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.

This study investigated the effects of exercise intervention with a 12-week slightly-weighted shoe on lower-limb skeletal muscle and gait patterns in the elderly. A total of 29 healthy elderly Japanese who had irregular walking habits were randomly assigned to either slightly-weighted-shoe (WS group, n = 14; Age, 70.6 ± 5.7 years; WS, 493 g) or normal-shoe (NS group, n = 15; Age, 69.3 ± 6.9 years; NS, 293 g) intervention groups. The participants were instructed to maintain their normal daily physical activity (PA) during the intervention period. Segmental intracellular water (ICW) and muscle thickness (MT) were measured as an index of skeletal muscle mass in the lower limb, and kinematic gait data were acquired by motion analysis. Walking stability was assessed as a standard deviation of the vertical fluctuation in whole-body center of mass (COM fluctuation). The daily PA was monitored using an accelerometer and an activity record. ICW in the upper leg and MT of rectus femoris increased significantly in the WS group compared with the NS group (ICW: 13.8% vs. 2.2%, MT: 12.1% vs. 1.3%), while COM fluctuation was significantly reduced in the WS group (p<0.05) during normal walking. The present study demonstrated that interventions with a slightly-weighted-shoe may be able to increase muscle volume in the upper leg and change gait patterns in the healthy elderly.