Maximal contraction lessens impact response in a muscle contusion model

The effect of muscle contraction on a contusion injury model was studied in the gastrocnemius muscle of anesthetized rats. Both limbs of 18 rats received a contusion injury with a blunt non-penetrating impact. One hind limb was relaxed during impact and the other was electrically stimulated to tetanic contraction. The impact was produced using a drop-mass technique (mass = 171 g, height = 101 cm, spherical radius of impactor tip = 6.4 mm). The impact response was determined by sampling (10 kHz) the transmitted impact force and the displacement of the impactor. In a subgroup of nine rats, the severity of the contusion injury was measured by recording contractile tension in twitch and tetanus within two hours of injury. We found that the peak impact force was significantly less (p < 0.01), while the peak impact displacement was significantly greater (p < 0.01) in the contracted limb. Correspondingly, the impact stiffness of the contracted limb was significantly less (p < 0.01) than the impact stiffness in the relaxed limb. Both impacts produced significant injuries relative to an uninjured control group. The tetanic tension (31 +/- 4 N) generated by the muscles that were contracted during impact was significantly (p < 0.03) greater than that generated by the muscles that were relaxed during impact (27 +/- 4 N). The findings from this specific model indicate that the impact response of the limbs with relaxed muscle was dominated by the underlying bone, while maximally contracted muscle decreased the influence of the bone and lessened the impact response. Maximally contracted muscle was not more susceptible to injury and may act as protective mechanism against some impacts.