title: Effects of submaximal activation on the determinants of power of chemically skinned rat soleus fibres.
abstract: Reducing the activating calcium concentration with skinned fibres is known to decrease isometric force and maximal shortening velocity, both of which will reduce the peak power. However, power is also a function of the curvature of the force-velocity relationship, and there is limited information on how changes in activating calcium affect this important property of muscle fibres. Force-velocity relationships of permeabilized single type I fibres from rat soleus muscle were determined using isotonic contractions at 15°C with both maximal (pCa 4.5) and submaximal activation (pCa 5.6). The rate of tension redevelopment (k(tr)), which provides a measure of sum of the apparent rate constants for cross-bridge attachment and detachment (f(app) + g(app)) following a rapid release and restretch, was also measured. Compared with pCa 4.5, specific tension (P(o)) at pCa 5.6 declined by 22 ± 8% (mean ± s.d.) and the maximal velocity of shortening (V(max)) fell by 44 ± 7%, but curvature of the force-velocity relationship (a/P(o)) rose by 47 ± 31%, indicating a less concave relationship. The value of k(tr) declined by 23 ± 7%. The change in a/P(o) reduced the impact of changes in P(o) and V(max) on peak power by approximately 25%. Fitting the data to Huxley's model of cross-bridge action suggests that lower activating calcium decreased both the rate constant for cross-bridge attachment (f) and that for detachment of negatively strained cross-bridges (g(2)). The fact that V(max) (and thus g(2)) changed to a greater extent than k(tr) (f(app) + g(app)) is the reason that reduced activation results in a reduction in curvature of the force-velocity relationship.
authors.affiliation: Institute for Biomedical Research into Human Movement and Health, Manchester Metropolitan University, John Dalton Building, Oxford Road, Manchester, UK. s.f.gilliver@mmu.ac.uk
authors.forename: S F
authors.initials: SF
authors.lastname: Gilliver
authors.forename: H
authors.initials: H
authors.lastname: Degens
authors.forename: J
authors.initials: J
authors.lastname: Rittweger
authors.forename: D A
authors.initials: DA
authors.lastname: Jones
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D004558:Electric Stimulation; D009119:Muscle Contraction / Q000502:physiology*; D018657:Muscle Fibers, Slow-Twitch / Q000502:physiology*; D053580:Muscle Strength / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley