Skeletal muscle fiber cross-sectional area: effects of freezing procedures

The cross-sectional area (CSA) of individual fibers is an important measure of skeletal muscle plasticity. To investigate the effects of different freezing procedures on CSA measurements, the CSA of type-identified fibers in the cat tibialis anterior were determined following quick-freezing the muscle at a fixed physiological length (ipsilateral, frozen at length) and compared to the fiber CSA following quick-freezing a mid-portion of the muscle where the fibers were allowed to freely shorten during tissue preparation (contralateral, frozen as a block). The mean CSA of each fiber type was significantly smaller in the muscles frozen at length vs. frozen as a block in both a deep (close to the bone) and a superficial (away from the bone) region of the muscle, except for the slow oxidative (SO) fibers in the superficial region. The percent difference in mean fiber CSA was smaller for the SO compared to the fast oxidative glycolytic and fast glycolytic fibers in both the deep (41, 47 and 56%, respectively) and superficial (20, 36 and 48%, respectively) regions. In addition, the differences in the mean CSAs of the fast fibers between the two freezing procedures were approximately 10% larger in the deep compared to the superficial region. The fiber-type differential responses may be related, at least in part, to the architectural features of the fibers. These data indicate that the freezing procedures used to prepare the muscle tissue are an important consideration when determining the CSA of individual skeletal muscle fibers and consequently the specific tension, tension per unit CSA, of the muscle units.