Mechanical and metabolic determination of V̇O₂ and fatigue during repetitive isometric contractions in situ

Repetitive isometric tetanic contractions (1/s) of the canine gastrocnemius-plantaris muscle were studied either at optimal length (L(o)) or short length (L(s); ~0.9 · L(o)), to determine the effects of initial length on mechanical and metabolic performance in situ. Respective averages of mechanical and metabolic variables were (L(o) vs. L(s), all P < 0.05) passive tension (preload) = 55 vs. 6 g/g, maximal active tetanic tension (P(o)) = 544 vs. 174 (0.38 · P(o)) g/g, maximal blood flow (Q̇) = 2.0 vs. 1.4 ml · min^-1 · g^-1, and maximal oxygen uptake (V̇O₂) = 12 rs. 9 μmol · min^-1 · g^-1. Tension at L(o) decreased to 0.64 · P(o) over 20 min of repetitive contractions, demonstrating fatigue; there were no significant changes in tension at L(s). In separate muscles contracting at L(o), Q̇ was set to that measured at L(s) (1.1 ml · min^-1 · g^-1), resulting in decreased V̇O₂ (7μmol · min^-1 · g^-1), and rapid fatigue, to 0.44 · P(o). These data demonstrate that 1) muscles at L(o) have higher Q̇ and V̇O₂ values than those at L(s); 2) fatigue occurs at L(o) with high V̇O₂, adjusting metabolic demand (tension output) to match supply; and 3) the lack of fatigue at L, with lower tension, Q̇, and V̇O₂ suggests adequate matching of metabolic demand, set low by short muscle length, with supply optimized by low preload. These differences in tension and V̇O₂ between L(o) and L(s) groups indicate that muscles contracting isometrically at initial lengths shorter than L(o) are working under submaximal conditions.