Changes in perfusion related to muscle length affect the pressor response to isometric muscle contraction

To test the hypothesis that the dependence of the pressor response on muscle length is caused by changes in perfusion, we compared the cardiovascular responses to static contraction at short and long muscle lengths during free perfusion with those during circulatory arrest. Five males performed 2-min static knee extension exercise at 30% of maximal voluntary torque at each of two muscle lengths at a knee angle of 40 degrees (short) and 90 degrees (long). The subjects performed two trials - a free perfusion trial and a circulatory arrest trial. For circulatory arrest, an occlusion cuff placed around the proximal portion of the thigh was inflated to 250 mmHg 2 min before exercise. Mean arterial pressure (MAP), minute ventilation (V(E)), and the muscle oxygenation index in the vastus lateralis muscle were measured using near-infrared spectroscopy. In the free perfusion trial, MAP and V(E) were significantly greater during contractions at 90 degrees than at 40 degrees (p < 0.05). The muscle oxygenation index was significantly lower during contractions at 90 degrees than at 40 degrees (p < 0.05). Circulatory arrest diminished these differences. These results suggest that the relationship between muscle length and the pressor response can be explained by changes in perfusion, which are related to muscle length.