Exercise physiology

In short

Exercise physiology studies how the body's systems respond to a single bout of exercise (acute response) and how they remodel themselves when the bout is repeated for weeks (chronic adaptation). It supplies the measurable basis of training: energy systems, VO2 max, heart rate zones, lactate threshold, fatigue, recovery, hydration and environmental stress. In Indian universities it is a core paper of B.P.Ed, M.P.Ed and B.Sc Sports Science, and is examined in UGC-NET Physical Education, SAI/NSNIS Patiala diploma and coaching entrance tests. On the job it is used by strength and conditioning coaches, sports scientists at SAI centres and Khelo India Accredited Academies, physiotherapy and rehab teams, corporate wellness and fitness-industry roles, and by anyone running the Khelo India Fitness Assessment or a lab VO2 max test.

Explained

Scope, homeostasis and the ATP currency

• Exercise physiology - the study of functional changes caused by physical activity; sports physiology applies it to athletic performance and training prescription.
• Homeostasis is the steady internal state at rest; exercise disturbs it and the body defends a new steady state through feedback control of HR, ventilation, temperature and pH.
• ATP is the only immediately usable energy currency. Muscle stores just ~5 mmol ATP/kg wet muscle - enough for 1-2 s of maximal work, so ATP must be resynthesised continuously.
• Energy release is coupled: exergonic catabolism (fuel breakdown) drives endergonic work (cross-bridge cycling, Ca²⁺ pumping, Na⁺-K⁺ pumping).
• Fuels: phosphocreatine, muscle and liver glycogen, blood glucose, intramuscular and adipose triglyceride, and (in prolonged starvation/ultra events) protein as amino acids.

The three energy systems and their interplay

1. ATP-PCr (phosphagen) system - anaerobic, alactic. Creatine kinase: PCr + ADP → ATP + Cr. PCr store ~20-25 mmol/kg wet muscle. Highest power output, smallest capacity; dominant 0-10 s. Also the myokinase reaction (2 ADP → ATP + AMP).
2. Anaerobic glycolysis (lactic acid system) - glucose/glycogen → 2 pyruvate → 2 lactate; net 2 ATP from glucose, 3 ATP from glycogen (one less ATP spent in phosphorylation). Dominant ~10 s to 2 min; peak blood lactate 15-25 mmol/L after a 400 m.
3. Oxidative (aerobic) system - glycolysis → pyruvate → acetyl-CoA → Krebs cycle → electron transport chain in mitochondria, with O₂ as final electron acceptor forming water. Yields about 30-32 ATP per glucose (older texts say 38; the lower modern figure accounts for the cost of shuttling NADH and ADP/ATP across the mitochondrial membrane). Beta-oxidation of one palmitate yields about 106 ATP - far more energy per molecule, but fat needs more O₂ per ATP, so it cannot support high intensity.
4. Interplay: all three always run; only the *proportion* changes with intensity and duration. The "crossover" from fat to carbohydrate occurs as intensity rises; at >75% VO2 max carbohydrate dominates.
5. Lactate is not a waste product. It is shuttled to other fibres, heart, liver and kidney for oxidation or for gluconeogenesis (Cori cycle). Acidosis comes from accumulated H⁺, not from lactate itself.
6. Lactate threshold (LT1) - the workload where blood lactate first rises clearly above rest (~2 mmol/L). OBLA / LT2 - the 4 mmol/L reference point, close to the maximal lactate steady state; the single best predictor of endurance performance, often at 55-65% VO2 max untrained and 80-90% in elite endurance athletes.

Muscle fibre types and the neuromuscular response

• A motor unit = one motor neuron + all fibres it supplies. Force is graded by recruitment (small, slow units first - Henneman's size principle) and by rate coding (firing frequency).
• Types: Type I (slow oxidative), Type IIa (fast oxidative-glycolytic), Type IIx (fast glycolytic).
• Training can shift IIx → IIa and change the metabolic profile of any fibre; wholesale I ↔ II conversion is not normally seen in humans.
• Acute responses: increased motor-unit recruitment, raised muscle temperature (improving enzyme activity and contraction velocity - the physiological basis of warm-up), and post-activation potentiation.
• Strength gains in the first 4-6 weeks are mainly neural (better recruitment, synchronisation, reduced co-contraction); measurable hypertrophy (myofibrillar protein accretion, satellite-cell activity) follows.
• DOMS (delayed onset muscle soreness) peaks 24-48 h after eccentric work; caused by micro-damage and inflammation, not by lactate.

Cardiorespiratory responses to acute exercise

• Cardiac output Q = HR × SV. Rest ~5 L/min; maximal ~20-22 L/min untrained, 35-40 L/min in elite endurance athletes.
• HR rises linearly with intensity to HRmax; SV rises to about 40-50% VO2 max and then plateaus in untrained people, but keeps rising in trained athletes (greater ventricular filling and contractility).
• Blood pressure: systolic rises to 180-220 mmHg in heavy aerobic work while diastolic stays flat or falls slightly. In resistance exercise both rise sharply (pressor response), exaggerated by the Valsalva manoeuvre.
• Redistribution: at rest skeletal muscle receives ~20% of Q; in maximal exercise ~80-85%, by vasodilation in active muscle and vasoconstriction in gut, kidney and inactive muscle. Coronary flow rises; brain flow is protected.
• Oxygen extraction: arterio-venous O₂ difference widens from ~40-50 mL/L at rest to 150-170 mL/L maximally.
• Ventilation VE = tidal volume × respiratory frequency. Rest ~6 L/min; maximal 100-150 L/min (over 180 in large elite athletes). Early increase is neural (central command), later chemical (CO₂, H⁺, K⁺).
• In healthy people ventilation does not limit VO2 max; the limit is central (Q, O₂ delivery) and peripheral (mitochondrial and capillary density).
• Oxygen deficit - the gap between O₂ required and O₂ consumed at the start of exercise, met anaerobically. EPOC (excess post-exercise oxygen consumption, "oxygen debt") - the elevated VO₂ during recovery, used to restore PCr and O₂ stores, clear lactate, and pay for raised temperature, catecholamines and ventilation.
• VO2 max - the highest rate of O₂ uptake, the classic measure of aerobic power, identified by a plateau in VO₂ despite rising workload (secondary criteria: RER > 1.10, HR within 10 bpm of predicted max, RPE ≥ 19, blood lactate > 8 mmol/L). Typical values: 35-45 mL/kg/min untrained young men, 30-38 untrained women, 70-85 elite endurance athletes.
• Hormonal response: adrenaline and noradrenaline, cortisol, growth hormone and glucagon rise; insulin falls while muscle glucose uptake rises via contraction-mediated GLUT-4 translocation - the reason exercise is first-line therapy in type 2 diabetes.

Training principles, adaptations and reversibility

• Principles: overload, progression, specificity (SAID - specific adaptation to imposed demand), individual difference, reversibility, variation, recovery, and periodisation (macro-, meso- and microcycle; preparatory → competitive → transition phases), ending with a taper.
• FITT-VP prescription: Frequency, Intensity, Time, Type, Volume, Progression.
• Aerobic adaptations (central): larger ventricular cavity and eccentric hypertrophy, higher SV, lower resting and submaximal HR (bradycardia), expanded plasma and total blood volume, modest rise in haemoglobin mass.
• Aerobic adaptations (peripheral): more and larger mitochondria (PGC-1α driven biogenesis), higher oxidative enzyme activity (succinate dehydrogenase, citrate synthase), greater capillary-to-fibre ratio, more myoglobin, larger glycogen stores, better fat oxidation, higher lactate threshold. VO2 max in untrained adults typically improves 10-25% in 8-12 weeks; the threshold and economy keep improving long after VO2 max has plateaued.
• Anaerobic/strength adaptations: raised PCr and glycogen stores, higher glycolytic enzymes and buffering capacity, hypertrophy, better rate of force development, tendon and bone strengthening.
• Overreaching vs overtraining: planned short-term overreaching is followed by supercompensation; overtraining syndrome brings performance decline, raised resting HR, disturbed sleep and mood, frequent infection, and needs weeks to months of rest.
• Detraining: blood volume and plasma fall within days; VO2 max can drop up to ~10% in 2-3 weeks of inactivity and 15-20% in 2 months; strength is lost more slowly than endurance. Short breaks are better bridged with reduced-volume, maintained-intensity work.
• Methods: continuous, interval (work-relief ratios 1:3 for speed, 1:1 or less for aerobic intervals), fartlek, circuit, HIIT, plyometric, isometric/isotonic/isokinetic resistance, repetition and tempo runs.

Fatigue, recovery, nutrition and environmental physiology

• Fatigue mechanisms: PCr depletion (sprints), H⁺ accumulation and impaired Ca²⁺ handling (1-2 min events), glycogen depletion and hypoglycaemia (prolonged events - "hitting the wall"), dehydration and hyperthermia, and central fatigue (reduced motor drive).
• Recovery: active cool-down clears lactate faster than passive rest; glycogen resynthesis needs 1.0-1.2 g carbohydrate/kg/h for 4 h after exercise; ~0.3 g protein/kg supports repair; sleep is the main anabolic window.
• Fuel guidelines (ACSM/IOC consensus): carbohydrate 5-7 g/kg/day for moderate training and up to 8-12 g/kg/day for heavy endurance loads; protein 1.2-2.0 g/kg/day; fat ≥20% of energy. Indian diets are usually carbohydrate-sufficient but iron- and protein-short, and iron-deficiency anaemia is common in Indian female athletes.
• Hydration: thirst under-reports need. Sweat rate = (pre-exercise mass − post-exercise mass + fluid taken − urine) / time; in Indian summer conditions 1.0-2.5 L/h is common. Replace to within ~2% body mass; drinking plain water in excess during long events risks exercise-associated hyponatraemia.
• Heat: heat is lost mainly by evaporation of sweat (~580 kcal per litre); humid conditions blunt it. Acclimatisation over 10-14 days raises plasma volume, starts sweating earlier, makes sweat more dilute and lowers HR and core temperature. Heat illness ladder: cramps → heat exhaustion → exertional heat stroke (core >40 °C with CNS signs; treat by immediate cold-water immersion, then transport).
• Cold: vasoconstriction, shivering thermogenesis; risk of hypothermia and reduced dexterity.
• Altitude: falling barometric pressure lowers inspired PO₂; VO2 max drops roughly 1% per 100 m above about 1,500 m. Acclimatisation (2-3 weeks) brings hyperventilation, raised EPO and red-cell mass, but reduced plasma volume and training intensity - hence "live high, train low". SAI's high-altitude centre at Shilaroo, Himachal Pradesh is the main Indian venue.
• Ergogenic aids, legal: caffeine 3-6 mg/kg, creatine monohydrate (20 g/day for 5-7 days, then 3-5 g/day), beta-alanine, sodium bicarbonate, dietary nitrate (beetroot juice).
• Doping: anabolic-androgenic steroids, EPO and blood doping, hGH, stimulants, diuretics and beta-blockers (sport-specific) are on the WADA Prohibited List, revised each year. India's testing is governed by the National Anti-Doping Act, 2022, with NADA as the testing authority, the National Anti-Doping Appeal Panel, and the NDTL, New Delhi as the WADA-accredited laboratory. Athletes bear strict liability for whatever is in their sample, and Therapeutic Use Exemptions must be obtained in advance.

Testing, norms and Indian practice

• Lab tests: graded treadmill (Bruce, Balke) or cycle ergometer protocol with open-circuit indirect calorimetry (metabolic cart, Douglas bag) for VO2 max, RER and ventilatory threshold; Wingate 30-s test for anaerobic power; blood-lactate stepwise test; isokinetic dynamometry; DEXA/skinfolds for body composition.
• Field tests: Cooper 12-minute run/walk, 1.5-mile run, 20 m multistage shuttle (beep) test, Yo-Yo intermittent recovery test, Harvard step test, Queens College step test, Rockport 1-mile walk, 50 m dash, standing broad jump, vertical jump (Sargent), sit-ups, sit-and-reach, 4 × 10 m shuttle.
• Indian batteries: the Khelo India Fitness Assessment (school ages, logged on the Khelo India mobile app) uses BMI, 50 m dash, 600 m run/walk, standing broad jump, sit-ups, sit-and-reach, 4 × 10 m shuttle run, plate tapping, and flamingo balance for the youngest group; the Fit India School certification runs on the same idea. Older curricula still teach the AAHPER youth fitness test, JCR and Barrow motor ability tests.
• Intensity monitoring: %HRmax, %HR reserve (Karvonen), %VO2 max, Borg RPE 6-20 (and CR-10), talk test, blood lactate, power meters.
• Health thresholds often quoted in Indian exam answers: ≥150 min/week moderate or 75 min/week vigorous aerobic activity plus 2 days of resistance training (WHO 2020 and ACSM); Asian-Indian BMI cut-offs from the 2009 national consensus - overweight ≥23 kg/m², obese ≥25 kg/m², waist ≥90 cm men and ≥80 cm women - are stricter than global WHO cut-offs.
• Screening: pre-participation questionnaire (PAR-Q+), and medical referral for cardiac symptoms, uncontrolled hypertension or diabetes before maximal testing. Always have an emergency plan and a cool, shaded recovery area for testing in Indian field conditions.

Key points

• ATP - the immediate energy currency; muscle stores last only 1-2 s of maximal work.
• ATP-PCr system - anaerobic and alactic, maximum power, dominant 0-10 s.
• Anaerobic glycolysis - 2 ATP per glucose (3 per glycogen glucosyl unit), dominant 10 s-2 min, produces lactate and H⁺.
• Oxidative system - ~30-32 ATP per glucose, ~106 per palmitate; needs O₂, dominant beyond ~2 min.
• Lactate threshold / OBLA - workload at ~4 mmol/L lactate; the best single predictor of endurance performance.
• VO2 max - maximal aerobic power in mL/kg/min; limited mainly by O₂ delivery (cardiac output), not by the lungs.
• Fick equation - VO2 = cardiac output × a-vO₂ difference; training improves both terms.
• Q = HR × SV - endurance training raises SV and lowers resting HR (bradycardia).
• Oxygen deficit and EPOC - anaerobic cover at the start, elevated O₂ uptake in recovery.
• Fibre types - Type I slow oxidative, IIa fast oxidative-glycolytic, IIx fast glycolytic; IIx → IIa shifts with training.
• Size principle - slow motor units are recruited first; fast units only at higher force demands.
• Early strength gain is neural; hypertrophy appears later.
• Specificity (SAID) - adaptation matches the energy system, muscle group and velocity trained.
• Reversibility - VO2 max can fall ~10% in 2-3 weeks of inactivity; strength declines more slowly.
• Karvonen formula - target HR = HRrest + % of (HRmax − HRrest); more individual than %HRmax.
• 1 MET = 3.5 mL O₂/kg/min, the resting metabolic rate; intensity and energy cost are expressed in METs.
• Heat loss in exercise is mainly evaporative; humidity, not just temperature, limits it - critical in Indian conditions.
• Strict liability - under the National Anti-Doping Act, 2022, an athlete is responsible for any prohibited substance found in their sample.

Formulas and examples

Example 1 - Fick equation and oxygen pulse. A 70 kg athlete at VO2 max has HR 190 bpm, SV 130 mL and an a-vO₂ difference of 160 mL/L.

• Q = 190 × 130 = 24,700 mL/min = 24.7 L/min.
• VO₂ = 24.7 × 160 = 3,952 mL/min ≈ 3.95 L/min.
• Relative VO2 max = 3,952 ÷ 70 = 56.5 mL/kg/min.
• Oxygen pulse = 3,952 ÷ 190 = 20.8 mL O₂/beat.

Example 2 - Karvonen target zone. A 20-year-old with resting HR 60 bpm is prescribed 60-80% HR reserve.

• HRmax = 220 − 20 = 200 bpm; HRR = 200 − 60 = 140 bpm.
• Lower = 60 + (0.60 × 140) = 60 + 84 = 144 bpm; upper = 60 + (0.80 × 140) = 60 + 112 = 172 bpm.
• Using Tanaka instead: HRmax = 208 − (0.7 × 20) = 194 bpm, HRR = 134, zone = 140-167 bpm - a reminder that predicted HRmax carries real error, so RPE should be used alongside.

Example 3 - Cooper 12-minute run. A 21-year-old male covers 2,600 m.

• VO2 max = (2,600 − 504.9) ÷ 44.73 = 2,095.1 ÷ 44.73 = 46.8 mL/kg/min.
• For a young adult male this is an average-to-good rating; a value above about 55 mL/kg/min would be needed for competitive middle-distance running.

Example 4 - Step tests. A subject completes the Harvard step test for the full 5 min (t = 300 s); 30-s recovery pulse counts are P1 = 45, P2 = 38, P3 = 32.

• Long form: PFI = (100 × 300) ÷ (2 × 115) = 30,000 ÷ 230 = 130.4 → excellent (above about 96 is high/excellent).
• Short form: PFI = (100 × 300) ÷ (5.5 × 45) = 30,000 ÷ 247.5 = 121.2 - the same conclusion from one pulse count.
• If the same subject's Queens College recovery HR (15-s count of 38 → 152 bpm) is used: VO2 max = 111.33 − (0.42 × 152) = 111.33 − 63.84 = 47.5 mL/kg/min.

Example 5 - MET cost and cycling efficiency. A 70 kg man cycles at 150 W for 30 min; measured VO₂ = 2.00 L/min and VCO₂ = 1.70 L/min.

• RER = 1.70 ÷ 2.00 = 0.85 (mixed fuel, roughly half carbohydrate and half fat), caloric equivalent ≈ 4.86 kcal/L O₂.
• Energy expenditure = 2.00 × 4.86 = 9.72 kcal/min → 30 min = 292 kcal.
• Relative VO₂ = 2,000 ÷ 70 = 28.6 mL/kg/min → 28.6 ÷ 3.5 = 8.2 METs. Cross-check by the MET formula: 8.2 × 3.5 × 70 ÷ 200 = 10.0 kcal/min - slightly above the measured 9.72 kcal/min, because the MET formula assumes 5 kcal per litre of O₂ whereas the measured RER of 0.85 gives 4.86.
• Work output = 150 J/s × 60 = 9.0 kJ/min; energy expended = 9.72 × 4.184 = 40.7 kJ/min → gross efficiency = 9.0 ÷ 40.7 = 22.1%, the usual 20-25% for cycling.

Where it is used

• SAI and NSNIS Patiala sports science departments, National Centres of Excellence and Khelo India Accredited Academies - VO2 max and lactate testing, load monitoring, talent identification and rehabilitation of national campers under the TOPS scheme.
• Strength and conditioning / sports science roles with IPL, ISL, Pro Kabaddi and state associations - periodised programming, GPS and HR load monitoring, return-to-play testing.
• Fitness industry - gym and studio trainers (Cult.fit, Anytime Fitness, Gold's Gym), online coaching; certification through ACSM, NSCA, ACE or REPs India-recognised bodies, with Karvonen zones, METs and PAR-Q+ screening used daily.
• Clinical and rehabilitation settings - cardiac rehab and pulmonary rehab in tertiary hospitals, exercise prescription for type 2 diabetes, hypertension and obesity; physiotherapy and exercise-is-medicine programmes.
• Armed forces, police and paramilitary selection - running, shuttle and endurance standards derive directly from aerobic and anaerobic capacity testing; also railway and PSU sports quotas.
• School and university teaching - running the Khelo India Fitness Assessment and Fit India activities, physical education teaching posts, and NET/SET-qualified college lectureships.
• Anti-doping and sports medicine support - NADA dope control officers, athlete education on the WADA Prohibited List and TUEs, and sports nutrition consulting.

Interview questions

• Q: Which energy system powers a 400 m sprint? A: Predominantly anaerobic glycolysis, with the ATP-PCr system covering the first few seconds and a growing aerobic contribution after about 40-50 s.
• Q: What exactly does VO2 max measure, and what limits it? A: The maximal rate of oxygen uptake and use per minute; in healthy people it is limited mainly by oxygen delivery - cardiac output and haemoglobin - rather than by the lungs or the muscle's enzymes.
• Q: Why is lactate threshold often a better race predictor than VO2 max? A: VO2 max sets the ceiling, but the threshold defines the fraction of that ceiling an athlete can sustain, which is what a race demands; it also keeps improving with training long after VO2 max plateaus.
• Q: Is lactate the cause of muscle soreness? A: No. Lactate clears within an hour; DOMS at 24-48 h comes from eccentric micro-damage and the inflammatory response.
• Q: State the Fick equation and say how training changes each term. A: VO₂ = cardiac output × a-vO₂ difference; training raises maximal cardiac output through a larger stroke volume and widens the a-vO₂ difference through more capillaries and mitochondria.
• Q: Why does stroke volume plateau in untrained people but not in athletes? A: At high heart rates filling time shortens; athletes have a larger, more compliant ventricle and greater blood volume, so end-diastolic filling - and hence stroke volume - keeps increasing.
• Q: What is EPOC and what is it spent on? A: The oxygen consumed above rest after exercise, used to resynthesise PCr and ATP, reload myoglobin and haemoglobin, clear lactate, and support raised temperature, catecholamines, ventilation and heart rate.
• Q: Why is Karvonen preferred to a flat percentage of HRmax? A: It works from heart rate reserve, so it accounts for the individual's resting heart rate and corresponds more closely to the same percentage of VO2 max.
• Q: An RER above 1.0 is recorded during a test - what does it mean? A: Carbon dioxide output exceeds oxygen uptake because bicarbonate is buffering lactic acid, so the work is above the threshold and the subject is near maximum; RER cannot exceed 1.0 from substrate oxidation alone.
• Q: How would you prescribe training for a client in Chennai in May? A: Train early morning or late evening, allow 10-14 days of heat acclimatisation with reduced intensity, monitor by RPE rather than pace, weigh before and after to compute sweat rate, and replace fluid with electrolytes rather than plain water alone.
• Q: How much fitness is lost in a two-week injury break, and what do you do about it? A: VO2 max can fall up to about 10%, largely from reduced plasma volume, while strength declines little; maintain some intensity through cross-training or low-volume high-intensity work instead of stopping completely.
• Q: An athlete tests positive for a substance in a cough syrup - what is the position in India? A: Strict liability applies under the National Anti-Doping Act, 2022; the athlete is responsible for the sample regardless of intent, though the degree of fault can reduce the sanction, and any medically needed substance must have a prior Therapeutic Use Exemption.

Common mistakes

• Treating the three energy systems as switching on and off in sequence instead of operating simultaneously in changing proportions.
• Blaming lactic acid for DOMS, or calling lactate a waste product rather than a usable fuel and gluconeogenic substrate.
• Quoting 38 ATP per glucose without noting that the accepted modern figure is about 30-32.
• Confusing absolute VO2 max (L/min) with relative VO2 max (mL/kg/min) - a heavy athlete can lead on one and trail on the other.
• Using HRmax zones instead of HR reserve, or trusting 220 − age as exact when its standard deviation is about ±10-12 bpm.
• Unit slips in calculations: mL with L, kgm/min with watts, kcal with kJ, or forgetting to divide by body mass.
• In practical labs: not standardising step height and cadence, mistiming recovery pulse counts, testing maximally without PAR-Q+ screening, or ignoring humidity when testing outdoors in India.

Related topics

Anatomy and physiology essentials · Biomechanics and kinesiology · Sports nutrition and weight management · Sports training methods and periodisation · Test, measurement and evaluation in physical education · Sports medicine, injury prevention and rehabilitation