Influence of 4 weeks of downhill running on calcium sensitivity of rat single muscle fibers.

Improved Ca 2+ sensitivity has been suggested as a mechanism behind enhancements in muscle mechanical function following eccentric training. However, little is known regarding the effects of eccentric training on single muscle fiber Ca 2+ sensitivity. Adult male Sprague-Dawley rats (sacrificial age ~18 weeks; mass = 400.1 ± 34.8 g) were assigned to an eccentric training (n = 5) or sedentary control group (n = 6). Eccentric training consisted of 4 weeks of weighted downhill running 3×/week at a 15° decline and 16 m/min for 35 min per day in 5-min bouts. After sacrifice, vastus intermedius single muscle fibers were dissected, chemically permeabilized, and stored until testing. Fibers (n = 63) were isolated, and standard Ca 2+ sensitivity, force, rate of force redevelopment (k tr ), and active instantaneous stiffness tests were performed using [Ca 2+ ] ranging from 7.0 to 4.5. Following all mechanical testing, fiber type was determined using SDS-PAGE. There was no difference in pCa 50 (i.e., [Ca 2+ ] needed to elicit half of maximal force) between groups or between fiber types. However, when comparing normalized force across pCa values, fibers from the control group produced greater forces than fibers from the trained group at lower Ca 2+ concentrations (p < 0.05), and this was most evident for Type I fibers (p = 0.002). Type II fibers produced faster (p < 0.001) k tr than Type I fibers, but there were no differences in absolute force, normalized force, or other measures of mechanical function between fibers from the trained and control groups. These findings indicate that eccentric training does not appear to improve single muscle fiber Ca 2+ sensitivity.