Plantar flexion force and associated EMG signals and V/F-waves generated by vibratory stimuli modulated in amplitude by a triangular envelope. A) Schematic representation of the triangular envelope of the vibratory stimulus, showing the time course of a 100-Hz sinusoidal vibration that linearly increased in amplitude from zero to a peak during 10 s of stimulation and then linearly decreased to zero during the next 10 s. The arrows below VFc, VF1, VF2, VF2′, VF1′ and VFc’ represent the times at which supramaximal electrical stimuli were delivered. T, and T’ represent the torque areas used for data analysis. A representative acceleration signal is shown at the right, with part of the signal (in expanded time scale) in the inset. B) Plantar flexion torque (eight superposed recordings) and C) EMG from the soleus muscle (typical recording) in response to the stimulation paradigm shown in (A). The thin white line in (C) is the envelope of the rectified EMG. D) The same data shown in (B) and (C) in graphs of force and EMG envelope as a function of vibration-amplitude (note the marked hysteresis in the force generation and EMG). The increasing and decreasing phase of the vibration amplitude are represented by the arrows pointing to the right and to the left, respectively. E) V/F-waves (eight superimposed recordings) obtained at different times during the vibratory stimulation. Note the larger V/F-wave amplitudes during the second half of the stimulation (VF1′,VF2′) and immediately after the stimulation (VFc’). Data in this figure are from a representative subject that showed clear increases in force, EMG and V/F-wave amplitudes (see also Figure 3) during the second half of the triangular-shaped vibratory stimulation.