We developed a rat model of fast-strain muscle injury to examine the relationship between functional deficit and the degree of muscle damage. A single fast strain was applied to the right plantaris (Plt) muscle of adult male rats. A tetanic contraction was induced by stimulating the sciatic nerve. Three types of strain injuries were produced by manipulating the timing of strain and contraction conditions, including applying the strain at: (1) the shortening phase (SP), (2) the full contraction phase (FCP) during tetanic contraction and (3) when the muscle is not contracting, i.e. non-contraction (NC). The contralateral Plt muscle was used as the control. Morphological and functional analysis were performed and the severity of strain injury was estimated by measuring [3H]thymidine labelling 48 h after the strain was applied. The results show a significant decrease in tension output in all three groups at 5 min after strain application (P < 0.01). The functional deficit lasted for 3 weeks in the NC group, while rats of the SP and FCP groups showed recovery 1 week after strain. Increased uptake of [3H]thymidine was similar in SP and FCP groups, but 3.5-fold higher in NC than in SP and FCP groups (P < 0.01). Histological analysis revealed an increase in the size of interstitial spaces of the muscle in NC compared to SP and FCP groups (P<0.05). Our results suggest that the contraction of muscle fibres limits the severity of connective tissue damage, and that muscle damage accompanied by disruption of the muscle connective tissue network requires a relatively longer recovery time.