III. ISOMETRIC, STATIC EXERCISE
Isometric means ‘‘equal measure.’’ The muscle fiber length remains the same when muscular tension is exerted against a fixed resistance; that is, static exercise involves the development of tension within muscle fibers and results in little or no movement of bones and joints. Weightlifting and pushing against a wall are both examples of isometric exercise. Such exercise is also called resistance exercise.
Resistance exercise is generally not recommended for individuals who have hypertension, heart failure, or known heart disease, because it causes a marked increase in blood pressure during the effort. The pressure of tensed muscles squeezes blood vessels, and therefore, less blood passes through these arteries. Also, the constriction of blood vessels causes an increase in blood pressure. The heart works harder to pump against the resistance in the constricted arteries, but the amount of blood the heart pumps only increases slightly because the few muscles that are used require less oxygen than would be required during isotonic exercise. Despite this lower oxygen requirement of the exercising muscle during static exercise, the heart work is increased and the heart muscle requires more oxygen. During a double leg press in which a trained lifter is allowed to hold his breath, blood pressure can increase to greater than 250 mmHg systolic and greater than 130 mmHg diastolic. Pressures increase to overcome the high intramuscular pressures to maintain effusion of muscle tissue. The response to an acute resistance effort is a marked elevation of diastolic blood pressure, which is relatively unchanged during aerobic exercise. If atheroma-tous coronary artery disease is present, the heart muscle may suffer from a shortage of oxygen that can result in chest pain, angina, or precipitation of a heart attack (see the chapter Angina). Hypertensive individuals should not engage in isometric exercises. During static exercise there is an increased heart rate and blood pressure, but only a mild increase in cardiac output and rate of breathing. Therefore, physical conditioning may be achieved, but the cardiopulmonary conditioning training effect cannot be obtained. Weightlifting produces increased strength of those skeletal muscles that are used but does not improve the capacity of the heart to work and does not increase blood circulation to the heart. Weight-lifting and power training do not maintain an increased heart rate that is more than 70% of maximal heart rate for 30 consecutive minutes. Only aerobic exercise in which several strength exercises performed sequentially without rest periods in between produces some heart fitness.
Страницы: 1 | 2