The resting tension (= force) in muscle is thought to reside mainly in the I-band region of the titin (= connectin) containing gap-filament that spans between the thick filament and the Z-line (Horowits et al. 1986). Indeed, recent experiments have shown that the steady-state force-extension relation of single titin molecules shares the same general features as that of the resting muscle/muscle fibre (Kellarmayer et al. 1997; Tskhovrebova et al. 1997). Furthermore, the force-extension relation of the single titin molecules shows a time-dependent force hysteresis loop, which may arise from differences in the unfolding and refolding kinetics of the molecule relative to the stretch and release rates (Kellarmayer et al. 1997). Although a resting force hysteresis exists in the intact muscle/muscle fibres, its characteristics have not been fully examined.

In a previous study, we reported the effects of small ramp stretches (~2-3 % initial muscle length; L_o) on the tension and sarcomere length responses of resting intact fast and slow muscle fibre bundles isolated from adult rat muscles. Our results show that, recorded at a moderately fast to fast stretch speed, the tension response to a ramp stretch consists of three components – a viscous, visco-elastic and elastic tension (Mutungi & Ranatunga, 1996). We have started to investigate the effects of large (~40 % L_o) stretch/release cycles on the resting tension of relaxed fast and slow muscle fibres isolated from 7-day-old rats. Unlike in adult rats where the muscle fibres are 10-14 mm long, they are only 3-4 mm long in the neonate. Plotting the instantaneous tension against the length change yields a hysteresis loop at all stretch/release speeds examined. The peak tension reached at the end of the stretch and the area of the resultant hysteresis loop increases with stretch speed in a complex manner. The aim of this demonstration is to illustrate these observations.

The experiments will be performed at 20 °C on small muscle fibre bundles isolated from neonatal rats killed with an overdose of sodium pentobarbitone given I.P. A preparation will be mounted horizontally between a tension transducer and a servo-motor in a flow-through muscle chamber. The average sarcomere length, over a 1 mm region, will be monitored using an He/Ne laser and used to set the initial sarcomere length of the preparation. The preparation will then be stretched and released using triangular length changes with different rise times (range 10-1000 ms). The tension responses will then be recorded and the force-extension curves displayed for viewing.

We thank The Wellcome Trust for financial support.

Horowits, R., Kemner, E.S., Bisher, M.E. & Podolsky, R.J. (1986). Nature 323, 160-164.

Kellermayer, M.S.Z., Smith, S.B., Granzier, H.L. & Bustamante, C. (1997). Science 276, 1112-1116.

Mutungi, G. & Ranatunga, K.W. (1996). J. Physiol. 496, 827-836. abstract

Tskhovrebova, L., Trinick, J., Sleep, J.A. & Simmons, R.M. (1997). Nature 387, 308-312.