Preload release increases blood flow and decreases fatigue during repetitive isotonic muscle contractions

The effects of preload on blood flow (Q), O2 uptake (VO2), and fatigue were investigated in the canine gastrocnemius-plantaris muscle in situ. 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 comparisons) and Q2 delivery (8, 13, and 17 mumol.min-1.g-1, P < 0.05 for all comparisons) increased significantly with decreasing preload. The maximal VO2 of HP was 7.2 mumol.min-1.g-1, which is significantly lower than both LP (10.5 mumol.min-1.g-1, P < 0.05) and PR values (11.4 mumol.min-1.g-1, P < 0.05); these differences were sustained 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 demonstrate that the preload, present as the level of passive tension maintained between contractions, can influence Q, VO2, and fatigue during repetitive isotonic tetanic contractions of muscle in situ by a mechanically determined metabolic modulation of dynamic muscle performance.