The influence of fiber composition, recruitment order and muscle temperature on the pressor response to isometric contractions in skeletal muscle of the cat.

The blood pressure response to isometric exercise was examined in the medial gastrocnemius muscle of the cat at muscle temperatures of 28 and 38 degrees C. Contractions were sustained at tensions of between 10 and 100% of the muscle's initial strength (tetanic tension of the unfatigued muscle) with recruitment proceeding from either the fastest to the slowest or from the slowest to the fastest motor units, respectively. The results of these experiments showed that the blood pressure response throughout the duration of contractions at tensions greater than 10% of the initial strength was constant by either order of recruitment and for both muscle temperatures examined, the mean blood pressure rising linearly from an average value of 101.2 +/- 10.3 mm Hg at the onset of the contraction to 146.3 +/- 14.2 mm Hg at the point of fatigue. In contrast, during contractions at 10% of the initial strength at both muscle temperatures, the blood pressure was lower during the first 20% of the contractions when recruitment proceeded from the slowest to the fastest motor units but stayed the same throughout the remainder of the contractions. The results of these experiments indicate the importance of muscle fiber composition in determining the blood pressure response for weak isometric contractions.