Length-dependent twitch contractile characteristics of skeletal muscle

Abstract

The length dependence of force development of mammalian skeletal muscles was evaluated during twitch, double-pulse, and tetanic contractions, and the relation between muscle length and the time-dependent characteristics of twitch and double-pulse contractions were determined. In situ isometric contractions of the rat gastrocnemius muscle were analyzed at seven different lengths, based on a reference length at which the maximal response to double-pulse contractions occurred (Lopt-2P). Twitch and double-pulse contractions were analyzed for developed tension (DT), contraction time (tC), average rate of force development (DT·tC–1), half-relaxation time (t50%R), peak rate of relaxation (DT·dtmin–1), and 90%-relaxation time (t90%R). Considering the length at which maximal tetanic DT occurred to be the optimal length (Lo-TET), the peak DT for twitch contractions and double-pulse contractions was observed at Lo-TET+ 0.75 mm (p < 0.05) and Lo-TET+ 0.1 mm (p > 0.05), respectively. When measured at the length for which maximal twitch and double-pulse contractions were obtained, tetanic DT was 95.2 ± 3 and 99.0 ± 2% of the maximal value, respectively. These observations suggest that double-pulse contractions are more suitable for setting length for experimental studies than twitch contractions. Twitch and double-pulse contraction tCwere 15.53 ± 1.14 and 25.0 ± 0.6 ms, respectively, at Lopt-2P, and increased above Lopt-2Pand decreased below Lopt-2P. Twitch t50%Rwas 12.18 ± 0.90 ms at Lopt-2P, and increased above Lopt-2Pand below Lopt-2P. Corresponding changes for double-pulse contractions were greater. Stretching the muscle leads to slower twitch contractions and double-pulse contractions, but the mechanisms of this change in time course remain unclear.Key words: muscle activation, force–length relation, gastrocnemius muscle, Ca2+sensitivity.