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

The effects of preload on blood flow (Q̇), O₂ uptake (V̇O₂), 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 Q̇ values (1.0, 1.6, and 2.1 ml · min^-1 · g^-1, P < 0.05 for all comparisons) and O₂ delivery (8, 13, and 17 μmol · min^-1 · g^-1, P < 0.05 for all comparisons) increased significantly with decreasing preload. The maximal V̇O₂ of HP was 7.2 μmol · min^-1 · g^-1, which is significantly lower than both LP (10.5 μmol · min^-1 · g^-1, P < 0.05) and PR values (11.4 μmol · 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 Q̇, V̇O₂, and fatigue during repetitive isotonic tetanic contractions of muscle in situ by a mechanically determined metabolic modulation of dynamic muscle performance.