent, rising somewhat when carbohydrate is depleted.

Source: Exercise physiology reference

Fat oxidation peaks somewhere in the easy-to-moderate range and drops away as intensity climbs, so the hardest efforts run almost entirely on carbohydrate. Where an individual's peak sits varies enough that blanket 'fat-burning zone' prescriptions are unreliable.

Source: Exercise physiology reference

The aerobic share climbs steeply from the first seconds and passes the anaerobic contribution around the one-minute mark. Even a 400 m runner draws a substantial part of their energy aerobically.

Source: Exercise physiology reference

Beyond a few minutes the aerobic system supplies almost all of the energy, so even the shortest standard triathlon is an aerobic event. 'Sprint' describes the distance relative to other triathlons, not the physiology.

Source: Exercise physiology reference

Anaerobic capacity is small in total energy but decisive in moments, because every acceleration has to be paid for before oxygen uptake has risen to meet it. Spending it repeatedly is what makes a surgy race so much costlier than an even one.

Source: Exercise physiology reference

Muscle and liver glycogen together amount to a couple of hours of hard work at most, which is why long-course fuelling is about carbohydrate. Even a lean athlete carries far more fat energy than any race could use.

Source: Exercise physiology reference

Rebuilding phosphocreatine takes oxygen, so a bigger aerobic engine gives better repeatability of short hard efforts. It is a clear example of the systems not being independent of each other.

Source: Exercise physiology reference

Economy is a cost per unit of speed, so the more economical runner uses less oxygen at the same pace, or equivalently runs faster for the same oxygen cost.

Source: Exercise physiology reference

VO2max sets the size of the engine; economy sets how much of it a given pace asks for. Differences in economy between runners of similar capacity are easily large enough to account for minutes over 10 km.

Source: Exercise physiology reference

Roughly four fifths of the energy you burn on a bike leaves as heat, and even between elite riders efficiency spans only a few percentage points. Small differences therefore matter, but there is little room to move.

Source: Exercise physiology reference

Efficiency is a ratio of work out to energy in, and it needs a measurable work output. Running involves so much internal work swinging limbs and absorbing impact that no clean denominator exists, so cost per distance is used instead.

Source: Exercise physiology reference

Laboratory testing consistently shows an economy benefit of a few per cent, which is large by the standards of almost anything else that alters economy. Individual response ranges from little to well above the average.

Source: Exercise physiology reference

Freely chosen cadence tends to sit at or near the least costly option for that individual, so an imposed change usually costs energy at first. Cadence is adjusted for injury or mechanical reasons, not as a shortcut to economy.

Source: Exercise physiology reference

A variable that barely varies within a group cannot explain differences inside that group. This is why economy is the interesting number among elites, while VO2max is the interesting number when comparing trained with untrained.

Source: Exercise physiology reference

Dozens of variables correlate with economy, and attempts to pin it on any single mechanism have not held up. That is also a large part of why it is hard to improve deliberately.

Source: Exercise physiology reference

Aerodynamics and metabolic cost pull in opposite directions once a position becomes extreme, and the net effect is individual. This is why a position change is judged on sustained speed rather than on drag figures alone.

Source: Exercise physiology reference

Drag rises roughly with the square of velocity and the power to overcome it faster still, so a swimmer who cannot get into a lower-drag shape pays enormously for small gains in speed. Nothing in running form offers a comparable return.

Source: Exercise physiology reference

The split is by site: nervous system drive versus the muscle's own contractile capacity. Both contribute to almost every effort, and central fatigue is measurable using techniques such as twitch interpolation.

Source: Exercise physiology reference

Peripheral fatigue is mainly a disturbance of excitation-contraction coupling by accumulating metabolites. Muscle ATP never approaches zero, and the contribution of acidosis is smaller at body temperature than the older textbook account implied.

Source: Exercise physiology reference

The end spurt implies capacity was still available, which is hard to square with fatigue as simple exhaustion. Whether that regulation is a protective reserve or an effort-based decision is exactly the argument covered later in this test.

Source: Exercise physiology reference

Fatigue is task-dependent: the dominant limiter changes with duration and intensity. Glycogen and heat barely matter over a minute, while metabolite accumulation barely explains hour four.

Source: Exercise physiology reference

Current models place perceived effort in the brain's awareness of the motor command it is issuing, rather than in a signal sent up from the muscle. This is why effort climbs at constant pace as fatigue develops.

Source: Exercise physiology reference

Mental fatigue studies decouple perception from the standard physiological markers and still change performance. That makes perceived effort a mechanism in its own right, not just a readout of the body's state.

Source: Exercise physiology reference

The model's core claim is anticipatory regulation of muscle recruitment in defence of homeostasis. Note that it argues against, not for, performance being a matter of willpower.

Source: Exercise physiology reference

This is an open argument in exercise physiology, not a resolved one. The broader idea that the brain regulates output is widely accepted; the specific governor formulation remains disputed.

Source: Exercise physiology reference

Localised glycogen pools appear to support calcium release from the sarcoplasmic reticulum, which is one reason force falls before stores are empty. This is a mechanism, not simply an empty tank.

Source: Exercise physiology reference

These studies show a two-way loop: the muscle reports its state, and the nervous system throttles drive accordingly. Removing the report lets fatigue run past its usual limit, with a cost.

Source: Exercise physiology reference

Plasma volume expands within the first days, easing the competition between skin and muscle for blood flow, and sweat rate goes up rather than down. Whether weeks of heat training also expand haemoglobin mass is an active and unresolved research question.

Source: Exercise physiology reference

Earlier onset means cooling starts before heat has accumulated, and better distribution means more of the sweat produced actually evaporates. Large between-athlete differences persist after acclimation.

Source: Exercise physiology reference

Aldosterone-mediated reabsorption in the sweat duct increases, so an acclimated athlete loses more water but proportionally less sodium. This is an adaptation of the sweat gland itself, not of the kidney.

Source: Exercise physiology reference

The signature of acclimation is reduced strain at the same external workload: heart rate down, core temperature down, and the effort perceived as more comfortable.

Source: Exercise physiology reference

Different adaptations arrive on different timelines, with the cardiovascular changes leading and sweat composition trailing. Reported timelines vary between studies, so treat these as ranges.

Source: Exercise physiology reference

Decay is real but not instant, and the published numbers vary widely. Plan top-up exposures rather than assuming a camp finished a month ago is still working.

Source: Exercise physiology reference

The adaptive signal is largely the rise in core temperature, which both routes can produce, but protocol design is unsettled. Any deliberate heat protocol should be planned with qualified guidance, not improvised.

Source: Exercise physiology reference

Acclimation shifts the cost of racing in heat downwards; it does not abolish it. The realistic gain is racing the heat better, not racing it as if it were cool.

Source: Exercise physiology reference

Risk is multifactorial and includes fit, young, well-trained athletes, who are in fact heavily represented in exertional heat stroke cases because they push hardest. Reduced sweating is a warning sign, not reassurance.

Source: Sports medicine reference

Any change in mental status in a hot event must be treated as exertional heat stroke until proven otherwise, and it can be fatal: cool first, transport second, alongside an immediate call for medical help. Core temperature is what distinguishes it from exercise-associated hyponatraemia, which presents similarly, so do not push fluids until medical staff have made that distinction.

Source: Sports medicine reference

Oxygen remains about 21% of the air at any altitude; what falls is the pressure driving it across the lung. The immediate responses are hyperventilation, higher submaximal heart rate, increased urine output and a reduced VO2max.

Source: Exercise physiology reference

Erythropoietin rises within hours and ventilation increases immediately, but building actual red cell mass is a slow process. Early haematocrit changes are largely a plasma volume effect, not new red cells.

Source: Exercise physiology reference

It is an attempt to separate the stimulus from the cost: get the hypoxic exposure while keeping training quality. Whether it delivers on that promise is a separate question.

Source: Exercise physiology reference

The athlete works just as hard internally but produces less watts or speed, so the specific high-intensity stimulus is diluted. Perceived effort rises rather than falls at a given pace.

Source: Exercise physiology reference

The responder/non-responder split is one of the more robust findings in this literature and complicates any group-average recommendation. Iron status explains some but not all of the variation.

Source: Exercise physiology reference

Three to four weeks is the common practical convention rather than a proven optimum, and post-camp benefits fade over weeks. Recommendations on when to race after descent conflict between practitioners.

Source: Exercise physiology reference

This remains a live scientific dispute, complicated by small samples, difficulty blinding athletes, and the confound of a training camp's own effects. Treat confident claims in either direction with suspicion.

Source: Exercise physiology reference

Practice has converged on a rough range, but that is convention plus limited evidence, not a settled dose-response curve. Going higher increases sleep disruption and illness risk without a guaranteed payoff.

Source: Exercise physiology reference

Altitude adds physiological stress on top of training stress, and iron deficiency can leave an athlete unable to respond at all. Assess iron status and plan exposure with a sports physician rather than alone.

Source: Sports medicine reference

Racing at altitude is about tolerating hypoxia on the day, which is a different objective from the disputed sea-level transfer. Endurance pace at altitude stays below sea-level pace even in acclimatised athletes.

Source: Exercise physiology reference

The categories are defined by the time course of recovery, so the label can only be applied looking backwards. That is a genuine limitation of the framework rather than a technicality.

Source: Exercise physiology reference

Hormonal, immune and autonomic markers all shift with heavy training in healthy athletes too, so none of them separates the syndrome from a hard block. That is why the diagnosis remains one of exclusion made by a physician.

Source: Exercise physiology reference

Training is one input into a shared recovery budget rather than the whole of it. A modest load on top of a heavy life can strain an athlete more than a large load on an empty schedule.

Source: Exercise physiology reference

Chronic underfuelling and the overtraining picture overlap so heavily that they can be hard to tell apart. One of them has a clear route to treatment, which is reason enough to look for it.

Source: Exercise physiology reference

The finding is consistent enough to be described in the literature, yet it says only that the athlete is heavily fatigued, not why or for how long. Like every other candidate marker, it fails on specificity rather than on reliability.

Source: Exercise physiology reference

Prevalence estimates vary widely because definitions differ and the diagnosis is retrospective, but nothing suggests the syndrome is common. Casual use of the term inflates an uncommon condition into an everyday one.

Source: Exercise physiology reference

What appears disturbed is the capacity to recover between efforts rather than any single performance. The protocol is informative in research but has not become a diagnostic test.

Source: Exercise physiology reference

The two-form model came from clinical observation and was never validated. Resting heart rate can rise, fall or stay put in affected athletes, which is exactly why it cannot carry a diagnosis.

Source: Exercise physiology reference

Each hypothesis explains part of the picture and none accounts for all of it. Any confident single-cause explanation you read is going beyond the evidence.

Source: Exercise physiology reference

The term is popular online but is not recognised by endocrinology, and reviews of cortisol testing have not supported it. Its practical danger is that it ends the search for a diagnosis that could be treated.

Source: Exercise physiology reference

The heart's pacemaker is continuously modulated by the vagus nerve, and that modulation appears as small differences between successive beat intervals. The number is a window on nervous system activity rather than on the heart muscle itself.

Source: Exercise physiology reference

Age, genetics and anatomy set where an individual's values sit, and those differences swamp any fitness-related difference between people. It is by nature an intra-individual measure.

Source: Exercise physiology reference

The respiratory swing in heart rate is a genuine vagal phenomenon, so a deliberately slowed breath lifts the number without anything about recovery having changed. Breathing naturally during the reading matters more than most athletes realise.

Source: Exercise physiology reference

Low-frequency power reflects vagal activity, baroreflex function and breathing as well as sympathetic tone, so the ratio does not mean what its name suggests. Time-domain measures are the defensible choice for short field recordings.

Source: Exercise physiology reference

The measurement is dominated by its conditions: lying versus sitting alone moves the value substantially. Inconsistent conditions turn the series into noise before any interpretation begins.

Source: Exercise physiology reference

The association across groups is real but loose, and it describes a different property from aerobic capacity. Suppressed reactivation immediately after hard work is the expected response, not a warning.

Source: Exercise physiology reference

The measure aggregates every stressor into one number, so it can flag that something has changed but never what changed. Attribution has to come from the athlete's knowledge of the fortnight.

Source: Exercise physiology reference

Very high vagal indices have been described alongside heavy fatigue, so a rising number is not automatically good news. This ambiguity is among the strongest arguments against reading the value in isolation.

Source: Exercise physiology reference

The idea is promising and has been tested, but mostly in small groups of moderately trained people using different decision rules. That is reason to treat it as an experiment on yourself rather than as settled practice.

Source: Exercise physiology reference

Optical sensors estimate pulse intervals rather than recording the electrical signal, and averaging windows differ, so values from different devices should not be merged into one trend. A composite score you cannot inspect also cannot be argued with.

Source: Exercise physiology reference

Family studies of trainability point to a large inherited component in how much aerobic capacity improves, but they do not make the environment irrelevant. Both halves of that are worth holding onto.

Source: Exercise physiology reference

A spread of individual results appears even when everyone responds identically, purely from test-to-test noise. Distinguishing real individual response from that noise is a study design problem rather than an observation.

Source: Exercise physiology reference

Work that raised the dose in apparent non-responders found many of them responded after all. Whether a true non-responder exists remains an open argument rather than a settled fact.

Source: Exercise physiology reference

Central capacity, threshold and economy are partly independent adaptations running on different timescales. Being labelled a poor responder usually means poor response in whichever variable happened to be tested.

Source: Exercise physiology reference

The oxygen-carrying differences are well documented as group averages, with large overlap between individuals. The claim that the gap reverses in very long events is not supported in ultra-running or ultra-cycling, though women do match or outperform men in ultra-distance open-water swimming, so it depends on the discipline.

Source: Exercise physiology reference

Audits of the sport science literature repeatedly find women a clear minority of participants. That is a gap in the evidence rather than proof the findings do not transfer, which is why it calls for caution rather than dismissal.

Source: Exercise physiology reference

Reviews of this literature describe small studies with inconsistent methods and low confidence in the conclusions. An athlete tracking her own symptoms and sessions has better information than the group averages currently offer.

Source: Exercise physiology reference

Continued training shifts the whole curve upwards and flattens it, but the downward slope remains. Much of the steep decline seen in the general population reflects activity dropping away rather than ageing alone.

Source: Exercise physiology reference

Maximal heart rate, muscle mass and high-intensity power decline earlier and faster than submaximal aerobic endurance. These are capacity changes: competitive performance declines in both short and long events, and record data show the steeper decline in the longer ones.

Source: Exercise physiology reference

A group mean conceals the range of individual results underneath it, and significance speaks to whether an effect exists rather than to how big it is. Reading the description of the participants is usually more informative than reading the conclusion.

Source: Exercise physiology reference