The present study focused on the effect of the newly motor unit (MU) recruitment on mechanomyographic signal (MMG) by the analysis on motor unit mechanical signal (MUMS) during prolonged isometric constant contraction (PICC) at low torque levels of the knee extension. The mechanical and myoelectric signals (MES) from m. vastus medialis or lateralis were recorded by condenser microphone and disc electrode, respectively. In order to recruit the objective MU during the PICC, the target torque set at several levels below the recruitment threshold torque of the MU (≦ 7.4 %MVC). 1) iMMG and iMES sustained constant for initial several minutes and then increased during the PICC. 2) MUMS superimposed on MMG from back ground MUs activities and iMUMS increased significantly at the timing of MU recruitment. Subsequent iMUMS decreased according to the decrement of MUMS amplitude depend on the discharge trend of the MU. 3) Amplitude of MUMS (MS-V_positive) showed different trend depended the recruitment timing during PICC. At the iMMG constant phase, MS-V_positive sustained constant followed by the increment similar to iMMG trend. In contrast, at iMMG increment phase, MS-V_positive showed increment trend without the constant phase. The present results suggested that the newly MU recruitment increase the iMMG during the PICC. IMMG increment at later period of the PICC could interpret from the MU recruitment and MS-V_positive increment. It is necessary to investigate the factors to increase the MS-V_positive from the muscle and muscle fibers conditions.

The evoked force was observed during repetitive electrical stimulation for 3 min on m. vastus medialis. The stimulus frequency was 0.2 Hz, 10 Hz and 20 Hz. The time to peak of twitch was 90.8 ms at 0.2 Hz stimulation. The changes in the evoked force did not represent a constant or a monotonic pattern but were complex at 10 Hz and 20Hz stimulations. At 10 Hz the evoked force represented an initial transient increment (steep peak), then an abrupt decrement followed by a gradual increase (gentle peak) and then a gradual decrease. At 20 Hz the steep phase did not appeared. The magnitude of potentiation was not necessarily large at 20 Hz. These results suggest that a constant discharge rate of motor units cannot maintain constant force development, and “rate coding” is considered to be necessary for keeping a constant force.

In the present study, we investigated the wave form of human single motor unit potentials recorded monopolarly using a surface electrode. Each motor unit potential consists essentially of three phases. However, we found a non-conductive component in the motor unit potentials, defined as“late positive deflection”. This non-conductive component appeared in and overlapped the third positive phase of the motor unit potential and showed the following properties 1) When surface electrodes were placed on the skin surface overlying the m. vastus medialis in line with the direction of muscle fibers belonging to the observed motor unit, the peaks of the non-conducting components were synchronized with each other and their amplitude increased exponentially with the distance from the motor end plate. 2) When the action potential was conducted to the myotendinous junction, the potential spread to the tendon electrotonically. The peak of the non-conducting component was also synchronized with the electrotonic potential. 3) The amplitude of the non-conducting component increased depending on the developed tension.
These results suggest that the appearance of the non-conducting component was due to synchronization of motor unit potentials that had just arrived at the myotendinous junction with the observed motor unit potential. When the motor unit potentials arrived at the myotendinous junction, the current flow to the forward local circuit of the action potential was cut off because of the high impedance of the tendon. Therefore the forward current flow was to be flowing the backward local circuit. The number of recruited motor units increased depending upon the developed tension. When many motor units fired randomly, their volume conducted-potentials canceled each other in the region of the muscle fiber. At the myotendinous junction, however, the directions of current flow due to the action potentials elicited by many motor units coincided and intensified each other.
Therefore, it is considered that this“late positive deflection”carries information on the number of motor units activated i. e., “recruitment”.

A study was conducted to investigate the discharge pattern of single motor units during submaximal prolonged activity at the tension of the recruitment threshold, and the relationship between the discharge pattern and the conduction velocity of the motor unit action potential, which has been used as an index of muscle unit characteristics (Andreassen & Arendt-Nielsen, 1987) . The results were as follows :
1) In all motor units observed (32 units), the spike interval during prolonged activity increased in the first several minutes. However, there was some difference in the motor unit discharge pattern accoding to the degree of the initial increment in the spike interval and the discharge pattern after the initial elongation period. Therefore we divided the motor unit discharge pattern into four typical styles, i, e., 1 : units derecruited (7 units), 2 : units first derecruited and later rerecruited (9 units), 3 : units that fired continuously with gradual initial slowing (8 units), 4 : units that fired continuously with only slight initial slowing (8 units) .
2) Recruitment of the motor units appeared after 5 min of the “load” according to their recruitment thresholds.
3) In most of the motor units observed, spike intervals became shorter 15 min after the onset of the “load”, and the recruitment thresholds decreased immediately after the “load” in comparison with the value before the “load”. It was suggested that most units were gradually excited by this prolonged load.
4) Conduction velocity of the muscle fibers was in the range between 2.59 and 4.99 m⋅s^-1.
5) When the conduction velocity of single motor units was divided into four groups according to the discharge pattern, there was no difference in the conduction velocity among the four groups.
During submaximal prolonged activity, motor units showed individual discharge patterns, and their excitability was generally increased. It was concluded that the increased excitability was due to some “compensatory” mechanism for maintaining the target tension, which probably differed from the neural control mechanism during “maximal” prolonged activity.

In the present study, we attempted to reconstruct the surface myo-electric signals from monopolarly recorded motor unit action potentials (MUAPs), and to construct a method of analysis for extracting information from surface ME signals on the recruitment behavior among motor units.
1) The waveform of a single MUAP of a surface electrode recorded monopolarly consisted of three phases : first, a positive, second, a negative transient and, third, a positive phase except for the end-plate region. The appearance of each phase could be interpreted from the field potential in the volume conductor produced by conduction of action potentials from the end-plate to the myotendinous junction.
2) Waveforms of MUAPs indicated that the positive phase and the negative phase are the same in area. In surface ME signals, coincidence of the phase areas was observed. Therefore, it was inferred that all the motor units producing the interfered surface ME signal showed a tendency to coincide with respect to the area between the two phases.
3) The fact that MUAPs consisted of three phases during conduction means that the contribution potential for a recorded electrode changes according to the position of the action potential on the muscle fiber (s) . Therefore, the potential of a surface myoelectric signal represents the sum of the contributed potentials from activated motor units.
4) The amplitude of surface ME potentials tended to be other than 0V as tension increased.
5) Considering the reconstruction of a surface ME signal involving many activated motor units from the contribution potential, the surface ME potential depends on the number of recruited motor units with a different waveform, in addition to the magnitude of synchronization and grouping discharge among motor units.

When monopolarly-recorded surface myoelectric signals from the vastus medialis muscle were analyzed by FFT (Fast Fourier Transform) to obtain an amplitude spectrum, we found a“dip”in amplitude of around 80 Hz. Using the frequency at the“dip” (Turning Frequency : TF) as a cut-off point, the raw myo-electric signals were divided into high frequency components (HFC) and low frequency components (LFC) .
1) TF appeared in the amplitude spectrum from all recording electrodes when the tension reached 60%MVC.
2) TFs shifted to higher frequencies with increased tension.
3) Using cross-correlation analysis, LFCs showed conductivity along muscle fibers. In contrast, a pair of HFCs from nearly fastened electrodes appeared in phase, but a pair of HFCs from distant electrodes showed no relation.
HFCs and LFCs, derived by using TFs from surface myoelectric signals, showed different characteristics in conductivity. This suggests that TF can be a useful analyzing method extracting new information from surface myoelectric signals. Further study is needed to pursue the relation between HFCs and LFCs, and physiological parameters.

We applied Fast Fourier Transform to monopolarily recorded surface myoelectric signals from m. biceps brachii to obtain the turning frequency (TF) at which the signals divide into high and low frequency components. Then using the TF as a cut-off point, the raw myoelectric signals were divided into high frequency component (HFC) and low frequency component (LFC) .
1) TF values were constant at every recording position along muscle fibers. And there was no considerable relationship between developed tensions and TF values.
2) Cross-correlation analysis was done to obtain the phase relationship between LFCs, and between HFCs at four different recording positions along muscle fibers. LFCs appeared in phase in all combinations below the tension of 10%MVC and also in HFCs. Above 20%MVC, LFCs represented the time delay depending upon the electrode distance in six subjects. The conduction velocity calculated from the relationship between time delay and distance in LFC was too high (16.1 m·s^-1-33.3 m·s^-1) compared with the muscle fiber and/or motor unit conduction velocity. But LFCs in other subjects remained in phase. HFCs showed no considerable relationship above 20%MVC in six subjects, while other subjects remained in phase between HFCs.
The present results differ from the recent investigation in m. vastus medialis (Kamo & Morimoto, 2000) . Our proposal on the construction mechanism of surface myoelectric signals could not be adapted to the electrical signals from m.biceps brachii. The present results suggest the different in architecture of muscle fibers and the innervation of the motor nerve in a motor unit between two muscles.

In this study, we investigated the applicability of the analyzing method of surface myoelectric signal proposed by Kamo & Morimoto (2000) for children's (age groups: Grade 2, 4, and 6 of elementary school) monopolarily recorded surface mvoelectric signal from m. vastus medialis. The subjects (n=16) were requested to exert constant brief (10 sec) isometric knee extension at low tension (4-12%MVC) .
Next, to elucidate the age dependent change of the possible motor control mechanisms, the change of the contribution of the integrated value of low (LFC) and high frequency component (HFC) to integrated value of raw signal was tested. Index of the contribution was considered from the slope of the regression line between raw and LFC integrated values, and between raw and HFC integrated values.
Obtained results were as follows:
1. Turing frequency (TF) appeared in the children's amplitude spectrum of surface mvoelectric signals. And TF shifted toward higher frequency depending upon the developed tension.
2. For children, cross-correlation analysis characterized LFC as conducting, and HFC as non-conducting characteristics.
3. There was no significant difference in integrated value of raw signal between age groups at each target tension. Results were similar concerning LFC and HFC as well.
4. The comparison of regression slopes showed that HFC had significantly lower contribution to construct the raw signal than LFC. Furthermore, results showed no age related difference as far as the contribution of the LFC, HFC to the raw signal concern.
Results 1 and 2 were in good agreement with the results obtainend from university-students (n=5) . Therefore, it suggests that the analyzing method proposed by Kamo & Morimoto (2000) . can adapt to the mvoelectric signal from children.
Similar result between the age groups suggests that under low-tension brief contraction motor unit control mechanism showed no developmental stage difference from 2 nd grade to university student, yet muscular strength of the 2 nd grade did not reach to the university student level.

The aim of the present study was to investigate, by analysis of motor unit action potential (MUAP) and motor unit mechanomyogram (MUMS) wave-forms, whether the synchronized activity of motor units (MUs) is a factor in increasing the integrated value of a mechanomyogram during muscle contraction at relatively low tension levels. MUAP and MUMS of m. vastus medialis were recorded by Ag/AgCl disc electrode ( 5mmφ) and condenser microphone ( 10mmφ), respectively, during muscle contractions, brief isometric constant contractions (BICC) and prolonged isometric constant contraction (PICC) at the target torques from just above the decruitment threshold torque of the objective MU to 20% of maximal voluntary contraction (MVC). The degree of synchronization of MUs, defined from the amplitude of late positive deflection (VLPD), could be seen in MUAP wave-forms.The amplitude of the positive phase in MUMS (MS-V_positive) had no relationship with the increase of V_LPD in BICC condition. During PICC, MS-V_positive and V_LPD increased with time. Applying linear regression analysis on the relation between V_LPD and MS-V_positive, except for data at 20%MVC, there was significant correlation. However, the scale of the time increments, between V_LPD and MS-V_positive, were different comparing exponential and logarithmic figures, respectively. Therefore, in the present experiment, the meaningful relationship between the two parameters could not be introduced. It is necessary to further investigate the relationship between the two parameters including the firing frequency of MU, intramuscular pressure and extent of recording area of both sensors.