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Electrical stimulation for inducing muscle contraction can be divided into peripheral nerve stimulation (PNS) and motor point stimulation (MPS). Although the neural pathways activated by PNS have been well studied, those by MPS are still unclear. Here we investigated whether MPS activates Ia-sensory nerves and induces antidromic firing of motor nerves. Ten able-bodied males and females participated in this study. We confirmed that soleus MPS did not induce the H-reflex while soleus PNS did. Furthermore, MPS of the tibialis anterior muscle did not induce the reciprocal inhibition of soleus muscle while PNS did. For testing the effect of MPS on motor neuron excitability, we examined the H-reflex modulation by soleus MPS. When the conditioning and test interval was under 100-ms and the conditioning stimulus intensity was above 30-mA, soleus MPS induced the H-reflex inhibition. This suggests that soleus MPS produces antidromic firing that can induce after-hyperpolarization. These results suggest that MPS predominantly activates the motor nerve without depolarizing the Ia-sensory nerve. Since MPS is applicable to larger number of muscles compared to PNS, utilizing MPS can lead to more versatile neuromodulation of the spinal cord.

Human postural sway during quiet standing is reduced when a fingertip lightly touches a stable surface. The tactile feedback information from the fingertip has been considered responsible for this effect of light touch. Studies have shown that a noise-like minute stimulation to the sensory system can improve the system's weak signal detection. In the present study, we investigated whether a noise-like unperceivable vibration on the fingertip enhances its tactile sensation and facilitates the effect of light touch during quiet standing. Thirteen volunteers maintained quiet standing while lightly touching a touch surface with the index fingertip. Based on each subject's vibrotactile threshold (VT), a noise-like vibration was applied to the touch surface at amplitudes under (0.5VT) or at VT (1.0VT), in addition to the normal light touch condition (no vibration, 0VT). The results showed that the mean velocities of the foot center of pressure (CoP) in both the anteroposterior (AP) and mediolateral (ML) directions were significantly reduced at 0.5VT compared to 0VT and 1.0VT (P0.05), while there was no significant difference between 1.0VT and 0VT (P0.05). Frequency analysis of CoP revealed that the power of high-frequency fluctuation (1-10Hz) was significantly reduced at 0.5VT (P0.05), whereas no significant change was observed in that of low-frequency sway (below 1Hz) (P0.05). These results indicate that an unperceivable noise-like vibration can facilitate the effect of light touch on postural stability, by further reducing fast postural sway.

Whole-body vibration (WBV) is being used to enhance neuromuscular performance including muscle strength, power, and endurance in many settings among diverse patient groups including elite athletes. However, the mechanisms underlying the observed neuromuscular effects of WBV have not been established. The extent to which WBV will produce similar neuromuscular effects among patients with neurological impairments unable to voluntarily contract their lower extremity muscles is unknown. We hypothesized that modulation of spinal motorneuronal excitability during WBV may be achieved without voluntary contraction. The purpose of our study was to describe and compare the acute effects of WBV during passive standing in a standing frame on the soleus H-reflex among men with and without spinal cord injury (SCI). In spinal cord intact participants, WBV caused significant inhibition of the H-reflex as early as 6s after vibration onset (9.0+/-3.9%) (p0.001). The magnitude of the H-reflex gradually recovered after WBV, but remained significantly below initial values until 36s post-WBV (57.5+/-22.0%) (p=0.01). Among participants with SCI, H-reflex inhibition was less pronounced with onset 24 s following WBV (54.2+/-18.7%) (p=0.03). The magnitude of the H-reflex fully recovered after 60s of WBV exposure. These results concur with prior reports of inhibitory effects of local vibration application on the H-reflex. Our results suggest that acute modulation of spinal motoneuronal excitability during WBV can be achieved in the absence of voluntary leg muscle contractions. Nonetheless, WBV has implications for rehabilitation service delivery through modulation of spinal motoneuronal excitability in individuals with SCI.

When an inverted pendulum approximates quiet standing, it is assumed that the distance between the center of pressure and the vertical projection of the center of mass on the ground (COP-COG) reflects the relationship between the controlling and controlled variables of the balance control mechanism, and that the center of mass acceleration (ACC) is proportional to COP-COG. As aging affects the control mechanism of balance during quiet standing, COP-COG must be influenced by aging and, as a result, ACC is influenced by aging as well. The purpose of this study was to test the hypotheses that aging results in an increased COP-COG amplitude and, as a consequence, that ACC becomes larger in the elderly than the young. Fifteen elderly and 11 young subjects stood quietly on a force platform with their eyes open or closed. We found that (1) the standard deviations of COP-COG and ACC were larger in the elderly than in the young, irrespective of the eye condition; (2) COP-COG is proportional to ACC in both age groups, i.e., the inverted pendulum assumption holds true for quiet standing. The results suggest that a change in the control strategy that is due to aging causes a larger COP-COG in the elderly and, as a consequence, that ACC becomes larger as well.

Many researchers have investigated the variability of discrete goal-directed movements. However, a possible time-dependent property for them has been ignored. We investigated the time-dependent property of the kinematic variability in human rapid goal-directed movements by using fractal analysis. The variability of the peak acceleration had fractal time correlation characterized by a non-negligible frequency-spectral exponent ( β =0.3). This β value was reduced at the peak velocity ( β =0.2) and nearly reached zero at the peak movement amplitude, indicating that the variability became white noise with no time correlation. A similar result was observed in the progress of the positional trajectory. The functional significance of this phenomenon for the goal-directed task demand is discussed.