Mechanisms of Training Adaptation

Cardiac output equals heart rate times stroke volume, the blood ejected by the left ventricle with each beat.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

Ekblom and Hermansen (1968) measured maximal cardiac outputs near 40 L/min in elite endurance athletes, about double untrained values.

Source: Ekblom & Hermansen, J Appl Physiol 1968 · recall

Maximal heart rate barely changes with training, so the extra output comes almost entirely from a bigger stroke volume.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

A ventricle that fills with more blood is stretched further and contracts more forcefully, so stroke volume rises with venous return.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

The classic finding is a stroke volume plateau at about 40-60% of VO2max in untrained subjects; further rises in output come from heart rate.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

Gledhill (1994) found stroke volume continued to increase to maximal exercise in elite endurance athletes, helped by faster ventricular filling.

Source: Gledhill, Cox & Jamnik, Med Sci Sports Exerc 1994 · recall

Oxygen demand at a fixed pace barely changes; a larger stroke volume delivers the same cardiac output with fewer beats.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · applied

Upright, gravity pools blood in the legs and lowers venous return; horizontal, more blood reaches the heart and fills it further.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · applied

The arterial-venous oxygen difference is how much oxygen is removed from the blood as it passes through the tissues.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

Maximal heart rate is largely age-determined and stays the same or drops a few beats with training; the gain came from stroke volume and extraction.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · applied

During prolonged steady exercise, especially in heat, heart rate climbs gradually while stroke volume falls.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

Heart rate rises to offset the falling stroke volume, so cardiac output is roughly maintained.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

The classic view is that sweating lowers plasma volume and more blood is sent to the skin, so less returns to fill the heart.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

Their 2001 review argued that a heart rate rise driven by body temperature leaves less time to fill the ventricle, lowering stroke volume.

Source: Coyle & Gonzalez-Alonso, Exerc Sport Sci Rev 2001 · recall

Heat raises skin blood flow and body temperature, and dehydration lowers plasma volume; both increase drift.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

Heart rate and core temperature rose in proportion to the degree of dehydration: the less the riders drank, the greater the drift.

Source: Montain & Coyle, J Appl Physiol 1992 · recall

If heart rate drifts up at a constant effort, holding heart rate constant forces power down, which is why long-course athletes often cap power instead.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · applied

Decoupling measures how much the power-to-heart-rate ratio worsens from the first to the second half; a small figure suggests good aerobic endurance.

Source: TrainingPeaks documentation, Pw:Hr and Pa:Hr · recall

Their work showed drift in the heat coincides with a lower VO2max, so the same absolute workload becomes a higher relative intensity.

Source: Wingo, Lafrenz, Ganio, Edwards & Cureton, Med Sci Sports Exerc 2005 · applied

Limiting dehydration and heat storage limits the two main causes of drift, so heart rate stays closer to its early value.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · applied

Plasma volume can rise by about 10% or more within days of starting training, one of the fastest cardiovascular adaptations.

Source: Convertino, Med Sci Sports Exerc 1991 · recall

More plasma dilutes the red cells, so the concentration drops although the total amount of haemoglobin is the same or higher.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · applied

It is not true anaemia: red-cell mass is normal, only diluted by expanded plasma.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

Concentration depends on plasma volume; haemoglobin mass is the total quantity in circulation and tracks oxygen-carrying capacity directly.

Source: Schmidt & Prommer, Eur J Appl Physiol 2005 · recall

In the optimised CO-rebreathing method, a small dose of carbon monoxide binds to haemoglobin, and its dilution gives the total mass.

Source: Schmidt & Prommer, Eur J Appl Physiol 2005 · recall

Across studies, VO2max rises by about 4 mL/min for every gram of haemoglobin mass.

Source: Schmidt & Prommer, Med Sci Sports Exerc 2010 · recall

Each gram of haemoglobin binds about 1.34 mL of oxygen (Hufner's number), so a litre of normal blood carries roughly 200 mL.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

Untrained men average around 11 g/kg, elite endurance athletes around 14-15 g/kg or more.

Source: Heinicke et al., Eur J Appl Physiol 2001 · recall

Plasma is back within a day or two, but red-cell mass takes several weeks to rebuild, so oxygen-carrying capacity stays lower.

Source: Kiss et al., JAMA 2015 · applied

Red cells last about four months before the spleen and liver remove them, so red-cell mass changes slowly.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

Ferritin is the storage form of iron; its level in blood tracks body stores when there is no inflammation.

Source: Sim et al., Eur J Sport Sci 2019 · recall

Ferritin rises with inflammation, so it should be measured rested and healthy, ideally in the morning.

Source: Sim et al., Eur J Sport Sci 2019 · applied

Hepcidin blocks iron export from gut cells and stores, lowering absorption and circulating iron.

Source: Peeling et al., Eur J Appl Physiol 2008 · recall

Exercise raises interleukin-6, which drives hepcidin to peak roughly 3-6 hours later, briefly reducing iron absorption.

Source: Sim et al., Eur J Sport Sci 2019 · recall

Stoffel and colleagues showed each dose raises hepcidin for around 24 hours; alternate-day dosing increased fractional absorption.

Source: Stoffel et al., Lancet Haematol 2017 · applied

The repeated impact of footfalls ruptures red cells in the foot's vessels, one of several routes of iron loss in runners.

Source: Telford et al., J Appl Physiol 2003 · recall

Stores run down first; haemoglobin falls only later, when the deficiency becomes anaemia.

Source: Peeling et al., Eur J Appl Physiol 2008 · applied

Supplementing replete athletes brings no benefit and can build up excess iron, so it should follow a blood test.

Source: Sim et al., Eur J Sport Sci 2019 · recall

Vitamin C enhances non-haem iron absorption; polyphenols in tea and coffee and calcium inhibit it.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · applied

Haem iron is absorbed at roughly 15-35%, non-haem iron at a few percent to around 20%, depending on the meal.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

PGC-1alpha coordinates the genes that build new mitochondria and is activated by endurance exercise.

Source: Hood, Appl Physiol Nutr Metab 2009 · recall

As ATP is used, AMP rises; AMPK senses this and switches on pathways that increase energy supply and mitochondrial growth.

Source: Hardie, Genes Dev 2011 · recall

Holloszy showed endurance training about doubled the activity of mitochondrial respiratory enzymes, founding the field.

Source: Holloszy, J Biol Chem 1967 · recall

Holloszy and Coyle explained that a smaller disturbance per mitochondrion means less glycogen use and lactate at the same pace.

Source: Holloszy & Coyle, J Appl Physiol 1984 · applied

Two weeks of about 2.5 h of sprint intervals matched about 10.5 h of endurance training for muscle oxidative capacity markers.

Source: Gibala et al., J Physiol 2006 · recall

Citrate synthase activity correlates closely with mitochondrial volume and is the most common biopsy marker of it.

Source: Larsen et al., J Physiol 2012 · recall

Myoglobin binds oxygen within muscle cells, supporting diffusion to the mitochondria; oxidative fibres hold more of it.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

More capillaries per fibre slow transit time and shorten diffusion distance, improving oxygen and fuel delivery.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

Vascular endothelial growth factor rises with shear stress, stretch and low oxygen, driving angiogenesis.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

Mitochondrial and capillary adaptations occur in the muscles that did the work, which is one reason each discipline needs its own training.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · applied

Type I fibres contract slowly, have many mitochondria and capillaries, and are the main endurance fibres.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

Type IIx fibres have the fastest myosin and the lowest oxidative capacity; IIa fibres sit between I and IIx.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

A small muscle sample is stained for myosin ATPase or analysed for myosin heavy-chain isoforms.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

Elite distance runners in Costill's 1976 study averaged about 79% slow-twitch fibres in the gastrocnemius.

Source: Costill et al., J Appl Physiol 1976 · recall

The clearest shift is from IIx to the more oxidative IIa; a full type II to type I switch in humans is small at most.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

Low-force efforts use small, slow motor units; higher forces add progressively larger, faster units.

Source: Henneman et al., J Neurophysiol 1965 · recall

As type I fibres fatigue and lose glycogen, holding the same force needs more motor units, including type II ones.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · applied

Coyle et al. (1992) linked a higher type I percentage to higher gross efficiency in trained cyclists.

Source: Coyle et al., Med Sci Sports Exerc 1992 · recall

Twin and family studies suggest a large genetic share, with training and environment accounting for the rest.

Source: Simoneau & Bouchard, FASEB J 1995 · recall

Hybrid fibres such as I/IIa or IIa/IIx carry mixed myosin isoforms and become more common while fibres are changing type.

Source: Pette & Staron, Microsc Res Tech 2000 · recall

Brooks and Mercier described a crossover point beyond which carbohydrate provides most of the energy as exercise intensity increases.

Source: Brooks & Mercier, J Appl Physiol 1994 · recall

Achten and Jeukendrup found peak fat oxidation around 60-65% of VO2max in trained subjects, lower in untrained people.

Source: Achten, Gleeson & Jeukendrup, Med Sci Sports Exerc 2002 · recall

RER is VCO2/VO2; it is about 0.7 when burning fat and 1.0 when burning carbohydrate.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

Between the fat value (0.7) and the carbohydrate value (1.0), an RER of 0.85 corresponds to about half and half.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · applied

These fat droplets sit next to mitochondria; trained athletes store and use more of them.

Source: van Loon, J Appl Physiol 2004 · recall

Carbohydrate gives about 5.05 kcal per litre of oxygen against about 4.7 for fat, and at a much faster rate.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · applied

Around 400-500 g of glycogen holds roughly 1,600-2,000 kcal, against tens of thousands of kcal in fat.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

The ketogenic diet raised fat oxidation markedly but reduced economy and wiped out the performance gain seen on high-carbohydrate diets.

Source: Burke et al., J Physiol 2017 · recall

Insulin inhibits lipolysis, so fewer fatty acids are released into the blood for the muscle to use.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · applied

At higher intensity, reduced carnitine availability limits CPT1, the gateway that carries fatty acids into mitochondria.

Source: van Loon et al., J Physiol 2001 · recall

Hickson's 1980 study is the classic starting point for research on concurrent training.

Source: Hickson, Eur J Appl Physiol 1980 · recall

The combined group gained strength like the strength-only group until about week seven, then levelled off and declined.

Source: Hickson, Eur J Appl Physiol 1980 · recall

AMPK, activated by endurance exercise, can inhibit mTOR, the key pathway for muscle protein synthesis.

Source: Coffey & Hawley, J Physiol 2017 · recall

Running, with its impact and eccentric load, produced greater interference than cycling.

Source: Wilson et al., J Strength Cond Res 2012 · recall

Concurrent training did not compromise maximal strength or hypertrophy overall but blunted explosive strength gains.

Source: Schumann et al., Sports Med 2022 · recall

Reviews find added heavy strength work improves cycling and running economy without reducing VO2max.

Source: Ronnestad & Mujika, Scand J Med Sci Sports 2014 · recall

A gap of about six hours or more lets the signalling from the first session settle before the second.

Source: Coffey & Hawley, J Physiol 2017 · applied

Heavy loads (roughly 4-10 repetition maximum) gave the clearest economy and performance gains in endurance athletes.

Source: Ronnestad & Mujika, Scand J Med Sci Sports 2014 · recall

A stronger muscle works at a lower relative force per contraction, and may recruit fewer, more efficient fibres.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · applied

Studies adding heavy strength work to endurance training report little or no gain in body mass.

Source: Ronnestad & Mujika, Scand J Med Sci Sports 2014 · applied

In some athletes, arterial oxygen saturation drops noticeably during near-maximal exercise at sea level.

Source: Dempsey & Wagner, J Appl Physiol 1999 · recall

It is most common in highly trained endurance athletes with very high cardiac outputs, and may be more frequent in women.

Source: Dempsey & Wagner, J Appl Physiol 1999 · recall

A very high cardiac output speeds blood through the lung capillaries, shortening the time for oxygen to diffuse in.

Source: Dempsey & Wagner, J Appl Physiol 1999 · recall

The work of breathing at maximal effort takes roughly 10% of VO2max, up to about 15% in highly fit athletes.

Source: Aaron et al., J Appl Physiol 1992 · recall

Fatiguing diaphragm and breathing muscles trigger reflex vasoconstriction in the limbs, cutting leg blood flow.

Source: Harms et al., J Appl Physiol 1997 · recall

Breathing large volumes of dry or irritant air cools and dries the airways, which then narrow, often peaking after exercise.

Source: Parsons et al., Am J Respir Crit Care Med 2013 (ATS guideline) · recall

Years of heavy breathing just above chlorinated water expose the airways to irritant chloramines.

Source: Bougault et al., Sports Med 2009 · recall

A drop of at least 10% in forced expiratory volume in one second after the challenge is the usual diagnostic threshold.

Source: Parsons et al., Am J Respir Crit Care Med 2013 (ATS guideline) · recall

Meta-analyses report stronger breathing muscles and small performance gains, most visible in some sports and longer efforts.

Source: Illi et al., Sports Med 2012 · recall

The larynx narrows during hard exercise, causing a harsh sound on breathing in; it is often mistaken for asthma.

Source: Halvorsen et al., Allergy 2017 · recall

Hydrostatic pressure squeezes the legs and abdomen, shifting several hundred millilitres of blood centrally.

Source: Pendergast et al., Compr Physiol 2015 · recall

The central blood shift is sensed as too much volume, so the kidneys excrete more water: immersion diuresis. Cold adds to it.

Source: Pendergast et al., Compr Physiol 2015 · applied

Cold water on the face triggers vagal slowing of the heart plus peripheral vasoconstriction.

Source: Shattock & Tipton, J Physiol 2012 · recall

Water's thermal conductivity is about 25 times that of air, which is why cool water chills the body quickly.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

Studies consistently find a lower maximal heart rate in swimming, often by about 10-15 beats per minute.

Source: Magel et al., J Appl Physiol 1975 · recall

Horizontal posture and immersion raise venous return, cool water limits heat load, and the arms are a smaller muscle mass.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall

Because swim heart rate is lower at the same relative effort, zones copied from running would push the swimmer too hard.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · applied

Immersion loads the chest wall and pushes abdominal contents up, making inspiration more effortful.

Source: Pendergast et al., Compr Physiol 2015 · recall

The sympathetic cold shock and the vagal diving response at the same time can trigger heart rhythm disturbances.

Source: Shattock & Tipton, J Physiol 2012 · recall

Water pressure rises by roughly 10 kPa, or 0.1 atmosphere, per metre of depth, enough to shift blood even near the surface.

Source: Kenney, Wilmore & Costill, Physiology of Sport and Exercise (8th ed.) · recall