The muscle strain injury(MSI) is one of the most common form of occupational or sports related muscle injuries. Heat has been used to prevent the MSI for a long time. However, it has been little known about the pathophysiology of the MSI and heat action on it.

To clarify the effect of intramuscular temperature elevation on active(eccentric) strain injury, we stretched the muscle by the speed of 10cm per second to produce strain injury on tibialis anterior(TA) and extensor digitorum longus(EDL) in seventeen rabbits with their neurovascular supplies preserved. During the stretch, the peroneal nerves were electrically stimulated simultaneously to evoke muscle contraction with and without infrared ray irradiation on muscles. Also, to specify the location of lengthening during active strain, we attached markers on the muscles and did the motion analysis.

Following results were obtained. The length increments of stretch in heated TA and EDL at the time of total disruption were 40.3±13.76% and 43.6±16.62%, respectively. But those of non heated TA and EDL were 35.5±12.13% and 34.4±15.83%, respectively, which were significantly lower than those of heated TA and EDL(p<0.05). The absorbed energy in heated TA and EDL until the time of total disruption were 175±39.5N·% and 248±47.0N·%, respectively. But those of non heated TA and EDL were 134±36.1N·% and 184±43.2N·%, respectively, which were significantly lower than those of heated TA and EDL(p<0.05, p<0.01). But there was no difference in peak force on the time of total disruption between two groups. The distal muscle segments including distal musculotendinous junction were the most lengthened parts in both muscle groups but the distal muscle segments of heated muscle groups showed more length increment than that of non heated muscle groups statistically. The most frequent site of total disruption by active strain injury was the distal musculotendinous junction in both groups.

In conclusion, the result that the heated muscles showed larger musculotendinous length at the time of total disruption than non heated muscles supports that intramuscular temperature elevation has preventive effect on muscle strain injury, not by increasing contractile ability but by improving extensibility of musculotendinous units. And it is the distal muscle segment including distal musculotendinous junction that lengthens and absorbs the energy mostly.