Contractile behaviour in skeletal muscle-tendon unit during small amplitude sine wave perturbations.

The effects of sinusoidal vibrations (0.2 mm) on contractile behaviour of the medial gastrocnemius muscle of the rat (N = 6) were determined over a wide range of frequencies (10-210 Hz). The performance of the contractile element (CE) during maximal tetanic contractions and vibrations was calculated by correcting muscle performance for series elasticity. For one experimental muscle the contractions were simulated using a computer model based on mechanical characteristics of that particular muscle. It was found that 10 Hz movements increased the slope of the CE force-velocity curve, compared to its isokinetically determined characteristics. At higher frequencies (30 Hz and above) this slope decreased. It is hypothesised that these two changes in CE behaviour are based on the same phenomena in CE behaviour: force enhancement by active stretch and depression by shortening. The time constants of the decay of these processes may cause the different impact on CE force-velocity behaviour. It is concluded that CE performance is affected at all frequencies, but its impact on muscle-tendon performance shows at low frequencies only. At high-frequencies series elastic characteristics play a dominant role.