Effects of whole body vibration on motor unit recruitment and threshold

Abstract

Whole body vibration (WBV) has been suggested to elicit reflex muscle contractions but this has never been verified. We recorded from 32 single motor units (MU) in the vastus lateralis of 7 healthy subjects (34 ± 15.4 yr) during five 1-min bouts of WBV (30 Hz, 3 mm peak to peak), and the vibration waveform was also recorded. Recruitment thresholds were recorded from 38 MUs before and after WBV. The phase angle distribution of all MUs during WBV was nonuniform (P < 0.001) and displayed a prominent peak phase angle of firing. There was a strong linear relationship (r = -0.68, P < 0.001) between the change in recruitment threshold after WBV and average recruitment threshold; the lowest threshold MUs increased recruitment threshold (P = 0.008) while reductions were observed in the higher threshold units (P = 0.031). We investigated one possible cause of changed thresholds. Presynaptic inhibition in the soleus was measured in 8 healthy subjects (29 ± 4.6 yr). A total of 30 H-reflexes (stimulation intensity 30% Mmax) were recorded before and after WBV: 15 conditioned by prior stimulation (60 ms) of the antagonist and 15 unconditioned. There were no significant changes in the relationship between the conditioned and unconditioned responses. The consistent phase angle at which each MU fired during WBV indicates the presence of reflex muscle activity similar to the tonic vibration reflex. The varying response in high- and low-threshold MUs may be due to the different contributions of the mono- and polysynaptic pathways but not presynaptic inhibition.

Fig. 1.

Intramuscular EMG recording obtained during a 1-min period of whole body vibration (WBV). Top trace shows the motion of the WBV platform, the black bar above this represents the period where the platform is turned on. Middle traces shows the intramuscular EMG recorded during WBV. Bottom section shows a period of 2 s during WBV where individual motor units (MUs) can be identified.

Fig. 2.

Process for identifying the phase of vibration at which the MU fired. Top: motion of the platform during WBV. Equal portions of the data selected before and after the point where the MU fired (X on the top trace) representing 1 cycle of the sine wave. Bottom: artificially generated sine wave (gray) corresponding to the selected portion and the cosine wave (black) generated by integrating the sine wave. Ax and Ay are represented by the X on the cosine and sine wave, respectively.

Fig. 3.

Intramuscular EMG recording during recruitment threshold task before and after WBV. A: force recorded during recruitment threshold task. B: intramuscular EMG recording showing a greater number of MUs firing at higher levels of force. C: expanded section showing the intramuscular EMG recording before WBV. D: expanded section showing the intramuscular EMG recording after WBV.

Fig. 4.

Rose plot of the phase angles recorded during WBV. The direction of each bin in the rose plot indicates the phase angle at which that MU fired. The number of firings within a given bin are indicated on the inner circles of the rose plot. Displayed is a single MU selected from each subject (remaining MUs can be found in the online supplementary data, different colors represent each subject).

Fig. 5.

Mean MU recruitment threshold (of all pre- and post recruitment thresholds) plotted against the change in threshold before and after WBV. The trend line indicates a strong linear relationship between recruitment threshold and change in recruitment threshold (r = −0.68, P < 0.001). The shape of each point indicates the category in which the MU was placed for subsequent analysis (categories were based on the pre-WBV recruitment threshold resulting in overlap of some categories in figure).

Fig. 6.

Levels of presynaptic inhibition for each subject before, immediately, 10, and 20 min after WBV. The line with ● and error bars represents the mean (SE) level of presynaptic inhibition for all subjects.