Exercise Physiology

Cardiorespiratory adaptations to short- and long-term activity. In a single bout, heart rate and stroke volume rise, so cardiac output (Q = HR × SV) climbs from about 5 L/min at rest to about 20 L/min in the untrained and 35–40 L/min in elite endurance athletes; systolic pressure rises while diastolic barely changes; the a-vO2 difference widens; ventilation (tidal volume × frequency) rises. Fick: VO2 = Q × a-vO2 difference. Long-term endurance training brings a lower resting and submaximal heart rate (bradycardia), larger stroke volume, higher maximal cardiac output and VO2max, more blood volume and capillaries and a higher lactate threshold; maximal heart rate is unchanged or slightly lower.

Muscle types, microscopic structure and sliding filament theory. Skeletal muscle is striated and voluntary; cardiac is striated and involuntary, with intercalated discs; smooth is non-striated and involuntary. Properties: excitability, contractility, extensibility, elasticity; functions: movement, posture, joint stability, heat production. Thick filaments are myosin; thin filaments are actin with troponin and tropomyosin; a sarcomere runs Z-line to Z-line. The A band (dark) spans the thick filaments, the I band (light) holds only actin, the H zone only myosin. Sliding filament theory (Huxley and Hanson; Huxley and Niedergerke; 1954): Ca2+ released from the sarcoplasmic reticulum binds troponin, tropomyosin uncovers actin's binding sites, myosin heads bind and make the power stroke, and fresh ATP detaches and re-cocks them. Filaments do not shorten: the I band and H zone shorten while the A band stays constant. No ATP, no detachment: rigor mortis.

Muscle fibre types, sports performance and muscular adaptation. Type I (slow oxidative, red): rich in mitochondria, myoglobin and capillaries, fatigue-resistant, low force; dominant in marathoners. Type IIa (fast oxidative-glycolytic) is intermediate. Type IIx (fast glycolytic): high force, fast fatigue; dominant in sprinters. Fibre mix is largely inherited, and training mainly shifts IIx toward IIa. Henneman's size principle: motor units are recruited from small to large. Early strength gains are mostly neural and later ones come from hypertrophy; endurance training adds mitochondria, oxidative enzymes and capillaries.

Neuromuscular junction, nerve impulse and kinesthetic sense organs. A motor unit (one motor neuron and the fibres it supplies) obeys the all-or-none law. Resting potential is about −70 mV in a neuron; Na+ entry depolarises and K+ exit repolarises; myelinated fibres conduct faster by saltatory conduction. At the junction acetylcholine crosses to the motor end plate and acetylcholinesterase ends the signal. The muscle spindle senses length and drives the stretch reflex; the Golgi tendon organ senses tension and relaxes the muscle (autogenic inhibition); the vestibular apparatus senses balance. In motor control, the cerebellum times movement and corrects errors and the basal ganglia start and scale it.

Bio-chemical aspects: metabolism and energy systems. Carbohydrate yields 4 kcal/g, fat 9 and protein 4. ATP–PC: anaerobic and alactic, via creatine kinase, about 10 s of maximal effort. Anaerobic glycolysis: lactic, net 2 ATP per glucose, dominant from roughly 10 s to 2 min. Aerobic (Krebs cycle, electron transport, beta-oxidation): dominant beyond 2–3 min; classical texts give 36–38 ATP per glucose and newer ones about 32. At rest metabolism is aerobic and mostly fat; carbohydrate takes over as intensity rises. OBLA is set at 4 mmol/L.

Direct and indirect methods of measuring energy cost. Direct calorimetry measures heat output in a sealed chamber: accurate but impractical in sport. Indirect calorimetry measures O2 consumed and CO2 produced by open-circuit (Douglas bag) or closed-circuit (Benedict–Roth) spirometry, at about 5 kcal per litre of O2. RQ = VCO2 ÷ VO2: carbohydrate 1.0, fat 0.7, protein about 0.8. 1 MET = 3.5 mL O2/kg/min, and kcal/min = METs × 3.5 × kg ÷ 200.

Recovery process and nutritional aspects of performance. Fatigue is central or peripheral (PCr and glycogen depletion, H+ accumulation, dehydration, heat). A.V. Hill's "oxygen debt" is now called EPOC: its fast component restores ATP–PC and myoglobin O2 within minutes, and its slow component clears lactate, mostly by oxidation and partly by the liver's Cori cycle. Phosphagens are restored in about 3–5 min, but muscle glycogen may need up to two days; active recovery clears lactate faster than rest. Nutrition: carbohydrate loading for events over about 90 min, a pre-event meal 3–4 h before, fluids, and 1.2–2.0 g protein/kg/day for athletes.

Environmental influence on human physiology under exercise. Heat: sweat evaporation is the main cooling route and fails in humid air; heat stroke (core above about 40 °C with CNS dysfunction) is an emergency; acclimatisation over 1–2 weeks expands plasma volume and brings earlier sweating. WBGT (outdoor) = 0.7 wet-bulb + 0.2 globe + 0.1 dry-bulb. Cold: hypothermia is a core temperature below 35 °C. Altitude: O2 stays 20.93% of air but its partial pressure falls, so VO2max drops; acclimatisation raises EPO and red cells. Mexico City 1968 (about 2,240 m) hurt endurance events and helped sprints and jumps.

Women in sports and ageing. Women are as trainable in relative terms but on average have smaller hearts, lower haemoglobin, more essential fat (about 12% against 3%), lower VO2max, a larger Q-angle and a bigger upper-body strength gap; force per unit of muscle cross-section is similar. Special problems: the female athlete triad (ACSM, 2007 definition: low energy availability, menstrual dysfunction, low bone density), RED-S (IOC, 2014) and iron-deficiency anaemia. Ageing cuts VO2max by about 10% a decade in sedentary adults, lowers maximal heart rate and causes sarcopenia; WHO 2020 advises 150–300 min of moderate activity a week, strength work on 2 or more days, and balance work for older adults.

Therapeutic modalities and rehabilitation. Cryotherapy (ice for 15–20 min, RICE) constricts vessels and slows swelling, pain and nerve conduction, so it suits the acute phase. Thermotherapy dilates vessels and eases spasm, so it suits later phases. Ultrasound heats deep tissue at 1 MHz and shallow tissue at 3 MHz; short-wave diathermy uses 27.12 MHz; TENS rests on gate-control theory (Melzack and Wall, 1965). Rehabilitation follows healing (inflammation, repair, remodelling) toward return to play.

Ergogenic aids and massage. Nutritional aids include creatine, caffeine (off WADA's prohibited list since 2004) and sodium bicarbonate. Prohibited: anabolic steroids, EPO, blood doping, diuretics as masking agents, and beta-blockers in archery and shooting (WADA list as of 2026-10). Massage strokes are effleurage (stroking toward the heart), petrissage (kneading), friction, tapotement (percussion) and vibration; massage raises local blood and lymph flow and relaxes muscle, but is avoided over acute injury, infection or thrombosis.