A study was undertaken to investigate whether the amplitudes of reflex EMG components (M1, M2 and M3) induced by sudden muscle stretching in the wrist flexor, are modified according to the reaction movement during wrist flexion, and how the aspects of reflex EMG components are related to premotor time under conditions of stretch stimulus (SS-PMT) and light stimulus (LS-PMT) . Fifteen healthy men, ranging in age from 22 to 28yr, participated in the study. A DC torque motor was used to evoke the reflex EMG activities on the wrist flexor. Analysis of the surface electromyogram recorded from the wrist flexor showed that short and long latency reflex components appeared in response to muscle stretch. In almost subjects, the amplitude of the M2 component was higher during reaction task than during no reaction task. The subjects were classified into two groups (I, II) according to the presence or absence of reflex components and their EMG amplitudes. SS- and LS-PMTs in group I were significantly shorter than those in group II. The amplitude of M2 in group I was significantly higher than in group II. In group I the M3 component was not identified, since the M2 component was followed by a voluntary EMG burst.
These results showed that the amplitudes of long latency reflex components increased during the reaction task in the flexion direction, and suggest that long latency components contribute to the initiation of voluntary movement in subjects with a shorter premotor time.

A study was conducted to investigate the relationship between long latency activities evoked by a sudden muscle stretch of the right wrist flexor and a peformance during a target match task accompanying wrist movement. The index of performance was the target match time (TMT) from onset of a handle displacement untill the handle position output was matched with a traget line. The standard deviation of TMT (TMT-S.D.) was calculated. Sixteen healthy men, ranging in age from 20 to 27 yr., participated in the study. A DC torque motor was used to generate an angular displacement at the right wrist joint. Analysis of the surface electromyogram recorded from the wrist flexor showed that the short and long latency reflex components (Ml, M 2 and M3) were evoked by muscle stretching. In the target match task, the amplitude of the M 2 and M 3 components for all subjects were significantly smaller than the M 1 component (p<0.01) . In some subjects the M 3 component was not identifiable, since its amplitude was very small in the target match movement. There were significant correlations between the M 2 amplitude and performances (TMT : r=0.857, p<0.01 and TMT-S. D. ; r=0.651, p<0.01) during the target match task. In shoat, the smaller the M 2 amplitude was, the shorter the TMT became and the smaller the TMT-S. D, became. These results suggest that the change in the M 2 component is related to motor control for the target match task.

The relationships were examined between the relative force level and exertion strategy during rapid isometric contractions. Thirteen subjects exerted isometric pinchings as fast as possible at various force levels. Force curves and surface EMGs from the first dorsal interosseous muscle were obtained during the contractions. While the single-peaked force curves remained analogous up to a certain force level, they changed to multi-peaked ones at higher force levels. The critical force level, which could be determined in all subjects as relative force level to MVC, was positively correlated with the %MVC EMG magnitude for unit time, obtained below the critical level. EMG durations were significantly shorter blow than above the critical level. These results suggest that different exertion strategies are utilized in rapid contractions at different force levels, and that individual differences in the critical force level are related to difference in the manner of motor unit activation.

The purpose of this study is to examine the validity of muscle fatigue evaluation using maximum voluntary torque (MVT), and to identify the dependence of individual's tolerance for fatigue on the capacity to exert MVT. In 14 young male subjects (10 regular exercisers and 4 sedentary), MVT was measured during isometric knee extension, and voluntary activation (VA), which reflects motor unit activation, was evaluated using the twitch interpolation technique. In addition, the maximum endurance time (ET) was measured, and behavior of the mean power frequency (MPF) and the average rectified value (ARV) of surface EMGs from the vastus lateralis muscle were analyzed during constant force isometric contractions of 60% MVT (short-duration fatigue task; SDF task) and 20% MVT (long-duration fatigue task; LDF task) . Correlations were examined among these five variables.
The results were as follows:
1) Subjects were divided into a high voluntary activation group (HVA group) and a low voluntary activation group (LVA group) . Four sedentary subjects were included in the latter group.
2) MVT was significantly larger in the HVA group than in the LVA group (p<0.01) . A significant positive correlation (r=0.72) was found between MVT and VA (p<0.01) .
3) A significant negative correlation (r=-0.71) was found between MVT and endurance time (ET) for the LDF task (p<0.01) . The ET was significantly longer in the LVA group than in the HVA group (p<0.01) .
4) The MPF of voluntary EMG decreased consistently, as ARV increased during isometric contraction in both tasks (p<0.01), indicating the development of fatigue in the muscle. The final change of MPF relative to the initial value was significantly greater in the SDF task than in the LDF task (p<0.05) .
5) A significant correlation (r=-0.83) was seen between the relative change in MPF and ARV in the SDF task (p<0.01) .
6) For the SDF task, the final change of MPF and ARV relative to the initial value was significantly greater in the LVA group than in the HVA group (p<0.05) .
These results indicate that tolerance for local muscle fatigue usually evaluated as maximum endurance time, may depend on individual differences in VA, the VA, in turn, depending on adapta-tion to exercise, and that there appears to exist a corresponding adaptative strategy of the neuromuscular system during fatiguing contractions. Usefulness of our procedure using the twitch interpolation technique in evaluating muscle fatigue was also suggested.

The purpose of this study was to compare the motor control against the passive force between an ascend phase (AP) and a descend phase (DP), and to investigate the effect of a subject's athletic experience on it. Thirty-four subjects participated in the experiment, and they were classified into an athlete group and a control group. We used a kinetic-equilibrating (K-E) task. Result of K-E task depends on the kinesthesia, because subjects are required to maintain their balance against a passive force under conditions of limited visual and aural feedbacks. Therefore, subjects were instructed to resist the passive force. Their performance was evaluated using parameters such as absolute error, position fluctuation, and variable error that were calculated from position data. Significantly higher values on DP than AP for each parameter in the control group were found. However, the values of both AP and DP were similar in the athlete group. It was suggested that the athlete group could perform the same level of motor control against passive force between AP and DP, although it was more difficult in DP than in AP for the control group.

Electromyographic muscle activities of the shoulder muscles during shoulder external rotation with reference to load magnitude were studied in 8 healthy male subjects without history of shoulder injuries. In addition, we discussed the relationship between rotator cuff muscles and superficial muscles. The subjects performed shoulder external rotation from 60°internal rotation to 45° external rotation at 20° of shoulder abduction and 20° of flexion. A Cybex dynamometer used to prescribe angle and velocity of the shoulder movement. At the same time, intramuscular wire electrodes were inserted into the supraspinous, infraspinous and teres minor muscles, and then surface electrodes were placed over the posterior deltoid and middle trapezius muscles. The load magnitude ranged 2-18 Nm and the angle velocity was set at 15 deg/sec. The rotator cuff muscles became significantly to be activated from 2 Nm (the supraspinous and infraspinous m.) and from 4-5 Nm (the teres minor m.) compared with the muscle activity during non-load. The superficial muscles became significantly to be activated from 4-7 Nm (the posterior deltoid m.) and from 3-6 Nm (the middle trapezius m.) compared with the muscle activity during non-load. Within the load range (18 Nm 46.8% MVC), %iEMG of the rotator cuff muscles was always larger than that of the superficial muscles. Therefore, we concluded that the contribution of the rotator cuff muscles was larger than that of the superficial muscles up to 3-4 Nm, and activities of the superficial muscles increased gradually from 3-4 Nm during shoulder external rotation.

We studied the mechanism for slowing surface electromyography (EMG) during fatiguing contraction using superimposed M-wave analysis. Seven healthy male subjects exerted 60% maximum voluntary contraction of isometric abductions in the left first dorsal interosseous muscle (FDI) until exhaustion. Simultaneously with voluntary contractions, the ulnar nerve was electrically stimulated at supramaximal intensity, and volitional EMG and superimposed M-waves were obtained. We examined the behavior of muscle fiber conduction velocity (MFCV) and median frequency (MDF) for both EMG, with the following results:
1) MFCV calculated from volitional EMG of FDI was about 6 m/s during 60% MVC.
2) The waveform of voluntary EMG detected from FDI slowed in all subjects during fatiguing contraction at 60% MVC, indicating fatigue had developed in the muscle.
3) As fatigue progressed, the waveform of the superimposed M-wave tended to decrease in amplitude and increase in duration.
4) As fatigue progressed, MDF and MFCV in volitional EMG decreased significantly (p<0.04) . The rate of change was larger in MDF than in MFCV (p<0.01) .
5) As fatigue progressed, MDF and MFCV in the superimposed M-wave decreased significantly (p<0.01) . The rate of change was larger in NIDF than in MFCV (p<0.05) .
These results suggested that MFCV and other peripheral factors affected the slowing of volitional EMG. Elongation of the depolarization zone in muscle fiber is proposed as a peripheral factor.

Electromyographic activity of the shoulder muscle at 20 and 90°abduction (20 Abd, 90 Abd) during external rotation was investigated in seven healthy men with no history of injury or instability of the shoulder joint.
Electromyography (EMG) was recorded using intramuscular fine-wire electrodes inserted into the M. Supraspinatus, M. Infraspinatus and M. Teres minor, and with bipolar surface electrodes on the middle and posterior parts of M. Deltoid anti the upper and middle parts of M. Trapezius. To compare activity in different muscles, the integrated EMG (iEMG ) activity of each muscle was normalized.
M. Infraspinatus and M. Teres minor showed significantly higher activity at both the 20 Abd and 90Abd compared with the middle and posterior parts of M. Deltoid and upper parts of M. Trapezius. M. Supraspinatus, the middle and posterior parts of M. Deltoid, and upper and middle parts of M. Trapezius all showed a difference in activity level between the two positions.
These findings suggest that when M. Infraspinatus and M. Teres minor contribute to external rotation as a stabilizer and prime mover, consecutively, M. Supraspinatus, the middle and posterior parts of M. Deltoid, and upper and middle parts of M. Trapezius function according to the positions. Moreover, the activity of the upper and middle parts of M. Trapezius in 90Abd should influence stabilization, adduction and upward rotation of the scapula. Therefore, we conclude that the external rotation position is closely related to shoulder muscle activity and coordination.

The purpose of this study was to examine the effects of short-term immobilization on the maximum voluntary contraction (MVC) force. The first dorsal interosseus (FDI) of 10 healthy male adults was immobilized for 1 week using casting tape. Atrophy of the muscle was estimated from a cross sectional view of magnetic resonance images (MRI) . To clarify the factors of a peripheral neuromuscular system contributing to the change in the MVC force, twitch force at rest was measured. The contribution of central factors was estimated from a voluntary activation (VA) index, which was obtained by the twitch interpolation method.
The MRI showed no significant changes in the cross sectional area. The MVC force declined after immobilization (p<0.01), and recovered after 1 week from the termination of immobilization (p<0.01) . Both the twitch force at rest and the VA at MVC declined after immobilization (p<0.01), and recovered after 1 week (p<0.05) .
The results indicate that the temporary decline of the MVC force was not accompanied by atrophy of the muscle. Furthermore the decline of the MVC was caused both by the deterioration of peripheral and central functions in the neuromuscular system. Possible factors in the peripheral and central neuromuscular systems affected by the immobilization were discussed.

We evaluated motor unit (MU) fatigue in the first dorsal interosseous muscle (FDI) using the collision principle. Eight healthy men exerted 70% (short-duration fatigue task: SDF task) and 30% (long-duration fatigue task: LDF task) maximum voluntary contraction of isometric abductions in the left FDI until exhausted. Before and after voluntary contractions, the ulnar nerve was stimulated at the wrist and elbow with supramaximal intensity, and a pair of M-waves was obtained. Fatiguerelated changes were studied in mean power frequency (MPF), averaged rectified value (ARV) calculated from surface EMG, and motor nerve conduction velocity (MCV) and distribution of motor nerve conduction velocity (DMCV) calculated from M-waves. The MPF of voluntary EMG decreased, whereas ARV increased significantly during SDF and LDF tasks, indicating fatigue had developed in the FDI. Endurance was significantly shorter in the SDF task than in the LDF task (p<0.01), whereas differences between tasks were not seen in MPF and ARV changes. Tasks did not affect MCV, but lower components in DMCV increased for both tasks. Increased lower components were larger in the LDF task than in the SDF task. The shift in DMCV indicated that fatigued MUs stopped activity and enduring MUs, which had lower axon conduction velocity, were activated selectively. These results suggest that the collision principle is applicable in evaluating motor unit fatigability.