Markedly smaller myofibers including branched myofibers (SF) were contained in regenerated skeletal muscles. However, its functional properties are not yet sufficiently understood. The present study examined the metabolic, contractile properties and hypertrophy of SF included in rat skeletal plantaris muscles regenerated from eccentric contraction-induced muscle injury. Succinate dehydrogenase and alpha-glycerophosphate dehydrogenase activity of SF were respectively similar to that of other normal size myofibers (NF) within regenerated muscles. The shortening velocity of skinned fibers prepared from branched myofibers isolated from regenerated muscles was slightly slower than that of control muscles. In addition, the extent of glycogen depletion in SF was similar to that of NF after exhaustive running. Therefore, SF were innervated by motoneurons and recruited during the contractile activity of the skeletal muscle. No matter when the regenerated muscle was loaded by synergistic ablation, the continued existence of SF was observed. Therefore, the ability of hypertrophy in SF may be distinct from that in NF. The physiological properties of SF were similar to that of normal myofiber, except for the ability to regulate hypertrophy.

The contractile and morphological effects of chronic overload induced by synergist gastrocne mius and soleus ablation were studied in regenerating plantaris muscle of rats after injuries produced by eccentric exercise. Rats were randomly divided into control (C), eccentric exercise (E) synergistic ablation (S) and eccentric exercise+synergistic ablation (ES) groups. At 6 weeks after the experimental treatment, contractile or morphological muscle properties were measured. The maximum tetanic tension of ES group was significantly larger than that of S group (p<0.05), although there was not significant difference between C and E groups. There was not significant difference in contraction time between S group and ES group. While there was not significant difference in mean cross-sectional area of any fiber types between C group and E group, all fiber types of ES group were significantly smaller than that of S group (p<0.05) . It is concluded that the overload on injured muscle by eccentric exercise may restrain muscle hypertrophy.