Morphological effects of two protocols of passive stretch over the immobilized rat soleus muscle

Abstract

This study evaluated two different stretching protocols employed during a period of hind-limb immobilization in terms of their effects on muscle morphology. Quantitative data regarding the soleus muscle were obtained based on the clinical hypothesis that a high frequency of this exercise would improve the recovery of muscle structure. Twenty-four male Wistar rats were divided into four groups (n = 6 each): the control group (C); the immobilized group, in which the left hind limb was immobilized in order to maintain the soleus muscle in a fully shortened position for 3 weeks (I); the 'immobilized and stretched every 3 days' group, in which the left hind limb was immobilized as in the immobilized group, but with the soleus muscle stretched every 3 days for 40 min (Ist3); and the 'immobilized (as in the immobilized group) and stretched every 7 days' group (ISt7). All soleus muscles were excised 21 days after the beginning of the experiment, and were processed for (1) haematoxylin and eosin and myosin ATPase to evaluate muscle morphology and cross-sectional area and the proportions of the different fibre types, and (2) ultrastructural analysis. The cross-sectional area was found to have decreased in all fibre types (I, II and C), mainly in ISt7, when compared with the C group and ISt3 group. The proportion of the different fibre types did not show statistical difference between groups. Light and electron microscopy examination revealed signs of cell degeneration that was more intense in the group immobilized and stretched three times a week. In conclusion, sessions of passive stretching applied to the soleus during immobilization induce muscle fibre injury, suggesting that this therapeutic tool should be applied carefully to disused muscles.

Figures

Fig. 1

Effect of stretch on muscle fibre morphology of rat soleus muscle. (A–C) Cross-sections stained with haematoxylin and eosin. In panel A(group I) note fibres with lesion-like central core (arrow) and rarefaction areas beneath the sarcolemma (arrowhead) (scale bar = 31 µm). Panel B(ISt3 group) shows fibres of small size, rarefaction areas in the cytoplasm (arrow) and enhanced perimysial connective tissue (asterisks) (scale bar = 45 µm). Panel C (ISt7 group) shows discrete enhancement of perimysial connective tissue and increased capillary density in the perimysium (arrow) (scale bar = 18 µm). (D–F) Ultrastructure of rat soleus muscle. Note in panel D (I group) sarcomere disruption associated with degeneration of the myofilaments (arrow), loss of Z-line (arrowhead) and swollen and degenerated mitochondria (*) (scale bar = 2.6 µm). In E (ISt3 group) note streaming of Z-line (arrowhead), rupture of the myofilaments (arrow), lesion-like central core (*), and swollen and degenerated mitochondria (scale bar = 2.1 µm). Panel F (ISt7 group) shows longitudinal disorganization of the Z-line and some amorphous and electron-lucent spaces beneath the sarcolemma (*) (scale bar = 1.2 µm).