Comparison of blood pressure and heart rate responses to isometric exercise and passive muscle stretch in humans

Abstract

The responses of mean arterial blood pressure (BP_a) and heart rate (f _c) to isometric contraction and passive stretch were compared in seven healthy male subjects at identical external forces. They were investigated in the sitting position with the hip and knee joint flexed to 90°. Each subject performed two tests, separated by a day, in which the stimuli were applied in random order. After 5 min of rest they performed either 10-min static plantar flexion of one calf (200 N) or 10 min of passive calf muscle stretch at the same load. After 5-min rest, the second stimulus was applied for a further 10 min followed by 5-min rest. The second test was identical except for the sequence of the stimuli. The BP_a was measured by a noninvasive and continuous method. Contraction of the vastus lateralis, gastrocnemius lateralis, and soleus muscles were determined by the myo-electric activity (electromyogram, EMG) by means of surface electrodes. The EMG activity of the vastus lateralis muscle remained at resting values throughout the experiments. Increases in EMG activity could only be detected for the triceps surae muscles during isometric contraction. During the initial 2 min of stimulation the BP_a and (f _c), responses to active contraction and passive stretch were comparable. Thereafter, both parameters showed significantly higher values during contraction. It was concluded that mechanical stress may have contributed to the early response of BP_a during both passive stretch and voluntary contraction but that chemical stimuli were needed to maintain the peripheral cardiovascular drive.