Active Stretch does not Affect the Rate of Force Redevelopment in Rabbit Skinned Fibres

Abstract

University of Calgary, Calgary, AB, Canada. The steady state force produced by a muscle following active stretch is greater than the corresponding purely isometric force. This phenomenon is called “Residual Force Enhancement” (RFE). Most of the proposed mechanisms of RFE were either related to the development of structural non-uniformities, or with the engagement of a passive structural element upon activation. So far there have been hardly any experiments investigating the possibility for a change in cross bridge kinetics after active stretching. The aim of this study was to investigate if RFE was accompanied with changes in the rate of force redevelopment in rabbit psoas and soleus skinned fibres. The rate of force redevelopment (Ktr) was evaluated after a quick release-stretch cycle. Furthermore, stiffness and the time that the cross bridges require to start redevelop tension (Δt) were measured after a purely isometric contraction and active stretch. Ktr and stiffness were not changed after active stretch compared to a purely isometric contraction in both psoas and soleus fibres. Δt after active stretch was smaller than Δt after a purely isometric contraction in psoas and soleus fibres. Using a two-state cross bridge model, as described by Brenner (PNAS, 1987), we estimated that the apparent rate constants of cross bridge attachment and detachment were not altered in the force enhanced state. Consequently, RFE might be explained, in part, in terms of changes in cross-bridge based forces. The strain of a passive element, likely the molecular spring titin, during active stretch and then its recoil when the fibre is released might be responsible for the observed decrease in Δt after stretch.