The effects of ramp stretches on active contractions in intact mammalian fast and slow muscle fibres

The effects of a ramp stretch (amplitude < 6% muscle fibre length (L-0), speed < 13L(0) s(-1)) on twitch tension and twitch tension re-development were examined in intact mammalian (rat) fast and slow muscle fibre bundles. The experiments were done in vitro at 20 degreesC and at an initial sarcomere length of 2.68 mum. In both fibre types, a stretch applied during the rising phase of the twitch response (including the time of stimulation) increased the re-developed twitch tension (15-35%). A stretch applied before the stimulus had little or no effect on the twitch myogram in fast muscle fibres, but it increased the twitch tension (similar to5%) in slow muscle fibres. A similar stretch had little or no effect on tetanic tension in either muscle fibre type. In general, the results indicate that the contractile-activation mechanism may be stretch sensitive and this is particularly pronounced in slow muscle fibres. Recorded at a high sampling rate and examined at an appropriate time scale, the transitory tension response to a stretch rose in at least two phases; an initial rapid tension rise to a break (break point tension, P-1(a)) followed by a slower tension rise (apparent P-2(a)) to a peak reached at the end of the stretch. Plotted against stretch velocity, P-1(a) tension increased in direct proportion to stretch velocity (viscous-like) whereas, P-2(a) tension (calculated as peak tension minus P-1(a) tension) increased with stretch velocity to a plateau (visco-elastic). Examined at the peak of a twitch, P-1(a) tension had a slope (viscosity coefficient) of 1.8 kN m(-2) per L-0 s(-1) in fast fibres and 4.7 kN m(-2) per L-0 s(-1) in slow muscle fibres. In the same preparations, P-2(a) tension had a relaxation time of 8 ms in the fast muscle fibres and 25 ms in the slow muscle fibres. The amplitudes of both tension components scaled with the instantaneous twitch tension in qualitatively the same way as the instantaneous fibre stiffness. These fast/slow fibre type differences probably reflect differences in their cross-bridge kinetics.