Myofibres of muscles removed from the carcass shorten during rigor mortis. Whether myofibres of stretched and restrained muscle tissues shorten during post mortem storage was examined. Muscle strips removed from the semimembranosus muscle of sheep before rigor onset were stretched and restrained. The muscle strips were fixed separately in formalin solution from 1-5hr to 12 days post mortem, and embedded in paraffin. Sections were stained with Mallory-azan. Some myofibres in the pre-rigor-excised and stretched-restrained muscle strips had contraction nodes at 1 day post mortem. The contraction node did not increase in frequency hereafter. Two to several successive sarcomeres shortened regularly to form the nodes parallel to the Z-line. In addition, several successive sarcomeres shortened partly and successively from one side to the other side to form the nodes oblique to the Z-line. Oblique and irregular contraction nodes appear to cause kinks or bends of myofibres. Sarcomere lengths were not always uniform in individual myofibres. Formation of contraction nodes and shortening of partial sarcomeres in the stretched-restrained myofibres indicate that the myofibres shorten individually during rigor mortis and vary in shortening states as in muscles left on the carcass.
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