Fiber-type susceptibility to eccentric contraction-induced damage of hindlimb-unloaded rat AL muscles

Slow oxidative (SO) fibers,of the adductor longus (AL) were predominantly d amaged during voluntary reloading of hindlimb unloaded (HU) rats and appear ed explainable by preferential SO fiber recruitment. The present study asse ssed damage after eliminating the variable of voluntary recruitment by teta nically activating all fibers in situ through the motor nerve while applyin g eccentric (lengthening) or isometric contractions. Muscles were aldehyde fixed and resin embedded, and semithin sections were cut. Sarcomere lesions were quantified in toluidine blue-stained sections. Fibers were typed in s erial sections immunostained with antifast myosin and antitotal myosin (whi ch highlights slow fibers). Both isometric and eccentric paradigms caused f atigue. Lesions occurred only in eccentrically contracted control and HU mu scles. Fatigue did not cause lesions. HU increased damage because lesioned- fiber percentages within fiber types and lesion sizes were greater than con trol. Fast oxidative glycolytic (FOG) fibers were predominantly damaged. In no case did damaged SO fibers predominate. Thus, when FOG, SO, and hybrid fibers are actively lengthened in chronically unloaded muscle, FOG fibers a re intrinsically more susceptible to damage than SO fibers. Damaged hybrid- fiber proportions ranged between these extremes.