Biomechanical and histological evaluation of muscle after controlled strain injury

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Abstract
This study correlates force generation and healing in muscle after controlled strain injury. Right tibialis ante rior (TA) muscles from 30 rabbits were strained to approximately 80% of failure while the left TA muscles served as control. Both injured and control muscles were then tested for ability to generate force. Seven animals were sacrificed immediately after testing and the muscles were examined grossly and histologically. Remaining animals were retested at 24 hours (N = 7), 48 hours (N = 8), and 7 days (N = 8). Contractile ability following injury was 70.5% of control immediately, 51.1% at 24 hours, 74.5% at 48 hours, and 92.5% at 7 days. Immediate histology showed limited distal fiber rupture and hemorrhage. By 24 hours, histology showed fiber necrosis, infiltration of inflammatory cells, edema, and hemorrhage. At 48 hours there was com plete fiber breakdown and intense inflammatory cell proliferation. At 7 days inflammation was reduced and collagen fibrosis more advanced. Our findings demon strate that injured muscle begins functional recovery by 48 hours despite inflammation and active healing. This suggests that decreasing muscle function seen clinically between 24 and 48 hours following strain injury may result from pain due to inflammation. Scarring and fibrosis seen at 7 days may explain the frequent recur rence of injury to strained muscles.