The purpose of this study was to determine whether initial muscle length influenced the extent of isometric force deficit following 20 in vitro lengthening contractions of the soleus muscle from Fischer 344 rats. Force deficit was evaluated following one of five protocols: (1) lengthening contractions from optimal muscle length (Lo) to 120% Lo; (2) lengthening contractions from 80% Lo to Lo; (3) lengthening contractions from Lo to 120% Lo but with a stimulation frequency that elicited the same force as protocol 2; (4) 20 isometric contractions at Lo; (5) 20 stretches +/- 20% Lo in inactive muscle. Following lengthening contractions, extent of force deficit significantly differed between protocols 1, 2, and 3 (P < 0.05). Maximal isometric force (Po) was decreased by approximately 32%, approximately 8%, and approximately 15% in protocols 1, 2, and 3, respectively. In contrast, neither isometric contractions nor passive stretching (protocols 4 and 5) resulted in any reduction in Po. Irrespective of muscle length, the extent of force deficit was highly correlated (R = -0.774, P < 0.001) with peak force during active lengthening. Thus, the magnitude of isometric force deficit following lengthening contractions is influenced by both initial muscle length and peak force development. These findings have important practical implications for both exercise conditioning and rehabilitation, which are discussed.
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