Factors & Physiological Mechanisms Affecting Cardiorespiratory Fitness

Factors & Physiological Mechanisms Affecting Cardiorespiratory Fitness

Background

In exercise physiology, it is a long-held belief that cardiopulmonary fitness is the foundation of an individual's exercise capacity. Indeed, improved fitness and performance with training are in part the product of an increased capacity to supply exercising muscle with oxygen and nutrients owing to adaptations of the cardiorespiratory system. However, other significant changes in exercising muscle can result in a reduction of required cardiorespiratory effort at a given level of exercise. These adaptations are discussed in the following sections of this entry (Bassett, 1997).

Cardiovascular Effects

At the onset of exercise, heart rate and stroke volume both increase, resulting in an increase in cardiac output (the amount of blood pumped through the heart in 1 minute, usually expressed in units of liters [L] per minute) and increased systemic blood supply. Over time, the body accommodates to the stress of chronic exercise by improving stroke volume (the volume of blood pumped with each contraction of the heart), which translates into improved cardiac output. Several factors combine to produce this increased stroke volume (Bassett & Howley, 1997).

Lower resting heart rate as well lower heart rate at a given exercise workload: This allows increased time for the heart to fill with blood, thus increasing the amount of blood that can be pumped back out of the heart. For endurance athletes, the heart rate is often bradycardic, or less than the 50 to 55 beats/minute (bpm) considered the lower limit of “normal.” The mechanism of the observed heart rate reduction appears to be reduced sympathoadrenal system activation as well as a decline in the withdrawal of vagal nerve activity during the same level of exercise in the trained versus untrained subject. The mechanistic pathway for these changes in neural stimulation remains unclear.

Increased blood volume: Blood volume in a trained individual is about 15% higher than in an untrained individual. Interestingly, this adaptation occurs very rapidly, resulting in improved exercise capacity after only a week of training. Unfortunately, it also disappears quickly, after only days of inactivity.

Cardiovascular hypertrophy: Hypertrophy is the increase in size of the heart muscle fibers, not an increase in the number of fibers. This change is evident on electrocardiogram (EKG) as well as imaging (chest X-ray, echocardiography, etc.). With exercise, hypertrophy is eccentric. This means that the volume of the ventricle increases substantially, resulting in an increase in the volume of the lumen of the chambers of the heart, while the increase in the thickness of the wall of the heart chambers is relatively minor. Hypertrophy is most evident in the left ventricle (Bergh, 2000). The consequence of a larger pump is the ability to pump more blood. The eccentric hypertrophy that occurs with chronic exercise is in contrast to the concentric hypertrophy seen when the heart does more work in the setting of systemic hypertension. In hypertension, although the heart works harder, the lumen does not increase in size and the heart rate does not ...