Abstract: | Repetitiveisometric tetanic contractions (1/s) of the caninegastrocnemius-plantaris muscle were studied either at optimal length(Lo) or shortlength (Ls;~0.9 · Lo),to determine the effects of initial length on mechanical and metabolicperformance in situ. Respective averages of mechanical and metabolicvariables were(Lo vs.Ls, allP < 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 ( ) = 2.0 vs. 1.4 ml · min 1 · g 1,and maximal oxygen uptake( O2) = 12 vs. 9 µmol · min 1 · g 1.Tension at Lodecreased to0.64 · Po over20 min of repetitive contractions, demonstrating fatigue; there were nosignificant changes in tension atLs. In separatemuscles contracting atLo, was set to that measured atLs (1.1 ml · min 1 · g 1),resulting in decreased O2(7 µmol · min 1 · g 1),and rapid fatigue, to0.44 · Po. Thesedata demonstrate that 1)muscles at Lohave higher and O2 values than those at Ls;2) fatigue occurs atLo with high O2, adjusting metabolic demand (tension output) to match supply; and3) the lack of fatigue atLs with lowertension, , and O2 suggestsadequate matching of metabolic demand, set low by shortmuscle length, with supply optimized by low preload. Thesedifferences in tension and O2 betweenLo andLs groupsindicate that muscles contracting isometrically at initial lengthsshorter than Loare working under submaximal conditions. |