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Exercise Physiology Review: Muscular, Skeletal, Cardiac, Respiratory, Neuromuscular, Endocrine Systems and Energy Pathways (Vocabulary Flashcards)

Muscular System

The recruitment of muscle fibres during exercise follows a specific order: slow-twitch Type I fibres are recruited first, followed by fast-twitch fibres (Type IIa, then Type IIx). Recruitment level is generally driven by the demand placed on the muscle. Even at maximal effort, the nervous system does not recruit all fibres; only a fraction are activated at a time to minimise the risk of muscle damage and injury.

During activity, energy for muscles comes from fuels such as carbohydrates and fats, and heat is produced as a by-product. The more intense the exercise, the more heat is generated as the amount of work performed increases. As muscles warm, the blood flowing through them is warmed too, contributing to a rise in core body temperature.

Skeletal muscle fibres number in the hundreds of thousands per muscle. When muscles contract and relax during exercise, microscopic tears form in the fibres. Recovery involves healing with proteins to fill the gaps, which can increase strength and, depending on the exercise type, muscle size. The energy demands of all muscles rise during exercise.

Cardiovascular adjustments accompany muscular activity. Blood distribution is not even; blood is directed to active muscles via vasodilation and vasoconstriction, regulated by hormones and other chemicals. Cardiac output (the volume of blood the heart pumps per minute) is about 5–6 L·min⁻¹ at rest in an adult male, but during exercise it can rise to 15–20 L·min⁻¹, with as much as about 86% of circulating blood directed to muscles. This redistribution helps meet the metabolic needs of muscles during exercise.

Muscle metabolism also produces heat, contributing to an increase in body temperature. Heat production scales with work rate: higher intensity yields more heat.

During exercise, energy is derived from fuels such as carbohydrates and fats. The utilisation of these fuels leads to heat production, and the heat contributes to the overall temperature rise.

Skeletal System

Bone is a dynamic tissue continually reshaped by osteoblasts (bone-forming cells) and osteoclasts (bone-resorbing cells). In midlife, osteoblast and osteoclast activity are generally balanced, but ageing shifts the balance toward osteoclast activity, releasing calcium and other minerals into the bloodstream.

Weight-bearing exercise stimulates osteoblast activity and suppresses osteoclast activity, supporting bone density. Adequate calcium intake and vitamin D are important for bone health and ossification. Long-term exercise helps slow skeletal ageing and promotes greater bone mass. Moderate-intensity weight-bearing exercise is particularly effective for maintaining bone density.

Synovial fluid, a thick straw-coloured lubricant found in synovial joints, increases with exercise. This reduces joint viscosity, helps protect cartilage from drying out, and may increase joint range of motion due to greater synovial fluid availability.

Weight-bearing activities (e.g., running, walking) are particularly beneficial to the skeletal system when calcium intake is adequate.

Cardiac System

Vasodilation and vasoconstriction regulate blood flow to tissues during exercise. Vasodilation increases arteriolar diameter, increasing blood flow to exercising muscles, while vasoconstriction decreases flow to other tissues to conserve blood for active muscles. For instance, at rest, kidney blood flow is about 20% of cardiac output; during maximal exercise, it can fall to about 1% as the kidneys effectively shut down to prioritise muscle perfusion.

The pH of blood is a measure of acidity/alkalinity. Blood pH typically lies between 7.2 and 7.5 (slightly alkaline). During intense exercise, blood pH can drop (become more acidic) due to metabolic by-products such as carbon dioxide and lactic acid. In anaerobic metabolism, lactic acid is produced and can contribute to acidosis if not adequately buffered.

During exercise, diffusion rates increase to move more oxygen from capillaries into working muscles, and carbon dioxide is exchanged into the blood for exhalation. With long-term aerobic training, diffusion becomes more efficient, improving oxygen uptake and carbon dioxide removal.

The arteriovenous oxygen difference (a-vO₂ difference) increases with exercise: as muscle demand for oxygen rises, arterial oxygen is extracted, reducing venous oxygen content.

Respiratory System

Chemical Control

Lowered oxygen and increased carbon dioxide levels in the blood stimulate the respiratory center to increase breathing rate. This chemoreceptive control helps regulate ventilation in response to metabolic needs.

Neural Control

At exercise onset, receptors in muscles and joints provide rapid feedback that increases breathing rate. Forced breathing is used during strenuous activity, with expiration aided by contraction of internal intercostal muscles and abdominal muscles, increasing intra-abdominal pressure and elevating the diaphragm to expel air more forcefully.

Tidal Volume and Minute Ventilation

Tidal volume (the amount of air per breath) increases substantially during exercise. Typical resting tidal volumes are around 0.4 L, but elite athletes can reach around 2 L. Minute ventilation (VE) is the product of tidal volume and breathing rate: VE = V_T imes RR.
During exercise, alveolar ventilation increases, with deeper breathing contributing to higher efficiency of gas exchange. Alveolar ventilation can rise from about 70% of total ventilation at rest to over 85% during exercise.

As exercise intensity rises, tidal volume initially increases; at high intensities, tidal volume reaches a peak and further increases in ventilation come from higher breathing rates. Moderate exercise elicits minute volumes of roughly 40–60 L·min⁻¹.

Oxygen Haemoglobin Dissociation Curve

The oxygen dissociation curve shows the relationship between oxygen saturation and the partial pressure of oxygen. In exercise, higher temperature and lower pH shift the curve to the right, facilitating greater unloading of oxygen to active muscles. During prolonged high-intensity exercise, large lactate loads can drive pH toward very acidic values (blood pH can approach 6.8). After exercise, pH gradually returns toward normal (around 7.4).

Neuromuscular System

Muscles contract when stimulated by nerves. There are three basic types of contraction:

  • Isotonic: muscle shortens while developing tension.
  • Isometric: muscle develops tension without length change.
  • Isokinetic: muscle contracts maximally at a constant speed through the range of movement.

Proprioceptors in skeletal muscles (intrafusal muscle fibres) detect stretch and help prevent injury. Muscle spindles (primary receptors) respond to both the velocity and extent of stretch, sending information to the brain via the spinal cord. Secondary receptors provide information about the degree of stretch to the CNS. This information triggers motor neuron activation to contract the muscle and reduce stretch.

GTOs (Golgi tendon organs) are proprioceptors within tendons that sense tendon stretch and inform the CNS about contraction strength. They work with muscle spindles to promote smooth movement; they tend to cause the muscle to relax (inhibition) when necessary to prevent injury.

Chemoreceptors monitor chemical stimuli such as carbon dioxide levels in the blood; rising CO₂ triggers an increase in breathing rate. Thermoreceptors detect temperature changes and contribute to thermoregulation. Baroreceptors, located in blood vessels, sense blood pressure and inform the brain to maintain stable cardiac output during exercise.

  • Sense muscle stretch; cause muscle contraction (via reflex arcs).
  • GTOs sense tendon stretch; cause muscle relaxation to prevent damage.

Energy Systems and Fatigue

The body stores only a small amount of ATP, enough for only a few seconds of maximal exertion, so ATP must be continually resynthesised. ATP is formed by the reaction between ADP and a phosphate group to yield ATP; during use, the last phosphate is split off, releasing energy.

The ATP-PC system (phosphocreatine) provides rapid energy. PC breakdown releases energy and a phosphate that is used to rebuild ATP. The enzyme creatine kinase controls PC breakdown. The ATP-PC system can operate without oxygen (anaerobic) and is dominant for the first 5 seconds of exercise, sustaining all-out activity for about 3–15 seconds.

If activity continues beyond this period, other energy systems must resynthesise ATP. Glycolysis breaks down glucose to form ATP and pyruvic acid; under anaerobic conditions, pyruvate is converted to lactic acid, generating lactate and H⁺ and contributing to muscle acidosis. The accumulation of lactate and hydrogen ions impairs muscle function. NAD⁺ accepts H⁺ during glycolysis and becomes NADH; when oxygen is insufficient, NADH cannot unload H⁺, leading to H⁺ buildup. Pyruvic acid can accept H⁺ to form lactic acid, which dissociates to lactate and H⁺. Lactate may diffuse into the bloodstream. Muscle pH at rest is about 7.1; during intense exercise it can drop toward 6.5, impairing contraction.

Aerobic glycolysis involves glycogen → glucose, then oxidation with oxygen to produce ATP. Net gain is: 2 ext{ ATP net from glycolysis} = 4 ext{ produced} - 2 ext{ used} = 2 ext{ ATP}. Pyruvate enters the mitochondria and is converted to Acetyl CoA, feeding the Krebs cycle and the Electron Transport Chain (ETC).

Krebs cycle (citric acid cycle) occurs in the mitochondrial matrix with Acetyl CoA as the input. It produces 2 ATP per cycle, with CO₂ and hydrogen ions as by-products. Hydrogen is transferred to NAD⁺ to form NADH, which feeds the ETC.

The ETC is a sequence of redox reactions in the inner mitochondrial membrane where energy from hydrogen ions is used to phosphorylate ADP to ATP. The ETC yields approximately 34 ATP per molecule of glucose. Oxygen is the final electron acceptor at the end of the chain, combining with hydrogen to form water (the final by-product).

In summary, the major steps are: (1) glycolysis yields pyruvate and some ATP, (2) the Krebs cycle processes acetyl-CoA to CO₂ and hydrogen for NADH, and (3) the ETC uses the hydrogen carried by NADH to generate the bulk of ATP with water produced as a by-product.

Energy Systems (Consolidated)

  • ATP-PC system: dominant in the first 3–15 seconds; anaerobic; no oxygen required; rapidly replenishes ATP stores.
  • Glycolysis (anaerobic): breakdown of glycogen to ATP and lactate; provides energy for high-intensity efforts up to about 60 seconds; lactate accumulates, lowering pH.
  • Aerobic glycolysis, Krebs cycle, and ETC (aerobic): requires oxygen; yields most ATP (up to ~34 ATP per glucose); supported by mitochondria and oxygen delivery; fuels prolonged, lower-intensity exercise using glycogen and free fatty acids.

Endocrine System

Adrenaline (epinephrine) and noradrenaline (norepinephrine) enhance cardiac output by increasing heart rate and promoting vasodilation in active muscles, with vasoconstriction elsewhere to maintain blood pressure. Cortisol increases blood glucose for quick energy but is catabolic and can disrupt anabolic processes if chronically elevated. Testosterone and human growth hormone (HGH) drive muscle hypertrophy and related adaptations; training increases their secretion frequency and volume, supporting strength and muscular adaptations.

Tests of cardiovascular fitness include VO₂ max, an indicator of oxygen uptake capacity. Higher VO₂ max allows training at higher intensities before muscle tissue demand exceeds supply, aiding endurance and recovery.

Anaerobic threshold (percentage of VO₂ max) reflects the point where lactate and hydrogen ions accumulate faster than can be buffered or cleared. OBLA (onset of blood lactate accumulation) occurs at higher percentages of VO₂ max in trained individuals (roughly 70–80%) than in sedentary individuals (roughly 50–60%).

Anaerobic power can be increased through sprint or power training, while maximal strength is measured by 1RM (one-repetition maximum) and muscular endurance by 15RM tests.

Nutrition, Recovery, and Overtraining

  • Protein: Adequate intake is essential for muscle repair and recovery after resistance work; commonly about 20% of daily energy intake.
  • Carbohydrates: Around 70% of dietary intake; carbohydrates are essential for fueling exercise due to limited glycogen stores.
  • Hydration: Athletes should begin well hydrated and drink according to thirst after exercise; avoid excessive rapid intake.
  • Supplements and recovery strategies: Creatine, energy gels, electrolytes, whey protein, etc., are used by some athletes.
  • Overtraining: Four commonly cited factors include endocrine imbalance, excessive adrenaline and cortisol, insufficient rest, and sleep disruption. Consequences include decreased performance, impaired immune function, higher injury risk, and sleep disturbances. Adequate rest is essential for adaptation and recovery.

High Altitude Adaptations

Short-term responses

Hypoxia due to reduced oxygen availability increases cardiovascular and respiratory demands. Hyperventilation may occur as a response to impaired gas exchange. Tachycardia (RHR > 100 bpm) can occur. There is little change in stroke volume, but a decrease in maximal heart rate reduces cardiac output. Approximately for every 300 m above 1500 m, VO₂ max decreases by about 2%. This leads to reduced endurance performance.

Long-term responses

To counter lower oxygen levels, capillary density increases, improving diffusion of oxygen to tissues. Myoglobin and mitochondrial content increase, along with oxidative enzyme activity in mitochondria, supporting better oxygen utilisation by Type I and some Type IIa fibres. RBC efficiency improves, aiding oxygen delivery. OBLA tends to increase, allowing higher lactate tolerances. Altitude adaptation also supports improved aerobic performance; some endurance advantages are observed with acclimatisation.

Sleep High, Train Low

A strategy to acquire altitude adaptations by sleeping at high altitude but training at lower altitude to maintain training intensity.

Thermoregulation and Heat Management

The body loses heat by several pathways: conduction, convection, radiation, and evaporation. Heat transfer depends on the temperature difference between the body and the environment and the speed of moving air. Evaporation of sweat is a major cooling mechanism. When overheating occurs, sweat production increases to promote cooling, but electrolytes must be replaced to maintain neural and muscular function.

Heat acclimatisation increases plasma volume, improving cardiac output and VO₂ max during heat exposure. Venous blood flow to the skin increases during heat stress, which can temporarily divert some blood from exercising muscles. When the environment is hot, the body relies more on skin blood flow for cooling, which can reduce oxygen delivery to working muscles and affect both aerobic and anaerobic performance.

Body’s Response to Extreme Cold

Shivering thermogenesis increases heat production through involuntary muscle contractions, especially in brown adipose tissue, where chemical energy from ATP is converted into heat.

Hyperthermia occurs when heat loss is insufficient, leading to symptoms such as headache, nausea, dizziness, sweating, and potentially organ damage, with core temperature exceeding approximately 40°C. Dehydration results from excessive water loss and can lead to reduced plasma volume and impaired CV function; electrolyte balance must be maintained.

Hypothermia occurs when heat production fails to match losses; reduced breathing rate, blood pressure, and heart rate, along with drowsiness, can ensue and may be fatal as core temperature drops.

Frostbite results from prolonged exposure to freezing temperatures and vasoconstriction to extremities, risking tissue damage that may require surgical intervention if blood flow is not restored.

Additional Notes on Practical Implications

  • Training programs should consider energy system contributions to tailor duration and intensity (e.g., aerobic vs anaerobic emphasis).
  • Hydration and nutrition strategies greatly influence performance and recovery, particularly overtraining risks and altitude exposure.
  • Monitoring HR, BP, VO₂ max, OBLA, and lactate can guide training progression and adaptation.
  • Understanding thermoregulation and heat/cold responses helps athletes train safely in extreme environments.