Stretch speed‐dependent myofiber damage and functional deficits in rat skeletal muscle induced by lengthening contraction

Exercise involving lengthening contraction (LC) often results in delayed myofiber damage and functional deficits over the ensuing days. The present study examined whether the stretch speed of LC is a determinant of damage severity. Under isoflurane anesthesia, LC was repeatedly induced in rat ankle extensor muscles at different stretch speeds (angular velocities of 50, 100, 200, and 400 deg/sec) over a fixed stretch range of motion (90°). The number of muscle fibers labeled with Evans blue dye, a marker of muscle fiber damage associated with increased membrane permeability, increased with the angular velocity of LC (by 20% of all myofibers at 400 deg/sec). Muscle fibers with cross‐sectional areas in the range of 3600–4800 μm(2), corresponding to type IIb fiber size, exhibited the most severe damage as revealed by the largest decrease in the number of fibers 3 days after LC at 200 deg/sec, suggesting that muscle damage occurred preferentially in type IIb myofibers. Isometric torque of dorsiflexion measured 2 days after LC decreased progressively with LC angular velocity (by 68% reduction at 400 deg/sec). The angular velocity of muscle stretch during LC is thus a critical determinant of the degree of damage, and LC appears to damage type IIb fibers preferentially, resulting in a disproportionate reduction in isometric torque. This LC response is an important consideration for the design of physical conditioning and rehabilitation regimens.