PRELOAD RELEASE INCREASES BLOOD-FLOW AND DECREASES FATIGUE DURING REPETITIVE ISOTONIC MUSCLE CONTRACTIONS

The effects of preload on blood flow (Q), O-2 uptake (VO2), and fatigu e were investigated in the canine gastrocnemius-plantaris muscle in si tu. Repetitive (1 contraction/s, 200 ms duration) afterloaded (0.25-0. 3 maximal active isometric tension) isotonic tetanic contractions were performed in high-preload (HP; 69 g/g, n = 5), low-preload (LP; 35 g/ g, n = 6), and preload-release (PR; 0 g/g, n = 5) experiments. Maximal Q values (1.0, 1.6, and 2.1 ml.min(-1).g(-1), P < 0.05 for all compar isons) and O-2 delivery (8, 13, and 17 mu mol.min(-1).g(-1), P < 0.05 for all comparisons) increased significantly with decreasing preload. The maximal VO2 of HP was 7.2 mu mol.min(-1).g(-1), which is significa ntly lower than both LP (10.5 mu mol.min(-1).g(-1), P < 0.05) and PR v alues (11.4 mu mol.min(-1).g(-1), P < 0.05); these differences were su stained through 20 min of contractions. Fatigue, measured as a loss of power production, was 63, 37, and 23% at 20 min of contractions in HP , LP, and PR, respectively, indicating significantly less fatigue with decreasing preload (P < 0.05 for all comparisons). These data demonst rate that the preload, present as the level of passive tension maintai ned between contractions, can influence Q, VO2, and fatigue during rep etitive isotonic tetanic contractions of muscle in situ by a mechanica lly determined metabolic modulation of dynamic muscle performance.