Whole body vibration (WBV) may improve muscular strength, power and balance (1). Improvements may be related to reflex muscular contractions (2,3) although this has never been verified. Motor unit (MU) recruitment threshold of the vastus laterals was recorded in 7 healthy subjects (34 ± 15.4yr), using fine wire EMG, before and after 5 1min bouts of WBV separated by 30s (30Hz, 3mm peak to peak). Recruitment thresholds were recorded during ramp contractions of the knee extensors during which the force of contraction was recorded. Fine wire EMG was recorded during WBV, along with the vibration waveform by a strain gauge. From this the phase angle at which each MU fired during the vibration cycle was determined. In a further 8 subjects (29 ± 4.6yr) presynaptic inhibition of the soleus was assessed using the same WBV protocol. This was done by eliciting 15 unconditioned and 15 conditioned H-reflexes at a current producing H-reflexes at an amplitude of 30% Mmax. 32 single MUs were discriminated during WBV. The phase of the vibration cycle at which each MU fired was not uniform (P<0.001 for all MUs), indicating that a single MU fired at a consistent phase of the vibration cycle, but the phase differed between units (Fig. 1). There was no difference in MU recruitment threshold before and after WBV (P > 0.05). There was however a strong relationship (r = -0.68, P<0.001, Fig.2) between average recruitment threshold and change in threshold. Further analysis revealed that recruitment threshold of the lowest threshold units significantly increased (P=0.008) while the highest threshold decreased (P=0.031; Fig. 2) after WBV. Due to high levels of variability there was no change in the levels of presynaptic inhibition after WBV (P=0.93). This is the first study to record activity from single MUs during WBV. The results indicate that MU firing is phase-locked to the vibration cycle and suggests that activity is reflexive. Variation in the phase at which each MU fired during WBV could be due to whether the MU was activated via mono- or poly- synaptic pathways. Overall no effect on recruitment threshold was found, although lower threshold MUs increased while higher threshold units decreased threshold after WBV. Lower threshold units are more likely to be influenced by monosynaptic pathways, which are thought to be inhibited after vibration (4), while higher threshold units also have a polysynaptic component. If WBV elicits reflex activity similar to the tonic vibration reflex, which is controlled by mono- and polysynaptic pathways, this could explain the preferential effect on higher threshold MUs. This effect is not related to presynaptic inhibition, which was unchanged after WBV, however other presynaptic mechanisms such as post activation depression cannot be ruled out.