Oxygen uptake before, during, and after light steady-state exercise.

Oxygen uptake before, during, and after light steady-state exercise.

useful, during dynamic exercise that uses a large muscle mass, each person has a maximal oxygen uptake, a ceiling up to 20 times basal consumption, that cannot be exceeded, although it can be increased by appropriate training. This maximal oxygen uptake is a useful but imperfect predictor of the ability to perform prolonged dynamic external work or, more specifically, of endurance athletic performance. Maximal oxygen uptake is decreased, all else being equal, by age, bed rest, or increased body fat.

Maximal oxygen uptake is also used to express relative work capacity. A world champion cross-country skier obviously has a greater capacity to consume oxygen than a novice. However, when both are exercising at intensities requiring two thirds of their respective maximal oxygen uptakes (the world champion is moving much faster in doing this, as a result of higher capacity), both become exhausted at roughly the same time and for the same physiological reasons (Fig. 30.2). In the discussion that follows, relative as well as absolute (expressed as L/min of oxygen uptake) work levels are used to explain physiological responses. The energy costs and relative demands of some familiar activities are listed in Table 30.1.

What causes oxygen uptake to reach a ceiling? Historically, many arguments claim primacy for either cardiac output (oxygen delivery) or muscle metabolic capacity (oxygen use) limitations. However, it may be that every link in the chain taking oxygen from the atmosphere to the mito chondrion reaches its capacity at about the same time. In practical terms, this means that any lung, heart, vascular, or musculoskeletal illness that reduces oxygen flow capacity will diminish a patient's functional capacity.

In isometric exercise, force is generated at constant muscle length and without rhythmic episodes of relaxation. Isometric work intensity is usually described as a percentage of the maximal voluntary contraction (MVC), the peak isometric force that can be briefly generated for that specific exercise. Analogous to work levels relative to maximal oxygen uptake, the ability to endure isometric effort, and many physiological responses to that effort, are predictable when the percentage of MVC among individuals is held constant.

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Essentials of Human Physiology

Essentials of Human Physiology

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