Recovery from immobilisation: responses of fast-twitch muscle fibres to spontaneous and intensive exercise in rat calf muscles

Abstract

Four weeks of immobilisation with two types of re-mobilisation programmes (intensive concentric treadmill exercising during 6 days, and free exercising, and immobilisation without any re-mobilisation period were studied to clarify possible exercise-induced calf muscle damage especially in fast-twitch fibres used in running compared to non-immobilised rats housing freely in their cages. As markers of muscle injury, conventional histology, β-glucuronidase (β-GU) activity and fetal myosin heavy chain expression (MHC-d) were assessed on Days 0, 1, 3, 6 and 14 after the cast removal. Only minor focal hypercontraction, ruptures and necrosis of myofibrils, and weak inflammatory cell reactions were found in all samples examined, except in the controls. No MHC-d positive cells were found indicating absence of active regeneration after immobilisation or re-mobilisation. Minor increase in β-GU activity was observed in all three muscles studied, but statistically significant increase was observed only in the samples of the free exercising group on Day 14 after the cast removal. To conclude, intensive concentric treadmill exercise for 6 days did not cause significantly more muscle damage than did free exercising re-mobilisation.