Neutrophil accumulation following passive stretches contributes to adaptations that reduce contraction-induced skeletal muscle injury in mice.

Skeletal muscles can be injured by their own contractions, especially when the muscle is stretched during a lengthening contraction. Exposing a muscle to a conditioning protocol of stretches without activation (passive stretches) before lengthening contractions reduces contraction-induced injury. Although passive stretching does not damage muscle fibers, neutrophils are elevated in the muscle after passive stretches. Our purpose was to investigate the relationship between neutrophil accumulation following passive stretches and the protection from subsequent contraction-induced injury provided by the passive stretches. Our hypothesis was that passive stretch conditioning would not provide protection from subsequent lengthening contraction-induced injury under circumstances when the increase in muscle neutrophils in response to the conditioning was prevented. Extensor digitorum longus muscles of mice were conditioned with passive stretches 14 days before exposure to a protocol of damaging lengthening contractions. Mice were either untreated or treated with an antibody (RB6-8C5) that reduced the level of circulating neutrophils by over 95% before administration of passive stretches. Neutrophil levels recovered in treated mice by the time lengthening contractions were performed. Lengthening contractions were also administered to muscles with no prior exposure to passive stretches. Maximum isometric force, number of damaged fibers, and muscle neutrophil concentration were measured 3 days after lengthening contractions. Compared with nonconditioned control muscles, the severity of contraction-induced injury was not reduced by prior passive stretch conditioning when mice were treated with RB6-8C5 before conditioning. We conclude that neutrophils contribute to adaptations that protect muscles from injury.