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.

Disused rat hindlimb caused by sciatic neurectomy is characterized by skeletal muscle atrophy and bone loss accompanying alterations in trabecular bone architecture. We studied the time course responses of the two-dimensional architecture of trabecular bone and the relationships between bone loss and muscle atrophy using a unilateral sciatic nerve denervation model of disuse atrophy.
Fischer-344 male rats aged 11 weeks were denervated. The right and left tibiae, soleus and extensor digitorum longus muscle of denervated and control rats were obtained at 1, 3, 7, 10 days and 2, 3, 4, 5, 6, 7, 8, 10 weeks after surgery. Histomorphometric analyses were performed on longitu-dinal sections of proximal tibial metaphyseal secondary spongiosa.
Marked trabecular bone loss and muscle weight loss were observed mainly at 7-10 days and 1-3 days after denervation, respectively. Time courses of the percent decrease in trabecular width and length from 0 day (i, e., thinning rate; TR and fragmentation rate; FR) were not matched at 7-10 days and 5-6 weeks after denervation. The scatterplot for TR; FR showed that the FR changed stepwise with the first threshold between 12-25% of TR.

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.

The running behavior and muscle characteristics associated with voluntary running activity were studied in female Fischer 344 rats after 9 weeks of training in voluntary exercise wheel cages. The exercise wheel employed allowed a load to be added to the wheel axis. The running activity was recorded as the number of rotations every 10 s for 24 h, and the number of running bouts, running duration, running speed and distance run per day were varied. Addition of a load to the wheel axis allowed the running style to be changed, and reduced the running duration and speed in each bout. The voluntary running training was done with no load, 30% per body weight load, and 60% per body weight load, and as treadmill running (40 m/min, 60 min/day) .
In the voluntary training, a difference in the total running distance was observed with increased load. The total distance run under 0%, 30% and 60% load in the voluntary group was about 552, 475, and 438 km, respectively, after training for 9 weeks. As a result of training, the weights of dorsiflexor muscles tended to increase with treadmill running, whereas weights of plantar-flexor muscles increased significantly in the voluntary training groups. In the plantaris muscle, the cross-sectional area of all fiber types was not changed by treadmill training, but in voluntary training, almost every type of fiber was enlarged. Voluntary running with a 30% load increased the cross-sectional area of type I and type ha fibers in the plantaris muscle compared with no load exercise. However, no significant differences in the cross-sectional area of these fibers were observed between 30% and 60% load exercise. The fatigue tolerance of the plantaris muscle improved significantly in all training groups, and the 60% load group showed the highest value.
These results suggest that voluntary loaded running is more effective for muscle enlargement than treadmill training and/or voluntary training with no load.

Four myosin heavy chain (MHC) isoforms were detected in rat hind-limb fast muscles. MHC isoforms are particularly good candidates for fiber type-specific markers in myofibril proteins. We studied the effect of running or jumping training on MHC isoform composition in 18, 6 month-old female rats. The animals were divided into three groups : sedentary (S; n=6), running (R; n=5) and jumping (J; n=7) at 4 months of age. Animals in group R were trained with treadmill running (30 m/min, 60 min/day and 5 days/wk) for 8 weeks. Animals in group J were trained with vertical jumping (40 cm high, 100 repetitions/day, 5 days/wk) for 8 weeks.
There was no significant difference in body weight among the groups. Muscle weight and muscle weight/body weight for the plantaris were significantly increased in both trained groups, but there was no significant difference in the protein concentration. With regard to MHC isoform composition, there was no significant difference in the compositions of the type I and type ha MHC isoforms. In the type lid MHC isoform, the values for both trained groups were significantly higher than that of group S (p<0.05) . The values for both trained groups in the type lib MHC isoform were lower than that of group S. In particular, there was a significant difference between groups S and J (p<0.05) .
These results indicate that a relative increment of the type lid MHC isoform is a typical adaptation phenomenon of the olantaris muscle in rats riven runninr and iumoinr training.

The effects of testosterone on the structural and contractile properties of the denervated rat skeletal muscles that were not influenced by neurotrophic-effects were investigated. Male Wistar strain rats (3-week-old, n=28) were used in the present study. All rats were divided into four (n=7, for each) groups; control (C), testosterone control (TC), denervated (DN), and denervated with testosterone (DNT) groups. Sciatic nerves of DN and DNT groups' rats were resected at the 4-week-old after birth. Testosterone that was melted in corn-oil was administered (40 mg/kg body weight/week) for the TC and DNT groups. At the end of breeding period (15-week-old), soleus (SOL) and extensor digitorum longus (EDL) muscles were dissected from both hindlimbs and contractile profiles were analyzed in vitro. Relative weight (muscle weight/body weight) of SOL muscle in the DNT group was significantly (p<0.01) higher than that of the DN group. There is no significant difference in EDL muscle between the DN and DNT groups. Time to peak tension and half relaxation time in EDL muscle of the DNT group was significantly (p<0.05) higher than those of the DN group. Maximum twitch tension was prone to decrease following denervation, and significantly (p<0.01) difference between the DN and DNT groups in both muscles regarding the maximum twitch tension was observed. There is no significant difference of myosin and actomyosin ATPase activities comparison of each group in SOL muscle, however, the TC group were significantly higher than those of the C group in EDL muscle. These results suggested that the effects of testosterone for the hypertrophy in skeletal muscles were not synergistic effects with the neurotrophiceffects, however, for the ATPase in fast muscle act synergistically with the neurotrophic effect to change of myosin ATPase. Moreover, it was also suggested that the sensitivity to the testosterone was different in each type of fiber.

A study was conducted to investigate the influence of a series of quick stretches during tetanic stimulation of skeletal muscle fibers in growing mice.
Mice were anesthetized with pentobarbital sodium. The distal tendon of the extensor digitorum longus muscle was attached to a servomotor. The effects on the muscles of a series of isometric contractions (ISO), a series of quick stretches during contractions (ST), and a sham operation for 1 h were assessed in terms of changes in tetanic tension, serum creatine kinase and lactate dehydrogenase activities, and the histological appearance of the muscle.
The data indicated the following ; 1) Serum enzyme activities were not altered in any of the experiments. 2) ST resulted in a decrease in tetanic tension compared with ISO values in mice of all ages. The rate of decrease in tetanic tension for 1 h ST was higher in muscles of young mice than in those of mature mice. 3) Muscle histology demonstrated numerous areas of localized sarcomere widening immediately after the series of ST.
These observations suggest that muscle stretching during contraction causes greater injury to developing skeletal muscles.

The purpose of present study was to investigate the relationship between the increase of sarcomere length, sarcomere number and tetanic tension of the extensor digitorum longus muscles during postnatal growth in mice. The following results were obtained ; 1) The cross-sectional area, length of muscle and fiber length increased rapidly up to 7th week. 2) The sarcomere number increased up to 9 th week, 3) The tetanic tension per cross-sectional area decreased significantly in 7 th week, 4) The maximum value of sarcomere length which was obtained at middle region of the fiber correlated significantly with the tetanic tension per cross-sectional area (the correlation coefficient : r=-0.675) . These observations suggest that the local expansion of sarcomere length during postnatal growth affects the tension development.