INTRODUCTION

The day-to-day variability in oxygen consumption in most physical actions is about 5%, and differences of 10% of this percentage among individuals can be expected. Energy costs vary greatly in different sports; eg, for a 150-lb player, 4.4 kcal/minute in archery, 9.1 kcal/minute in field hockey, 13.3 kcal/minute in judo, and 18.6 kcal/minute in squash.

Energy output also varies in the same sport depending on such factors as intensity of competition, neuromuscular skill level, position demands, performance level, age, body type, atmospheric conditions, and field conditions. In exercise physiology, however, it has been shown to be valid to measure energy expenditure of muscle tissue in terms of oxygen consumption in liters/minute but not valid to convert such data into energy units of watts or kilocalories/minute.

Metabolic capacity, maximum oxygen-intake capacity, and maximum oxygen-debt capacity are the current priority concerns of exercise physiologists.

When muscular effort must be prolonged longer than a minute, performance becomes increasingly dependent upon the demands of holistic homeostasis and not just that of active tissues. Basically, this involves oxygen supply, carbon dioxide removal, heat balance, and the replenishment of nutrients.


Metabolism

To maintain health, stored resources (potential energy) must be kept in balance with power expenditures (kinetic energy). While carbohydrate and fat are normally oxidized almost completely in the human body, protein is not. Protein derivatives of uric acid, urea, and creatinine are excreted in the urine. In addition, not all food ingested is absorbed; that is, 97% of carbohydrate, 95% of fat, and 92% of protein ingested is absorbed, and these numbers do not consider the "coarseness" of foodstuffs such as coarse corn meal or roughly ground whole grains.

Metabolic Rate.   Metabolic rate is directly proportional to gross body weight. Such factors as lean body mass, age, diet, sex, height, surface area, and race do not have a significant influence on metabolic rate during physical activity. The greater the energy demands, the higher the requirement for oxygen consumption. Total energy is the result of the basal (waking state) metabolic rate plus the energy necessary for work. This offers a ratio that can be used as an index to measure exercise intensity and performance efficiency. In a given period of time, energy output intensity is directly related to mechanical performance, measured by oxygen consumption in a specified period. In this sense, oxygen consumption can be considered a reflection of metabolic power.

Metabolic Capacity.   Metabolic capacity is directly related to performance capacity, reflecting the quantity of energy-yielding nutrients available (2-3 hours) from body reserves under aerobic conditions. Thus, one's maximum aerobic power and metabolic capacity are closely related, yet there are many individual differences. Besides metabolic capacity, other indices may be used such as those of glycogen storage, cardiac output, and water-balance efficiency.

Aerobic Power

To produce necessary energy, the body uses an aerobic (oxygen) pathway and an anaerobic (nonoxygen) pathway. To maintain life, the primary factor is the continuous and adequate flow of oxygen.

Restricted oxygen flow quickly manifests in function deterioration as seen clinically following infarcts and strokes, underscoring why so much emphasis is placed on oxygen demands during physical, psychologic, and environmental stress. Life signs and the degree of life are routinely evaluated from detectable arterial pulsation, breathing quantity and quality and rhythm, temperature, and reflexes -all of which are related to oxygen flow.

When oxygen demands exceed supply (oxygen debt) during and following prolonged exertion, lactic acid accumulates within muscle tissue and encourages fatigue. The greater the exercise intensity, the greater the lactic acid accumulation. Following maximum exercise, it may take an hour or longer to attain resting levels. Oxygen debt must be repaid rapidly such as through hyperpnea.


Anaerobic Power

Short bursts of effort primarily using explosive strength requiring less than 120 seconds are considered anaerobic activities. Because blood, circulation, respiration, and all the other factors contributing to human function during effort cannot be produced on a moment's notice, nature provides certain limited anaerobic mechanisms to meet the metabolic demands of active cells. Even with minimal work intensity, there is a period of oxygen deficiency that disturbs homeostasis and sets in motion a call for restoration at a higher metabolic level.

Both aerobic and anaerobic mechanisms determine an individual's performance capacity, but anaerobic activity is maintained only for a short time. An anaerobic state exists when oxygen is not used to produce energy and when glucose and glycogen reserves are used. The greater the intensity of the effort, the greater the anaerobic energy contribution. This can be measured by the amount of oxygen intake during the recovery period, usually attaining its peak (maximum oxygen debt) in about 50 seconds after intense exercise begins. If performance demands are great enough to exceed maximum oxygen transport capabilities, performance proceeds only until anaerobic energy stores become exhausted.

An index of work capacity is mechanical power of an anaerobic nature. Common tests are (1) running staircases, as the energy requirement for maintaining speed in running a specified distance depends on mechanical performance during the period and (2) using a bicycle ergometer, where the mechanical work is calculated by recording through a photoelectric circuit the number of wheel revolutions. Activity examples also include weight lifting, throwing, 100-yard dash, 100-meter freestyle swim, a basketball fast break, or running bases in baseball.


Interval Training

Interval training was developed because of problems associated with lactic acid buildup. Workouts interspersed with rest periods diminish a large accumulation of lactic acid and delay fatigue. Sessions require strict administration. It consists of repetitive efforts in which distance is set and pace is timed with established intervals for recovery between efforts. Long runs increase aerobic capacity, and fast, short runs increase anaerobic power and strength. As conditioning progresses, the time is shortened, the number of runs is increased, and the number of rest intervals is decreased.

The interval pattern of effort and rest for a specific amount of work and time critically determines the rise of excessive lactate levels, which, as previously explained, is a major cause of fatigue. In long-term events, it is important for an individual to keep high energy demands met by anti lactic acid reserves and try to tactically have the competition exceed their reserves.

Pace and recovery time is usually determined by pulse rate rather than time. Some authorities state that heart rate must be 60% of the available range from rest to the maximum attainable (eg, 140+ beats/minute during running) to develop a rate decrease of the working heart. Thus, they claim, an athlete's pulse below 140/minute indicates a need for a faster run or swim. Once pulse rate decreases to a desired level, rest intervals are ended. Such conclusions, however, fail to consider many unique individual factors.


The Pulmonary Apparatus

The level of oxygen saturation greatly determines the oxygen-carrying capacity of the blood, and oxygen saturation depends on factors determining the quality and quantity of oxygen diffusion in the lungs. These factors include (1) the quality of pulmonary blood flow and neuromuscular mechanisms, (2) the lung area available for the diffusing process, (3) the time duration in which blood receives alveolar-capillary exposure, (4) the thickness of the alveolar-capillary membrane, (5) the alveolar air and pulmonary capillaries oxygen pressure differential, and (6) respiratory frequency, which is often linked in the athlete with the rhythm of movement. It therefore becomes apparent that the quality of oxygen transport is contingent on the blood, the cardiovascular system, and the pulmonary-respiratory system.

Ventilation.   Lung function is evaluated by physiologists by measuring pulmonary residual volume and vital capacity -the components of total lung volume. As an index to breathing capacity, vital capacity is calculated from the maximum amount of air exhaled after a maximum inhalation. About 20% of vital capacity is used during rest. About 70% might be used during prolonged exercise. Up to a quarter of external ventilation is "wasted" in pulmonary "dead space" due to the incomplete mixing of alveolar and airway air, enhanced by an athlete's or a laborer's typically diminished respiratory rate.

Ventilation efficiency is assisted as tidal volume increases with decreased respiratory frequency for a given total ventilation. More commonly, ventilation efficiency is judged by the quantity of air inhaled or exhaled in relation to the amount of oxygen absorbed. Such measurements must take into consideration varying atmospheric conditions and individual metabolic needs. Because adequate oxygen is essential for life, both oxygen demands and oxygen consumption must be considered.

Lactic Formation and "Choked" Performance.   It has been described that during heavy exercise lactic acid accumulates within muscle as a result of oxygen demands exceeding oxygen supply. Choking of performance because of excessive competition or poor pacing may lead to early anaerobic demands on metabolism. The result is lactate accumulation, witnessed as a premature distressing hyperventilation. Local muscle weakness may also induce premature breathlessness.

Hyperventilation from premature lactate accumulation can cause a person to exceed normal ventilation adjustments where oxygen delivered to the circulation is less than the corresponding demand for oxygen consumption. It is thus important for an athlete to avoid lactate accumulation until late in activity. If local muscle weakness is the cause, the situation can be corrected by strengthening exercises so the athlete can operate nearer aerobic power before lactate accumulates sufficiently. Marathon runners usually operate just under their lactate threshold until the final sprint.

Second Wind.   A "second wind" is considered an opposite reaction to that found with choked performance. While early lactate accumulation may be the result of physiologic forces (eg, cardiorespiratory maladjustment), with prolonged activity systemic blood pressure rises, movement pace is steadied, ventilation diminishes, and the respiratory muscles become "warmed-up", which reduces respiratory resistance and awareness of breathing, and the level of circulating lactate is lowered. Other mechanisms may also be involved.

Diffusing Capacity.   Many well-conditioned athletes, especially swimmers and other endurance-related participants, exhibit a large pulmonary diffusing capacity (larger pulmonary surface) that enhances oxygen transfer. These athletes also exhibit an increased ratio of oxygen intake to lung ventilation per minute, which decreases as exhaustion approaches. However, even with maximum effort, the equilibrium of pulmonary gases between the blood stream and alveolar spaces is fairly complete. Thus, a gain in diffusing capacity offers little benefit except for swimmers who deliberately hold their breath or for athletes performing at high altitudes.

Carbon-Dioxide Homeostasis.   Both low and high levels of carbon dioxide affect normal tissue function. Excessive carbon dioxide elimination may be encountered in high altitudes, witnessed by intermittent ventilation and symptoms of mountain sickness; ie, dyspnea, headache, blood pressure and pulse rate changes, and neurologic disorders due to maladjustment to reduced oxygen pressure at high altitudes. Accumulation of carbon dioxide is unusual except for the scuba diver due to the increased rate of carbon dioxide production, the decreased maximum voluntary ventilation, the added external dead weight, and the possible inefficiency of the carbon dioxide-absorbing canisters.

The Circulatory System

Blood transports oxygen, energy subtrates, and metabolic wastes. It also serves a vital role in temperature regulation. Reduced blood volume, reduced red cells, and reduced hemoglobin lower the body's capacity for aerobic activity. Each tissue has a range of functional response with definite limits of adaptation. In this sense, blood oxygen transport capability is limited by its capacity to carry oxygen (ie, hemoglobin content and oxygen saturation).

An individual's pulmonary blood flow, lung diffusing capacity, rate of oxygen removal, and total hemoglobin all have a close relationship with maximum oxygen intake. Total hemoglobin determines the potential arterial capacity to transport oxygen. For example, low hemoglobin levels in an athlete are often attributed to increased cell destruction, as shown by increases in circulating haptoglobins from increased rates in blood flow or extrinsic trauma (eg, runner's feet, boxer's abdomen). Dietary habits are more significant than the minute amounts of iron lost in perspiration.

Cardiac Output.   Blood oxygen transport also depends on cardiac output. While evaluation of cardiac output during exertion is helpful in diagnosis, stroke volume is difficult to determine directly. Cardiac output increases with work intensity and is directly related to the quantity of oxygen intake: maximum heart output parallels maximum oxygen intake. Such factors as heat exposure and/or dehydration influence stroke volume and change the relationship between heart rate and stroke volume that alters the relationship between oxygen consumption and heart rate.

Cardiac output effectiveness is also determined by relative circulatory distribution among active muscles, viscera, and skin. The maximum limits of stroke volume are determined by the type of exercise and body posture. For example, in comparison to a runner or swimmer who uses most of the body, a cyclist, in not using his upper extremities for propulsion, often pools a large amount of blood within upper extremity veins. The consequence of this is a reduced stroke volume in the cyclist.

Oxygen Pulse.   During exertion, cardiac stroke volume increases and the active cells take more oxygen from arterial blood. Both of these factors increase oxygen delivery to cells. The term "oxygen pulse" refers to the quantity of oxygen removed from the blood during each pulse. It is measured in a specified period by dividing oxygen intake by heart rate. Oxygen pulse increases during exertion, reaching its typical maximum of from 11 to 17 ml at about 135 pulses per minute and decreasing after further cardiac acceleration.

Heart Rate.   Heart rate is closely correlated with maximum oxygen intake. Typically, heart rate is parallel with performance intensity, but maximum cardiac rate decreases with advancing age. There is a linear relationship between heart rate and metabolic rate. Due to the wide variance in individual balance between sympathetic and vagal drives to the cardiac pacemaker, the resting heart rate of the endurance-trained athlete may reach lows of 30 per minute. The maximum sustained heart rate during competition is about 185-195 per minute or less. In activities of high stress and isometric exertion (eg, skiing), peak heart rates of 250 per minute or more may be briefly encountered.

Blood and Pulse Pressures.   Blood pressure and pulse pressure also have a lose relationship with maximum oxygen intake. To meet oxygen demands during prolonged exertion, the blood quantity in the muscles and the blood flow within the lungs must be increased. By increasing the force of heart muscle contraction, systolic blood pressure is raised as heart rate increases. This increase is minimized in the well-trained athlete. This is attributed to decreased peripheral resistance because of vasodilatation.

Pulse pressure, the difference between systolic and diastolic pressures, offers an index to the efficiency of cardiac contraction and stroke volume. Difficulties in the exchange of oxygen and carbon dioxide in active tissues are rarely anticipated except in specific types of events. For example, an overland cyclist may complain of pain and weakness in leg muscles during hill ascents. This is apparently caused by local circulatory obstruction resulting from vigorous quadriceps contractions. However, if activity can be continued in spite of the pain, increased systemic blood pressure tends to overcome the local vascular occlusion. This phenomenon is thought to be a manifestation of the heart failing to develop an immediate and adequate increase in blood pressure.