Metabolic and cardiorespiratory responses to maximal intermittent knee isokinetic exercise in young healthy humans

There have been many studies on the effects of isokinetic exercise on muscle performance in training and rehabilitative programmes. On the other hand, the cardiovascular and metabolic responses elicited by this type of exercise have been poorly investigated. This study was specifically designed to describe the relationships, if any, between metabolic and cardiorespiratory responses and power output during maximal intermittent knee isokinetic exercise when a steady state is reached. A group of 18 healthy subjects (10 men and 8 women, age range 25-30 years) were requested to perform at maximal concentric isokinetic knee extensions/flexions 60 degrees . s(-1) and 180 degrees . s(-1) for 5 min, with a 5-s pause interposed between consecutive repetitions. The power output ((W) over dot) was calculated; before and during the tasks heart rate (f(c)) and arterial blood pressure (AP(a)) were continuously monitored. Pulmonary ventilation ((V) over dot(E)) and oxygen uptake ((V) over dot O-2) were measured at the 4th and at the 5th min of exercise and blood lactate concentration at rest and at the 3rd min of recovery. From the 4th to the 5th min only a slight decrease in (W) over dot was observed, both at 60 degrees . s(-1) and 180 degrees . s(-1). The (V) over dot O-2, (V) over dot(E) f(c) and AP(a) showed similar values in the last 2 min of exercise, suggesting that a steady state had been reached. The (V) over dot O-2 increased linearly as a function of (W) over dot, showing a significantly steeper slope at 60 degrees . s(-1) than at 180 . s(-1). The f(c), in spite of a large interindividual variation, was linearly related to metabolic demand, and was not affected by angular velocity. Systolic and diastolic AP(a) were not related either to (V) over dot O-2 or to angular velocity. In conclusion it would appear that the metabolic response to maximal intermittent knee isokinetic exercise resembles that of dynamic exercise. Conversely, the cardiocirculatory responses would seem to reflect a relevant role of the isometric postural component, the importance of which should be carefully evaluated in each subject.