The influence of vibration frequency (40, 80, 100, 120, 150, or 200 Hz) at selected displacement amplitudes (0.2, 0.3 mm) on tonic vibration reflex (TVR) characteristics was investigated. The degree of synchronization of motor unit activity with vibratory stimuli in ten humans was determined using the electromyographic (EMG) activity of the finger and wrist flexor muscles when vibration was applied to the distal tendons of the hand flexor muscles. The EMG spectral analysis indicates that harmonic and subharmonic motor unit synchronization mechanisms contribute to the modulation of the amplitude of the TVR as the vibration frequency increases. Harmonic synchronization decreases while subharmonic synchronization increases as vibration frequency increases. It is suggested that the synchronization process influences muscle fatigue, since it forces the driving of motor units, leading to a decrease in contraction efficiency. This phenomenon most probably results from an impairment of excitation-contraction coupling. High-frequency vibration (> 150 Hz) tends to induce less motor unit synchronization in a frequency range beyond the known mechanical resonance of biological tissues. The findings of this study may be applied to the design of hand-held power tools, since their vibration triggers the TVR in active muscles.

Although upper limb movements are known to be slower and more variable in elderly persons, the extent to which these changes are associated with deficits in movement-related sensory feedback is poorly understood, despite the importance of proprioception in the control of skilled movement. Age-related changes were examined with 22 participants (10 of M age 27 years and 12 of M age 75 years) in performance of an elbow position-matching task which varied in terms of interhemispheric transfer and/or the need to retrieve memory-based proprioceptive information. Matching errors were significantly greater, and movements more prolonged, and irregular in their time course in the elderly group than in the young group. Impaired performance in conditions requiring interhemispheric transfer and retrieval of memory-based proprioceptive information reflected the importance of cognitive processing during complex sensorimotor tasks. This novel matching paradigm provided a sensitive means of manipulating the demands of the task and may be an effective method for as sessing both cognitive and sensorimotor declines associated with aging.

The present results suggest that occupational activities requiring prolonged standing are likely to contribute to lower-extremity and/or back disorders.

Asymmetries in upper limb position sense have been explained in the context of a left limb advantage derived from differences in hemispheric specialization in the processing of kinesthetic information. However, it is not clearly understood how the comparison of perceptual information associated with passive limb displacement and the corresponding matching movement resulting from the execution of a motor command contributes to these differences. In the present study, upper limb position sense was investigated in 12 right-hand-dominant young adults performing wrist position matching tasks which varied in terms of interhemispheric transfer, memory retrieval and whether the reference position was provided by the same or opposite limb. Right and left hand absolute matching errors were similar when the reference and matching positions were produced by the same hand but were 36% greater when matching the reference position with the opposite hand. When examining the constant errors generated from matching movements made with the same hand that provided the reference, the right and left hand matching errors (≈3°) were similar. However, when matching with the opposite limb, a large overshoot (P <0.05) characterized the error when the right hand matched the left hand reference while a large undershoot (P <0.05) characterized the error when the left hand matched the right hand reference. The overshoot and undershoot were of similar magnitude (≈4°). Although asymmetries in the central processing of proprioceptive information such as inter-hemispheric transfer may exist, the present study suggests that asymmetries in position sense predominantly result from a difference in the “gain of the respective proprioceptive sensory-motor loops”. This new hypothesis is strongly supported by a dual-linear model representing the right and left hand sensory-motor systems as well as morphological and physiological data.

Effective fall prevention technologies need to detect and transmit the key information that will alert an individual in advance about a potential fall. This study investigated advanced vibrotactile cuing that may facilitate trip recovery for balance-impaired individuals who are prone to falling. A split-belt treadmill that simulated unpredictable trip perturbations was developed to compare balance recovery without and with cuing. Kinetic and kinematic measures from force plates and full body motion capture system were used to characterize the recovery responses. Experiment I evaluated recovery adaptation resulting from repeated trip exposure without vibrotactile cuing. Experiment II investigated the effects of vibrotactile cuing as a function of cuing location (upper arm, trunk, lower leg) and lead time prior to a trip (250, 500 ms). Experiment I showed that trip recovery improved progressively from the fourth to the eighth trial. Experiment II showed that trip recovery was almost the same as the eighth trial in Experiment I, regardless of the location of the cuing stimulus and lead time. The results suggest that a combination of vibrotactile cuing and hazard detection technology could reduce the risk of trips and falls.