Mechanical and metabolic determination ofV˙o2and fatigue during repetitive isometric contractions in situ

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Abstract
Repetitive isometric tetanic contractions (1/s) of the canine gastrocnemius-plantaris muscle were studied either at optimal length ( Lo) or short length ( Ls; ∼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 ( Lovs. 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 (V˙o2) = 12 vs. 9 μmol ⋅ min−1⋅ g−1. Tension at Lodecreased to 0.64 ⋅ Poover 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 V˙o2(7 μmol ⋅ min−1⋅ g−1), and rapid fatigue, to 0.44 ⋅ Po. These data demonstrate that 1) muscles at Lohave higher Q˙ andV˙o2values than those at Ls; 2) fatigue occurs at Lowith highV˙o2, adjusting metabolic demand (tension output) to match supply; and 3) the lack of fatigue at Lswith lower tension, Q˙, andV˙o2suggests adequate matching of metabolic demand, set low by short muscle length, with supply optimized by low preload. These differences in tension andV˙o2between Loand Lsgroups indicate that muscles contracting isometrically at initial lengths shorter than Loare working under submaximal conditions.