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°·s-1 and 180°·s-1 for 5 min, with a 5-s pause interposed between consecutive repetitions. The power output (Ẇ) was calculated; before and during the tasks heart rate (f(c)) and arterial blood pressure (AP(a)) were continuously monitored. Pulmonary ventilation V̇(E) and oxygen uptake (V̇O2) 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 Ẇ was observed, both at 60°·s-1 and 180°·s-1. The V̇2, V̇(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 V̇O2 increased linearly as a function of Ẇ, showing a significantly steeper slope at 60°·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 V̇O2 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.