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.

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This study shows a way to develop web design guidelines through the quality function deployment (QFD) approach. The voice of the customer (VOC) and design elements were collected from a group of users and web design experts. Design elements that have an impact on the search performance and subjective sensibility of the users were identi® ed. Desirable levels of several design elements were then investigated by experiments. To quantify the eþ ects of the design elements on the search performance and subjective sensibility of the users, simulators were designed and used in experiments. Performance tests showed that the location of the table of contents and the method of menu display have signi® cant in¯uence on the search time. Sensibility assessment tests showed that brighter background makes the users feel more comfortable. Yellow header and scanning column makes users feel the page has simplicity and sensibility, while blue or green make the users feel balanced. The text style does not have an impact on the sensibility of the users. Our results clearly showed that QFD could produce speci® c data useful for such new business areas as web design.

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