The pressor response and muscle performance

This thesis examines how muscle perfusion affects contractile performance and how contractile performance evokes cardiovascular responses that modify perfusion.To determine how perfusion pressure affects force output of a working muscle, steady contractions of adductor pollicis were produced by ulnar nerve stimulation. The arm was raised or lowered to change muscle perfusion pressure within the physiological range. Force output fell within seconds of lowering pressure and recovered on restoring perfusion pressure.To determine how this reduced contractility affects muscle fatigue, subjects made repeated contractions of adductor pollicis at 50% maximal force. Fatigue, assessed by interposed twitches, was greater at lower perfusion pressures while central blood pressure rose in proportion to fatigue.The effect of changes in systemic blood pressure on muscle force output was determined by stimulating to evoke steady contractions. Force output declined with time. When central pressure was increased, force output rose so that 18% of the force lost to fatigue was recovered for each 10% increase in blood pressure.These studies were repeated in tibialis anterior, a large postural leg muscle normally exposed to high arterial pressures. The relationships between contractility and perfusion, and between fatigue and the evoked pressor response, were not different to adductor pollicis but extended over wider operating ranges. Muscle flow autoregulation is insufficient to maintain force output in the face of changing perfusion pressure. Contractility is highly sensitive to small changes in perfusion pressure and can vary by up to 30% across the physiological range of blood pressures. The time-course and directions of these perfusion-mediated changes in contractility can be explained by Na+/K+ shifts across the sarcolemma during sustained contraction. A control model is proposed in which sensory input concerning the adequacy of muscle performance exerts a feedback control over the increase in systemic blood pressure during muscular activity. From this, it is estimated that the rise in blood pressure that results from muscle fatigue, reduces fatigue by one-half. That is, muscles would fatigue twice as fast without the central pressor response. The leg muscles in particular rely on their high perfusion pressure during standing