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1 Anesthesiology, Medical College of Wisconsin and VA Medical Center, Milwaukee, WI, USA
2 Kinesiology, University of Western Ontario, London, Ontario, Canada
* To whom correspondence should be addressed. E-mail: pcliff{at}mcw.edu.
Nine healthy volunteers performed a series of single handgrip isometric contractions to test the hypothesis that the blood flow response to a contraction is determined solely by the tension-time index (isometric analog of work). Contractions were performed in duplicate at 15, 30, and 60% of maximal voluntary contraction (MVC) at durations of 0.5, 1, and 2s. Forearm blood flow (FBF) was measured beat by beat using Doppler ultrasound. Peak FBF responded in a graded fashion to graded increases in peak tension with contraction time held constant (35, 56, 90 ml/min for 15, 30, and 60% MVC for 1s, respectively). When tension was kept constant, peak FBF responded in a graded fashion to graded increases in duration (77, 90, 97 ml/min for 60% MVC for 0.5, 1, and 2 s). With a constant tension-time index, peak FBF responded in a graded fashion to graded increases in peak tension (48, 56, 77 ml/min for 15% MVC/2s, 30% MVC/1s, and 60% MVC/0.5s). Similar trends were also observed for total postcontraction hyperemia. Blood flow increased regardless of whether the change in tension-time index was accomplished by an increase in tension or duration of contraction. However, with a constant tension-time index the change in blood flow was related to the peak tension developed. Our results suggest that the blood flow response to a single muscle contraction is not determined solely by the work performed (tension-time index) but also by the number of muscle fibers recruited.
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