The physical exercise capacity can be measured using simple yet standardized techniques of ergometry. This may be desirable in athletes, for example, to assess the results of training, or in patients undergoing rehabilitation therapy. Ergometry assesses the effects of exercise on physio.logical parameters such as O2 consumption (VO2), respiration rate, heart rate (^ p. 74), and the plasma lactate concentration (^ A). The measured physical power (performance) is expressed in watts (W) or W/kg body weight (BW).
In bicycle ergometry, a brake is used to adjust the watt level. In "uphill" ergometry on a treadmill set at an angle a, exercise performance in watts is calculated as a factor of body mass (kg) X gravitational acceleration g (m-s-2) X distance traveled (m) X sin a X 1/ time required (s-1). In the Margaria step test, the test subject is required to run up a staircase as fast as possible after a certain starting distance. Performance is then measured as body mass (kg) X g (m ■ s-2) X height/time (m ■ s-1).
Short-term performance tests (10-30 s) measure performance achieved through the rapidly available energy reserves (creatine phosphate, glycogen). Medium-term performance tests measure performance fueled by anaerobic glycolysis (^ p. 72). The maximum O2 consumption (VO2max) is used to measure longer term aerobic exercise performance achieved through oxidation ofglucose and free fatty acids (^ p. 74).
In strenuous exercise (roughly 2/3 the maximum physical capacity or more), the aerobic mechanisms do not produce enough energy, so anaerobic metabolism must continue as a parallel energy source. This results in lactacidosis and a sharp increase in the plasma lactate concentration (^ A). Lactate concentrations of up to 2mmol/L (aerobic threshold) can be tolerated for prolonged periods of exercise. Lactate concentrations above 4mmol/L (anaerobic threshold) indicate that the performance limit will soon be reached. Exercise must eventually be interrupted, not because of the increasing lactate concentration, but because of the increasing level of acidosis (^ p. 74).
Physical training raises and maintains the physical exercise capacity. There are three types of physical training strategies, and most training programs use a combination of them.
Motor learning, which increases the rate and accuracy of motor skills (e.g., typewriting). These activities primarily involve the CNS.
Endurance training, which improves submaximal long-term performance (e.g., running a marathon). The main objectives of endurance training are to increase the oxidative capacity of slow-twitch motor units (^ p. 58), e.g., by increasing the mitochondrial density, increase the.cardiac output and, consequently, to increase Vo2 max (^ B, C). The resulting increase in heart weight allows higher stroke volumes (^ C) as well as higher tidal volumes, resulting in very low resting heart rates and respiratory rates. Trained athletes can therefore achieve larger increases in cardiac output and ventilation than untrained subjects (^ C). The Vo2 max of a healthy individual is limited by the cardiovascular capacity, not the respiratory capacity. In individuals who practice endurance training, the exercise-related rise in the lactate concentraton is also lower and occurs later than in untrained subjects (^ A).
Strength training improves the maximum short-term performance level (e.g., in weight lifting). The main objectives are to increase the muscle mass by increasing the size of the muscle fibers (hypertrophy) and to increase the glycolytic capacity of type motor units (^ p. 58).
Excessive physical exercise causes muscle soreness and stiffness. The underlying cause is not lactic acid accumulation, but sarcomere microtrauma, which leads to muscle swelling and pain. The muscle ache, is a sign of microinflammation (^ D).
Muscle fatigue may be peripheral or central. Peripheral fatigue ist caused by the exhaustion of energy reserves and the accumulation of metabolic products in the active muscle. This is particularly quick to occur during postural work (^ p. 66). Central fatigue is characterized by work-related pain in the involved muscles and joints that prevents the continuation of physical exercise or decreased the individual's motivation to continue the exercise.
|— A. Lactate concentration (phys.exercise)
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