Effect of muscle mass on the pressor response in man during isometric contractions.

1. Changes in blood pressure and heart rate were measured in six healthy male subjects during voluntary isometric contractions of the forearm and quadriceps muscles. Arterial pressure was measured directly via a catheter inserted into the radial artery of the non-contracting arm. Each subject exerted two types of contractions: (a) sustained contractions at 70% of the maximum voluntary contraction (MVC) until fatigue occurred and (b) sustained contractions starting at maximum tension (100% MVC), held for a total duration of 1 min. 2. During fatiguing contractions at 70% MVC, there was a progressive increase in blood pressure, reaching a peak level at fatigue. The same level of mean arterial pressure was achieved during contractions of the same relative tension, regardless of the muscle mass. The same trend was observed for the changes in heart rate. 3. During contractions which started with the maximum tension, where tension fell continuously during the 60 s of maximal effort, mean arterial pressure rapidly increased to high levels within a few seconds, and then increased further by 20-30 mmHg during the sustained maximal effort. There was no difference in the initial rapid increases in mean arterial pressure, nor in the final mean arterial pressures reached between contractions of the forearm or quadriceps muscles. There were no differences in the heart rates achieved during these contractions either. 4. There was no significant difference between the mean arterial pressures observed at fatigue of a 70% MVC contraction or at the end of the 60 s maximum effort during handgrip contractions, or during contractions with the quadriceps muscles. 5. These results support the view that muscle mass is not a determinant of the magnitude of the cardiovascular reflexes during fatiguing isometric contractions in man.