It is well known that the force produced during lengthening of an activated muscle is greater than the maximal isometric force, but there is less agreement as to the magnitude of this force enhancement (see Woledge et al. 1985). In experiments on isolated frog muscle, the force produced during lengthening has been shown to be up to twice the isometric force (Katz, 1939). During in vivo experiments on human muscle, however, the lengthening force shows only modest, if any, increase above the isometric force (Pinniger et al. 2000). While muscle temperature is known to influence the contractile process (Ranatunga, 1984), there has been no systematic investigation of the influence of temperature on lengthening force enhancement in isolated mammalian muscle. This was the purpose of this study.

Adult male rats were humanely killed with an intra-peritoneal injection (> 200 mg kg^-1) of an overdose of Sodium pentobarbitone (Euthatal). Small bundles of about five intact muscle fibres, dissected from the flexor hallucis brevis muscle, were mounted horizontally between a force transducer and a servomotor at an initial sarcomere length of 2.4-2.6 µm. The fibre bundle was stimulated to maximal tetanic tension and a ramp stretch of 5 % L₀ (muscle fibre length) was applied on the tension plateau. Ramp stretch velocities between 0.05 and 4.0 L₀ s^-1 were used and the force enhancement examined at 5°C increments between 10 to 30°C.

Force responses during constant velocity lengthening were qualitatively similar to those from frog muscle fibres (Lombardi & Piazzesi, 1990). Force responses were analysed for peak force at the end of stretch (P_k), the steady force 500 ms after the stretch (P₃) and the rate of force decay after stretch. To quantify the steady force during lengthening (P₂), P₃ was subtracted from P_k and the resultant was normalized to the tetanic force immediately prior to the stretch (P₀).

A small steady force after stretch (P₃) was present in all records and was independent of stretch velocity. When corrected for P₃, the enhancement of steady force during lengthening (P₂) increased with stretch velocity up to an ‘approximate’ plateau (above ~0.5 L₀ s^-1). In preliminary experiments (n = 6), the relative force enhancement decreased with increasing temperature. As a function of isometric force (P₀) the mean (± S.D.) lengthening force (at ~4.0 L₀ s^-1) was 2.22 (± 0.14) at 10°C, 1.78 (± 0.07) at 20°C and 1.59 ( ± 0.02) at 30°C. This decrease is probably due to a marked increase in isometric force (approximately 2-fold) in warming from 10 to 30°C. The temperature-dependent modulation of force transients may account, in part, for the differences in the force-lengthening velocity relationship reported for human and frog muscle.

We thank the Wellcome Trust for financial support.