Effects of hypoxia exposure on skeletal muscle in vivo / 体力科学

Hypoxic condition of skeletal muscle is caused not only by hypoxia exposure but also by exercise and disease etc. It is thought that clarifying a mechanism of response to hypoxia in vivo is useful for developing better training methods and treatment strategies. However, at present, research dealing with the effects of hypoxic exposure on skeletal muscle have not shown consistent results. Hypoxic exposure results in angiogenesis or muscle atrophy as morphological changes in skeletal muscle. Applications of hypoxic exposure include intermittent hypoxic exposure and hypoxic training, both of which may lead to angiogenesis in a mechanism different from normal hypoxic exposure. In this review, we present some findings on the effects of hypoxia exposure on skeletal muscle and discuss whether satellite cells are involved in promoting angiogenesis by hypoxia.

The effect of icing treatment on recovery process of damaged muscle in the rat / 体力科学

<p>Although icing treatment has been well accepted as aftercare in sports fields, the detailed mechanisms of the treatment is not fully understood. In this study, we investigated the effect of icing treatment on the recovery process of rat plantaris muscles with artificially induced muscle damage. Sixty male Wistar rats (8-weeks-old) were randomly assigned to three groups; control (CTL), bupivacaine-injected (BPVC), and icing treatment after BPVC (ICE). Icing treatment was applied for 20 min immediately after BPVC, and the treatment was used once per day for 3 days. The plantaris muscles were removed at 3, 7, 15, and 28 days after the muscle damage, then immunohistochemical and real time RT-PCR analysis were performed. In histochemical analysis, although significant changes were found in the relative muscle weight, cross-sectional area of muscle fiber, percentage of muscle fiber with central nuclei, and expressed immature myosin heavy chain isoforms after muscle damage, as compared to the CTL group, no differences were found between BPVC and ICE groups. In mRNA expression analysis, the ICE group had a significantly lower value of MyoD than the BPVC group at 3 days after the damage. Expression of IL-6 mRNA, which relates to muscle inflammation, indicated significantly higher value in BPVC, but not in ICE, than CTL groups at 7days after the damage. Furthermore, BKB₂ receptor, which relates to acute muscle soreness, indicated a significantly higher expression in BPVC than ICE groups at 3 days after the damage. These results suggest that icing treatment is effective to suppress muscle inflammation and soreness at an early stage of recovery from damage, but not effective for muscle regeneration at a later stage.</p>

Training and Detraining Effects on Satellite Cell Response after Exhaustive Exercise in Thoroughbred Horses / 体力科学

We investigate the effects of training and detraining on the satellite cell activation in thoroughbred horse muscles after an exhaustive exercise. Six horses were subjected to conventional training for 18 weeks and detraining for 6 weeks. Before training (Pre), after 10-week and 18-week training (10Tr and 18Tr), and after 6-week detraining (DTr), an incremental exercise test (IET) was performed on inclined treadmill to measure VO₂max and the velocity at a plasma lactate of 4 mmol/l (VLA4). Biopsy samples from gluteus medius muscle was obtained before and at 1 minute (1min), 3 hour (3hr), 6 hour (6hr) and 1 day after each IET. Number of muscle satellite cell were counted in type identified muscle fibers by immuno-histochemical stain images. The levels of mRNA expressions were determined using real time RT-PCR system. The number of satellite cells in 10Tr was significantly higher in type IIa fibers (0.31±0.10) than Pre (0.15±0.06). As compared to each value before IET, IL-6 mRNA expression (fold change) increased remarkably at 6hr after IET in 10Tr (x 2290.2) and 18Tr (x 2304.2), but not in both Pre (x 260.0) and DTr (x 853.3). IGF-I and Myogenin mRNA expressions were significantly increased at 1 day after IET in 18Tr (x 6.6 and x 3.3), but not in both Pre and DTr. These results suggested that the increased reactivity of satellite cells by training for 18 weeks is almost disappeared after detraining for 6 week, as well as VO₂max and VLA4.

Plasticity of skeletal muscle and variability of myonuclear domain / 体力科学

Skeletal muscle fiber has a great ability to hypertrophy during growth and in response to exercise stimuli, and atrophy during aging and in response to disuse. Because the muscle fiber is a multinucleated cell, the region of cytoplasm governed by a single myonucleus (myonuclear domain; MND) is a very important factor for understanding muscle plasticity. Although the MND size varies with fiber type, metabolic property and species, it was considered that the size was maintained constantly during muscle adaptation. Recently, however, there have been many studies demonstrating the variability of the MND size. In some of these studies, it is hypothesized that muscle hypertrophy is achieved by increase in protein synthesis rate until the ‘ceiling’ of MND size and subsequent addition of myonuclei by satellite cell activation. On the other hand, during muscle atrophy, the myonuclei seems to be long lasting as ‘muscle memory’ storing information about previous size to prepare for recovery of muscle fiber. Understanding the variability of MND size more deeply would provide fundamental insights into the mechanism of skeletal muscle plasticity.

MUSCLE ADAPTATION TO TRAINING IN THOROUGHBRED HORSES / 体力科学

The thoroughbred horse is a remarkable animal, with both speed and endurance abilities during running. Muscle adaptation associated with training and growth has been examined using various physiological, histochemical and biochemical measurements. We have emphasized the importance of type IIX fiber recruitment in training program for thoroughbred. Because type IIX fiber provides the greatest contribution to total muscle volume in the gluteus medius muscle and increases in oxidative enzymes of the fibers would markedly improve the whole-muscle oxidative capacity. Based on these concepts, we examined the adaptations of type IIX fiber including hybrid fiber to various training programs.It is considered that scientific information about optimal training of these excellent abilities can provide significant insights into exercise physiology and health science not only for thoroughbred horses, but also for human beings.

FUNCTIONAL AND METABOLIC ADAPTATION OF THE RAT DIAPHRAGM TO AGING AND COMPENSATORY INCREASED ACTIVITY / 体力科学

We examined age-related changes in functional, morphological and metabolic adaptations of the rat diaphragm muscle to compensatory increased activity. Forty-two male Wistar rats (6, 48 and 100 weeks after birth) were randomly divided into control (CTL) and compensatory activation (CAC) groups (n=7 in each group). In the CAC group, denervation surgery in the hemidiaphragm was performed to induce compensatory activation on the contralateral hemi-diaphragm muscle. Four weeks later (10 weeks, 1 year and 2 years after birth), isometric contraction properties by in-vitro physiological measurement, fiber type composition and metabolic properties (succinic dehydrogenase (SDH), 3-hydroxyacyl-CoA dehydrogenase (HAD) and phosphofructkinase (PFK)) were analyzed by histochemical and biochemical procedures.In the CTL groups, significant prolongations in the contraction and half-relaxation times were found in 2-year group, as compared to 10-week group. This age-related changes were attributed to changes in muscle fiber type composition; selective hypertrophy in Slow-twitch Oxidative fiber. In 2-year CAC group, as compared to age matched CTL group, significant improves were found in the half-relaxation time, SDH activities in each fiber type, HAD and PFK activities of the diaphragm muscle.These results demonstrated that the aged diaphragm muscle has a tendency toward slow muscle in functional and morphological properties, and that the muscle has a high adaptability to compensatory increased activity, at least in metabolic properties.

The effects of strength training on muscle fiber conduction velocity of surface action potential / 体力科学

We investigated the effects of strength training a muscle fiber conduction velocity in biceps brachii of 7 male students. The subjects were trained to exhaustion by 60% of maximum isotonic voluntary contraction with 3 sets/day, 3 days/week for 16 weeks. The muscle fiber conduction velocity was measured with a surface electorode array placed along the muscle fibers, and calculated from the time delay between 2 myoelectric signals recorded during a maximal voluntary contraction. Upper arm girth significantly increased (p<0.01), from 29.2±1.4 cm (means±S. D.) to 30.6±1.5 cm. On the other hand, training induced no significant changes in upper arm skinfold. A significant difference between pre- and post-training was found in maximum isotonic strength (p<0.01) . Although maximum isometric strength showed no significant changes with training, there was a tendency for an increase in maximum isometric strength. Muscle fiber conduction velocity increased by 3.5% during training period, but this was not significant. These results suggest no effects of strength training on muscle fiber conduction velocity.

STUDY ON OPTIMAL STARTING PERIOD OF POWER SPRINT TRAINING DURING GROWTH IN RATS / 体力科学

The purpose of this study was to investigate the optimal starting period of muscle power training during growth. Thirty-three Wistar strain male rats were used and classified into four groups, control group, 3T group started a training at 3 weeks after birth (weaning period), 8T group started a training at 8 weeks after birth (puberty period) and 13T group started a training at 13 weeks after birth (beginning of mature period) . Each training group was subjected to a power sprint training till 23 weeks after birth (mature period) .<BR>The results were summarlized as follows;<BR>1) Hypertrophy and increase of contractile properties in muscle were not seen in each training group.<BR>2) Muscle glycogen contents increased significantly in 3T group (p<0.05) and in 8T, 13T group (p<0.01) .<BR>3) Muscle fiber area increased significantly in 8T group only.<BR>4) Phosphofructokinase activity in muscle increased significantly in 8T group only.<BR>These results suggest that 8 weeks after birth, the puberty period may be the most effective starting period of power sprint training during growth in rats.

Muscle fiber conduction velocity in human vastus lateralis during isometric contractions. Relation to muscle fiber composition / 体力科学

Relationship between muscle fiber conduction velocity and muscle fiber composition was studied in vastus lateralis of well-trained athlete students, who differed considerably regarding their fiber type distribution with averaged 69.2% fast twitch (FT) muscle fibers for sprinters (12 male subjects) and 39.7% FT muscle fibers for distance runners (7 male subjects) . Measurement of muscle fiber conduction velocity during brief and maintained isometric contraction was made directly, using the zero-crossing time delay method between two surface myoelectric signals. Muscle fiber conduction velocities increased 4.40 to 4.84 m/sec for sprinters and 3.91 to 4.31 m/sec for distance runners as developing force increased from 25 to 100% maximum voluntary contraction (MVC) . The correlation coefficients between conduction velocity and % FT fibers were 0.59, 0.63, 0.64 and 0.84 for 25, 50, 75 and 100% MVC, respectively. During maintained at 50% of MVC conduction velocities in sprinters decreased linealy from 4.77 to 4.38 m/sec and in distance runners decreased from 4.11 to 3.96 m/sec at only last stage. It was suggested that the individual difference in conduction velocity was caused by different muscle fiber composition and that the changes of conduction velocity depended on the recruitment of muscle fibers.