# The contractile properties of single slow- and fast twitch muscle fibers: Alterations with endurance exercise-training

#### Abstract

Fast muscle is known to have 3 to 5-fold higher maximal shortening velocities (Vmax) and the difference is thought to reflect the higher myofibrillar ATPase activity of the fast muscle. One objective of this work was to use the single fiber technique to test the hypothesis that the fast type IIb fibers exhibit a higher Vmax and ATPase activity than the fast type IIa fiber. Vmax was determined by the slack test and the fiber ATPase was measured fluorometrically in the same fiber. The Vmax (7.9 $\pm$.18 fl/s) of the type IIb fibers was found to be significantly greater than the type IIa fibers (4.4 $\pm$.21 fl/s) and a similar difference existed in the ATPase activity (927 $\pm$ 70, 760 $\pm$ 60 $\mu$M$\cdot$min$\sp{-1}\cdot$mm$\sp{-3}$, respectively). Skeletal muscle is known to adapt to alterations in recruitment pattern, and regular exercise-training programs have been shown to induce a variety of biochemical and physiological adaptations in skeletal muscle. The majority of studies have been done on whole muscle and thus the effect of exercise-training on individual slow- and fast-twitch fibers is for the most part unknown. A second objective of this study was to determine the effect of regular endurance exercise on fiber Vmax, ATPase activity, the force pCa relationship and the force-velocity relationship. The training program consisted of treadmill running 2 hr/day, 5 day/wk up a 15% grade at a speed of 27m min for 6-12 wk. The exercise program had no effect on fiber diameter, peak tension per cross sectional area (Po/CSA), the force-pCa relationship, of force-velocity relationship. The Vmax of the trained soleus type I fibers was significantly greater (23%) than the control fibers. Concomitantly, there was a significant increase (23%) in the fiber ATPase activity following training. The exercise induced increase in the Vmax was highly correlated with (r = 0.76) and likely caused by the elevated fiber ATPase. The fast type IIa and IIb fibers were not significantly altered by the exercise program. This study clearly demonstrates a functional difference between the fast type IIa and IIb fibers of limb skeletal muscle, and establishes that endurance exercise-training increases the fiber ATPase and thus the Vmax of the slow type I fiber.

#### Recommended Citation

Schluter, Jane Marie, "The contractile properties of single slow- and fast twitch muscle fibers: Alterations with endurance exercise-training" (1991). Dissertations (1962 - 2010) Access via Proquest Digital Dissertations. AAI9133809.
https://epublications.marquette.edu/dissertations/AAI9133809

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