The force-velocity relationship at negative loads (assisted shortening) studied in isolated, intact muscle fibres of the frog

Aim: The study was undertaken to explore the force-velocity relationship under conditions where the myofilament system is subjected to an external force that serves as a negative load and assists the shortening movement. Methods: The experiments were carried out on single muscle fibres isolated from the anterior tibialis muscle of Rana temporaria. The fibres, being operated under load-clamp control, were released to shorten during tetanic stimulation at sarcomere lengths where the fibres carried different degrees of passive tension. The shortening thus occurred while the sarcomeres were subjected to a force that may be characterized as a 'negative load', that is, a force assisting the shortening movement. Results: The force-velocity relationship below zero load was found to be a smooth continuation of the force-velocity curve recorded at positive loads with the shortening velocity increasing steeply at loads below zero. A negative load amounting to merely 10% of the isometric force, thus raised the shortening velocity to a level two to three times higher than V-0, the velocity recorded at zero load. Conclusions: The results provide evidence that, even in the presence of a longitudinal compressive force, the speed of shortening of the muscle fibre is determined by the cycling rate of the interacting cross-bridges. The force-velocity relationship at negative loads may play a relevant part during fast movements of striated muscle as pointed out in the discussion.