Loss of muscle strength is not only associated with loss of muscle mass, but also affected by neural factors. It is well known that facilitatory and inhibitory responses of spinal motor neurons occur with cutaneous stimulation via spinal interneurons. The purpose of this study was to examine the neural adaptations associated with low load resistance training utilizing skin cooling (SC). 10 men trained both legs and each side was randomly assigned to SC training (SC-T) and non SC training (NSC-T). Subjects performed 30 isometric ankle dorsiflexion repetitions at 35% maximum voluntary contraction (MVC) 3 times weekly for 6 weeks. The skin cooling condition was defined as when skin temperature was 25°C while repetitive resistance training was being performed. Dorsiflexor MVC significantly increased in both SC-T (n = 9) and NSC-T (n = 9) by 12.8 and 3.8%, respectively. A significant increase in root mean square of EMG (rmsEMG) was observed for 30 isometric ankle dorsiflexion repetitions in SC-T both pre- and post-training. Lower leg girths did not significantly increase post-training. Therefore, the results of this study suggest that muscle strength might increase via changes in neural activation and that SC-T may lead to greater increases in muscle strength compared with NSC-T because of improved muscle activation during resistance training with SC. Therefore, we suggest that low load resistance training with SC is an effective method to increase muscle strength.

The effect of forearm and upper limb muscles vibration during extension and flexion movement of wrist and elbow was studied in 10 normal human subjects. In first experiment, a vibratory stimulation was applied to either the flexor carpi ulnaris (FCU) or the biceps brachii (BB) muscle during simple and simultaneous extension movement about wrist and elbow. In second experiment, vibratory stimulation was applied to either the extensor carpi radialis (ECR) or the triceps brachii (TB) muscle during simple and simultaneous flexion movement about wrist and elbow. The main new findings of the present study are as follows. During simple and simultaneous extension-flexion movements of the elbow, the application of vibration to the FCU or to the ECR produced an undershoot of the target position. However, no undershoot was observed by the application of vibration to the BB or the TB during simultaneous extension-flexion movements of the wrist. From these results, it was revealed that although there are cases where the phenomenon of undershoot resulting from vibration of the wrist and elbow during simple and simultaneous movements corresponds to the type of synaptic connection from muscle spindle group Ia sensory inputs to alpha motor neurons, as identified by Cavallari & Katz (1989) and Cavallari et al. (1992), the manifestation of undershoot is influenced by differences between the movement patterns of the wrist and elbow joints, as well as the differences between simple movement and simultaneous movement.

Object : The purpose of this study was to test the response of the decline in heart rate (HR) induced by compression on the eyeball (eyeball pressure : EP) and voluntary non breath (VNB) after pedaling exercise. Methods : EP ; Nine male subjects performed exercise for 3 min in a supine position using a bicycle ergometer. Immediately after the exercise all subjects received EP for 10 seconds. After that, subjects undertook the same protocol without EP (CON-E). VNB ; Four male and two female subjects performed exercise for 5 minutes using a bicycle ergometer. Immediately after the exercise subjects received VNB for 7 seconds. After that subjects undertook the same protocol without VNB (CON-V). Results : The slope of the decline in HR recovery (HR_DS) after exercise in EP increased significantly more than that in CON-E (p<0.05). However, time constant (HR_TC) in CON-E declined faster than that in EP. Thereby, the relationship between HR_DS in EP and HR_TC in CON-E correlated (r=-0.562). The HR_DS of VNB was greater than that of EP and CON-V. However the relationship between HR_DS in VNB and HR_TC in CON-V did not correlate. Conclusion : We suggest that EP affects vagal nervous activity and VNB affects strength of baroreflex sensitivity. Therefore HR_DS of EP might evaluate vagal nervous activity.

Vigorous exercise induces a muscular oxidative DNA damage due to increased generation of reactive oxygen species (ROS). However, it is not fully understood how ROS induces muscular DNA damage after vigorous exercise. This study examined the relation between muscle damage and the muscular oxidative DNA damage in the gastrocnemius (GAS) and soleus (SOL) muscles of rats after eccentric exercise. All rats performed level (0 degree ; L) or downhill (—17 degrees ; DH) running on a treadmill for 1h at 20 m/min. In GAS, the 8-hydroxy-2'-deoxyguanosine (8-OHdG) content, an indicator of DNA damage, showed a 2-3 times increase from that in the control after DH running, but not after L running (P<0.05). The 8-OHdG content was positively correlated with lipid peroxide (TBARS) (r=0.61, P<0.01). Moreover, plasma creatine kinase (CK) activity was positively correlated with 8-OHdG content (r=0.68, P<0.05) and TBARS (r=0.64, P<0.05) after both DH and L running. The 8-OHdG content in SOL showed larger changes than that in GAS during the 1-week recovery period following DH running. These findings show that the exercise-induced muscular DNA damage is reflected in plasma CK activity. Consequently, it is suggested that changes in plasma CK activity can also be useful as an indicator of muscular DNA damage.

The present study examined the recruitment threshold of motor units (MUs) and the cold reflex activation of the cutaneous receptors at the first turning point (TP1) and the second turning point (TP2) of decreasing skin temperature. The skin temperatures of the biceps brachii were continuously reduced using a cooling chamber fixed at -10°C. TP1 and TP2 appeared at 25.5±0.5°C and 18.5±2.21°C, respectively. The data were collected at±1°C of TP1 and TP2 (TP1-B, TP1-A, TP2-B and TP2-A) . The MUs was collected during a slow ramp contraction for 3 sec to 20% maximal voluntary contraction (20%MVC) at the each measure points (TE) . The rates of decrease in skin temperature were 1.242±0.349°C min at slope-1 (TP1-B), 0.627±0.284°C rain at slope-2 (TP1-A and TP2-B), and 0.201±0.045°C/min at slope-3 (TP2-A) . The difference of the threshold force value (ΔTF= TE-control value) of LT-MUs were positive value, on the other hand, ΔF of HT-MUs were negative value at TP1-B, TP1-A, TP2-B and TP2-A. The changes of ΔTF of LT-MUs were a little at TP1-B, TP1-A, TP2-B, and increased markedly more at TP2-A than at TP1-B (p<0.05) . On the other hand, the ΔTF of HT-MUs decreased significantly more at TP1-B than at TP1-A and TP2-B (p<0.05), however, it did not significantly differ at TP2-A. These results suggested that the threshold force of HT-MUs depend on skin temperature and LT-MUs depend on decrease speed of skin temperature.

A small amount of the myotube-like structures (chained nuclei) are present in skeletal muscle independently of disease. These experiments show that the incidence of the myotube-like structure increases with the advance of age in mice. Moreover, it is shown that the DNA in the myotube-like structure is not fragmented. That is, the myotube-like structures were TUNEL (Terminal deoxynuc-leotidyl transferase (TdT) -mediated dUTP-biotin Nick-End labeling) negative. Moreover, we detected the polyadenylated mRNA around the myotube-like structure by using an in situ hybridization technique. This suggests that the myotube-like structure has transcription activity.
This report indicates that the myotube-like structure is a useful indicator of aging in skeletal muscle.

The purpose of this study was to examine the effect of the different visual and vestibular inputs on EMG response of ankle muscles during movement to absorb impact in sliding down a slope. Thereby, our research focused on the relationship between preactivation (PA) in central program and stretch reflex induced by dorsiflexion immediately after impact. The subjects were nine healthy males. Movements were conducted using a special sliding apparatus. Conditions included sliding down a lower 15°slope with eyes open (Low) and with eyes closed (Low-Closed), and a higher 20°slope with eyes open (High) . PA prior to impact indicated the co-contraction of the medial gastrocnemius (MG) and tibialis anterior (TA) . PA levels in those muscles were significantly higher during High than during Low-Closed (p<0.05) . In contrast, PA of the coleus (Sol) was low in all test conditions. After impact, however, the stretch reflex of Sol during Low-Closed was greater than other test conditions. Because muscle stretch velocities and PA levels of Sol among all test conditions remained unchanged, these results suggest that different visual inputs could change the response of stretch reflex by modulations in reflex gain.

The present studies were performed to investigate changes of reaction time and increase of muscle strength by isometric training of biceps brachii muscle. The intensity of its training was the three types of maximal voluntary contraction (MVC), 70%MVC and 50%MVC. EMG reaction time (EMG-RT) as a function of motor control in center nervous system was measured by isometric contraction on fixed elbow.
After training for 8 weeks period, integrated EMG (IEMG) and power spectorum (mean power frequency, MPF) were increased as a component of muscle strength. IEMG and MPF were increased 83.6% and 16.6% in MVC training group, and were increased 86.2% and 20.8% in 70%MVC training group, and were increased 69.2% and 13.2% in 50%MVC training group. Increase of muscle strength in MVC, 70%MVC and 50%MVC training group increased 36.8%, 42, 3% and 30.6%, respectively. Consequently, decrease of EMG-RT was markdly associated with increase of muscle strength. The heavy intensity of isometric training, MVC and 70%MVC groups, decreased the EMG-RT greater than 50%MVC training group. Therefore, the decrease of training-induced EMG-RT correlated to development of muscle strength due to stimulation by training intensity. These results suggest that the decreased EMG-RT might be due to stimulation on motor central system by muscle training of heavy intensity.

The present studies were performed to investigate changes of reaction time and increase of muscle strength by isometric training of biceps brachii muscle. The intensity of its training was the three types of maximal voluntary contraction (MVC), 70%MVC and 50%MVC. EMG reaction time (EMG-RT) as a function of motor control in center nervous system was measured by isometric contraction on fixed elbow.
After training for 8 weeks period, integrated EMG (IEMG) and power spectorum (mean power frequency, MPF) were increased as a component of muscle strength. IEMG and MPF were increased 83.6% and 16.6% in MVC training group, and were increased 86.2% and 20.8% in 70%MVC training group, and were increased 69.2% and 13.2% in 50%MVC training group. Increase of muscle strength in MVC, 70%MVC and 50%MVC training group increased 36.8%, 42, 3% and 30.6%, respectively. Consequently, decrease of EMG-RT was markdly associated with increase of muscle strength. The heavy intensity of isometric training, MVC and 70%MVC groups, decreased the EMG-RT greater than 50%MVC training group. Therefore, the decrease of training-induced EMG-RT correlated to development of muscle strength due to stimulation by training intensity. These results suggest that the decreased EMG-RT might be due to stimulation on motor central system by muscle training of heavy intensity.

Neuromuscular fatigability of different muscle groups was studied under various physiological conditions in eight healthy male subjects by means of EMG fatigue curve analysis (E/F ratio defined as the rate of rise in IEMG/force as a function of time) which has been commonly used.
Under the experimental conditions and assumptions described in the present investigation, the following conclusions could be justified on the basis of the experimental results.
1. The E/F ratio for the biceps brachii muscle was significantly greater (p<0.0005) than that of the soleus muscle at 40% of MVC, suggesting that the biceps brachii with presumably greater %FT fibers were consistantly more fatigable than their own soleus muscle.
2. The exponential growth of the E/F ratio as a function of either the maximal sustaining time (T_s) or different fractions of MVC suggested a possible neurophysiological link between the fatigability of the biceps brachii muscle and their MU activities which might increase in an accelerated fashion.
3. The evaluation of static work capacity (W_s) described in the present study revealed that the differences in the critical force level and Ws during free circulation and arterial occlusion could be explained by the relative availability of muscle blood flow which might determine the rate of energy reconstitution.
4. The EMG data taken during a constant torque output on an electrically braked bicycle ergometer indicated that some shift in the MU recruitment and/or MU firing frequency may occur during the application of arterial occlusion causing local muscle hypoxia.