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° (short) and 90° (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 (VE), and the muscle oxygenation index in the vastus lateralis muscle were measured using near-infrared spectroscopy. In the free perfusion trial, MAP and VE were significantly greater during contractions at 90° than at 40° (p < 0.05). The muscle oxygenation index was significantly lower during contractions at 90° than at 40° (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.
Mean Arterial Pressure Maximal Voluntary Contraction Pressor Response Knee Extensor Oxygenation Index
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The authors gratefully acknowledge Dr. Taku Wakahara for invaluable technical support with the ultrasonic apparatus.
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