Mechanical and metabolic determination of VO2 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 (Lo) or short length (Ls; approximately 0.9 . Lo), to determine the effects of initial length on mechanical and metabolic performance in situ. Respective averages of mechanical and metabolic variables were (Lo vs. Ls, all P < 0.05) passive tension (preload) = 55 vs. 6 g/g, maximal active tetanic tension (Po) = 544 vs. 174 (0.38 . Po) g/g, maximal blood flow (Q) = 2.0 vs. 1.4 ml . min-1 . g-1, and maximal oxygen uptake (VO2) = 12 vs. 9 micromol . min-1 . g-1. Tension at Lo decreased to 0.64 . Po over 20 min of repetitive contractions, demonstrating fatigue; there were no significant changes in tension at Ls. In separate muscles contracting at Lo, Q was set to that measured at Ls (1.1 ml . min-1 . g-1), resulting in decreased VO2 (7 micromol . min-1 . g-1), and rapid fatigue, to 0.44 . Po. These data demonstrate that 1) muscles at Lo have higher Q and VO2 values than those at Ls; 2) fatigue occurs at Lo with high VO2, adjusting metabolic demand (tension output) to match supply; and 3) the lack of fatigue at Ls with lower tension, Q, and VO2 suggests adequate matching of metabolic demand, set low by short muscle length, with supply optimized by low preload. These differences in tension and VO2 between Lo and Ls groups indicate that muscles contracting isometrically at initial lengths shorter than Lo are working under submaximal conditions.