Exercise Physiology II: the Neuromuscular Junction and the Transmission of the Nerve Impulse, the Kinaesthetic Sense Organs and the Neural Control of Motor Skills, the Biochemistry of Exercise, Aerobic and Anaerobic Systems at Rest and in Exercise, Direct and Indirect Measurement of the Energy Cost of Exercise, the Recovery Process and the Nutritional Aspects of Performance

The kinaesthetic sense, the awareness of the position and movement of the body and its parts, comes from proprioceptors. Muscle spindles lie parallel to muscle fibres and detect stretch and its rate; a sudden stretch fires them and triggers the stretch (myotatic) reflex, a contraction of the stretched muscle, as in the knee-jerk, which also underlies the stretch-shortening cycle used in plyometrics. Golgi tendon organs, at the junction of muscle and tendon, detect tension; very high tension inhibits the muscle (autogenic inhibition), a protective reflex exploited in PNF stretching. Joint receptors signal joint angle and movement, and the vestibular apparatus of the inner ear (semicircular canals and otolith organs) signals head position and acceleration for balance. Together with vision and touch they supply the feedback on which skilled movement depends.

Movement is controlled at several levels. Spinal reflexes give fast, automatic responses. The motor cortex of the frontal lobe plans and initiates voluntary movement and sends commands down the corticospinal (pyramidal) tract; the premotor and supplementary motor areas plan sequences. The cerebellum compares intended with actual movement and corrects errors, coordinating timing, balance and smoothness; damage to it causes clumsy, ataxic movement. The basal ganglia help start and scale movements and suppress unwanted ones. The brainstem controls posture and muscle tone. As a skill is learned, control shifts from conscious cortical attention to more automatic control, which is the neural side of the autonomous stage of motor learning taught in Unit IV.

The only energy muscle can use directly is adenosine triphosphate (ATP); splitting it into ADP and phosphate releases energy for the cross-bridge cycle. Stores of ATP last only a few seconds, so it must be resynthesised continuously from food fuels. Carbohydrate is stored as glycogen in muscle and liver and circulates as blood glucose; it yields about 4 kcal per gram and is the only fuel usable anaerobically. Fat, stored as triglyceride in adipose tissue and in muscle, yields about 9 kcal per gram and is the major fuel at rest and in low-intensity, prolonged exercise, but can be used only aerobically. Protein yields about 4 kcal per gram but normally supplies only a small share of energy, rising in very prolonged exercise when glycogen runs low.

Three systems resynthesise ATP. The ATP–PC (phosphagen) system uses phosphocreatine to rebuild ATP almost instantly, without oxygen and without lactate; it dominates maximal efforts lasting up to about 10 seconds, such as a 100 m sprint, a jump or a throw. Anaerobic glycolysis breaks glucose or glycogen down to pyruvate and, when oxygen supply cannot keep pace, to lactate, yielding a net 2 ATP per glucose (3 from a glycogen unit); it dominates intense efforts of roughly 10 seconds to 2 minutes, such as 200–800 m running, and the accompanying hydrogen ions contribute to fatigue. The aerobic (oxidative) system in the mitochondria oxidises pyruvate and fatty acids through the Krebs cycle and the electron transport chain, yielding about 30–32 ATP per glucose (older texts give 36–38) and far more from a fatty acid; it is slow to reach full rate but practically unlimited, and dominates everything longer than a few minutes. All three work together at every moment; what changes with intensity and duration is their relative contribution. At rest almost all energy is aerobic, with fat supplying the larger share.

Direct calorimetry measures energy expenditure as heat: the subject exercises inside an insulated chamber (the Atwater–Rosa respiration calorimeter was the classic design) and the heat given off is measured by the temperature rise of water circulating through its walls. It is accurate but expensive, slow and unsuited to most sports. Indirect calorimetry estimates energy from oxygen consumption, because the body's energy comes ultimately from oxidation: each litre of oxygen consumed releases about 5 kcal (about 4.7 when fat is burned and 5.05 when carbohydrate is burned). Oxygen uptake is measured by open-circuit spirometry, collecting expired air in a Douglas bag or with a metabolic cart, and energy is also estimated in the field from heart rate, accelerometers and the doubly labelled water method.

The respiratory exchange ratio, RER = VCO2 ÷ VO2, shows which fuel is being burned: about 0.7 for pure fat, 1.0 for pure carbohydrate and about 0.8 for protein, with values in between for a mixture; in very intense exercise it can exceed 1.0 because extra carbon dioxide is blown off to buffer acid. (Measured at the cells the same ratio is the respiratory quotient, RQ.) The metabolic equivalent (MET) expresses intensity as a multiple of resting metabolism: 1 MET ≈ 3.5 mL of oxygen per kg of body mass per minute, roughly 1 kcal per kg per hour. Energy expenditure in kcal per minute can be estimated as METs × 3.5 × body mass (kg) ÷ 200. For a 70 kg person running at 8 METs: 8 × 3.5 × 70 ÷ 200 = 9.8 kcal/min, or 294 kcal in 30 minutes.

Fatigue is the decline in the ability to produce force or power. Peripheral fatigue arises in the muscle: depletion of phosphocreatine in short maximal efforts, accumulation of hydrogen ions and inorganic phosphate in intense efforts, glycogen depletion in prolonged ones ("hitting the wall"), impaired calcium release, dehydration and high body temperature. Central fatigue arises in the nervous system as reduced drive to the muscles. After exercise, oxygen uptake stays above resting level for some time: this excess post-exercise oxygen consumption (EPOC), once called the oxygen debt, has a fast component in the first minutes that restores ATP and phosphocreatine and reloads myoglobin and blood with oxygen, and a slow component that removes lactate and meets the cost of raised temperature, heart rate, ventilation and hormones.

The energy stores recover at different rates. Phosphocreatine is about half restored in 30 seconds and largely restored within about 2–5 minutes, which is why sprint repetitions need several minutes of rest. Lactate is removed within about an hour, and faster with light active recovery (easy jogging or cycling) than with complete rest. Muscle glycogen takes 24–48 hours to be fully replenished and needs a carbohydrate-rich diet; eating carbohydrate soon after exercise speeds the early phase. Sleep, hydration, massage, cold-water immersion and compression are used to speed recovery, with varying evidence.

Nutrition supports performance through energy balance, fuel and recovery. Athletes' carbohydrate needs rise with training load, commonly guided at about 5–7 g per kg of body mass per day for moderate training and 6–10 g/kg or more for heavy endurance training; protein needs are about 1.2–2.0 g/kg per day, higher than for sedentary people, spread across meals; fat should not be cut too low. A pre-event meal 3–4 hours before competition should be high in carbohydrate, moderate in protein and low in fat and fibre, and familiar. Carbohydrate loading in the days before an endurance event lasting more than about 90 minutes, by tapering training while eating a high-carbohydrate diet, raises muscle glycogen above normal (supercompensation). During long events carbohydrate drinks delay fatigue. Hydration matters because a fluid loss of about 2 per cent of body mass impairs endurance performance; athletes can weigh themselves before and after training to judge fluid needs, and should also avoid drinking so much that they develop hyponatraemia.