Prevention of histopathologic changes from 30 repeated stretches of active rat skeletal muscles by long inter-stretch rest times.

This study was performed to examine the protective effect of a long rest between stretches of activated rat plantar flexor muscles on immediate functional and delayed structural signs of injury. Plantar flexor muscles activated by nerve stimulation by ankle rotation from 1.57 to 0.70 rad under anesthesia (brevital) were stretched 30 times. Three groups of female Sprague Dawley rats were tested: (1) long inter-stretch (180 s, S180), (2) short inter-stretch (40 s, S40), and (3) short inter-isometric contraction (40 s, IC40) times. Isometric and peak stretch forces during the stretch protocols and force-frequency relationships before and 1 h after the stretching protocols were measured. Histopathologic changes were evaluated in medial gastrocnemius muscle samples using antibodies to cellular (actin, desmin, dystrophin, fast myosin) and extracellular (laminin, albumin, complement C3) proteins and markers for neutrophils (W3/13) and macrophages (ED1). Both stretch protocols produced the same force deficits [mean (SEM)] [S40 56.7 (2.1)%, S180 51.3 (3.7)%] with little recovery within 1 h. After 2 days, histopathologic changes were present only in samples from the S40 protocol including desmin negative fibers [162 (32)] which stained for albumin, complement C3, laminin, actin and fast myosin. Many of the necrotic fibers also contained infiltrating neutrophils and macrophages. In summary, histopathologic changes in rat gastrocnemius muscle following repeated stretch injury was prevented by long inter-stretch rests.