To obtain one of the possible factors on the abrupt increment of mechanomyographic (MMG) signal during prolonged isometric constant contraction (PICC; <20%MVC), the present study focused on the iMMG changes from m. vastus medialis (VM) and/or lateralis (VL) during the intermittent isometric constant contractions (IICC) at various condition of the duty cycle (or relaxation/contraction ratio: R-C ratio) and the target tension. Target tension set at 5, 10 and 15 %MVC and the duty cycle set at 16s-ON/4s-OFF, 56s-ON/4s-OFF and 116s-ON/4s-OFF. Simultaneous recording of myoelectrical signal (MES) was made on VM and/or VL. 1) IMMG increased weakly and monotonously with the cumulative load which is accumulated the acreage of tension and time through the IICC. Also, iMES sustained or increased weakly but the increment ratio was lower than iMES under PICC condition. 2) The increment ratio of iMMG (or gradient of linear regression formula with the cumulative load: G-iMMG) from both muscles increased depend on the increment ratio of the cumulative load (or gradient of linear regression formula with the time) in the stored data under the IICC conditions. But there could not be seen the distinct relation in that of iMES. 3) The G-iMMG decreased with R-C ratio. But G-iMES did not show distinct relationship with R-C ratio among subjects and between muscles. Linear incremental tendency of iMMG would originate from the inserted muscle relaxation during IICC. Therefore, the abrupt increase of iMMG during PICC suggests to be caused from the continuous muscle contraction itself.

To investigate factors that increase the mechanomyographic (MMG) signal during voluntary prolonged muscle contraction at relatively low tension levels, the MMG signal from the motor unit (MU mechanical signal: MUMS) was analyzed. In the present study, the author focused on the interval dependency of MUMS amplitude as one of the factors to increase MMG. From the m. vastus medialis or m. vastus lateralis, MUAP and myoelectrical signal (MES) were recorded by Ag/AgCl disc electrode and MMG and MUMS were respectively recorded by condenser microphone during various types of muscle contractions; brief isometric constant contraction (BICC), prolonged isometric constant contraction (PICC) and prolonged isometric contraction under constant MU firing interval (PIC-CFI).The amplitude of positive phase in MUMS (MS-V_positive) increased proportionally with the firing interval of the MU in BICC. The firing interval of MU showed an initial elongation, followed by shortening during PICC. During PICC and PIC-CFI, MS-V_positive sustained in an initial part of the contraction and then abruptly increased. There was no meaningful relationship between the firing interval and MS-V_positive during PICC and PIC-CFI. However, the increment ratio of the MS-V_positive in PIC-CFI was smaller than that in PICC.The present results on MS-V_positive suggested that the increment of MMG during PICC did not depend on the firing interval change in the activated MUs. Based on findings with different increment ratios in MS-V_positive between PICC and PIC-CFI, it is suggested that the hysteresis of muscle contraction is one of the factors causing an increase in MUMS and MMG.

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.

Manner of muscular relaxation in elbow flexion was observed on the subjects aged from 7 to 22 years old. Relation between surface electromyogram of m, biceps brachii and mechanogram of elbow flexion were used for the parameters of muscular relaxation.
Results were as follows: 1) At elder subjects, silence of surface electromyogram preceeded to the tension decrease. But discharge of surface electromyogram preceeded during muscular relaxation at younger subjects. 2) It could be observed the transient tension increase before muscular relaxation. The appearance rate of the transient tension increment increased as becoming elder.
From above results, it could be considered that the inhibitibility of spinal alpha motoneuron pool did not yet developed for about 7 years old.

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.

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.

It had been proposed by Kurata that the relative threshold value F_th of single motor units depends on gradient G (kg/sec) in the tension increase of muscle in such a manner as
F_th (=T₁⋅G) =ρ⋅ G^λ
Here T₁ is the time interval from the onset of E.M.G. to the moment of recruitment of a motor unit and ρ is the proportional constant. The motor unit with a positive/negative value of λ is characterized to be static/phasic.
In order to confirm the above relation and to see whether or not the relation is also valid in the relaxation period, single motor units of M. VAST US MEDIALIS were studied by the same method as that of Kurata.
The present findings are
1) At the contraction period the aove relation was confirmed.
2) At the relaxation period the above relation also holds, provided that Ftn and G are replaced by the tension at the moment of silence and the absolute value of G, respectively. The motor unit with a positive/negative value of λ is characterized to be phasic/static.
3) The value of Ftn and λ at the relaxation period are not the same as those at the contraction period.