The Acute Effect of Whole-Body Vibration on the Hoffmann Reflex

Abstract

The extent to which motoneuron pool excitability, as measured by the Hoffmann reflex (H-reflex), is affected by an acute bout of whole-body vibration (WBV) was recorded in 19 college-aged subjects (8 male and 11 female; mean age 19 +/- 1 years) after tibial nerve stimulation. H/M recruitment curves were mapped for the soleus muscle by increasing stimulus intensity in 0.2- to 1.0-volt increments with 10-second rest intervals between stimuli, until the maximal M-wave and H-reflex were obtained. After determination of Hmax and Mmax, the intensity necessary to generate an H-reflex approximately 30% of Mmax (mean 31.5% +/- 4.1%) was determined and used for all subsequent measurements. Fatigue was then induced by 1 minute of WBV at 40 Hz and low amplitude (2-4 mm). Successive measurements of the H-reflex were recorded at the test intensity every 30 seconds for 30 minutes post fatigue. All subjects displayed a significant suppression of the H-reflex during the first minute post-WBV; however, four distinct recovery patterns were observed among the participants (alpha = 0.50). There were no significant differences between genders across time (P = 0.401). The differences observed in this study cannot be explained by level or type training. One plausible interpretation of these data is that the multiple patterns of recovery may display variation of muscle fiber content among subjects. Future investigation should consider factors such as training specificity and muscle fiber type that might contribute to the differing H-reflex response, and the effect of WBV on specific performance measures should be interpreted with the understanding that there may be considerable variability among individuals. Recovery times and sample size should be adjusted accordingly.

Effect of an Acute Bout of Whole Body Vibration Exercise on Muscle Force Output and Motor Neuron Excitability

“…T he use of whole body vibration (WBV) as an exercise modality is relevant to sports scientists and strength and conditioning coaches due to the array of physiological and performance measurements which it affects (2)(3)(4)(5). In particular, acute exposure to WBV has been shown to result in increased respiration (19,20) and energy expenditure (9), increased cutaneous blood flow (19), increased anabolic hormone response (12), and increased activation of the tonic vibration reflex (16).…”

“…In particular, acute exposure to WBV has been shown to result in increased respiration (19,20) and energy expenditure (9), increased cutaneous blood flow (19), increased anabolic hormone response (12), and increased activation of the tonic vibration reflex (16). However, the use of an acute bout of WBV as a method to potentiate athletic performance measures remains unclear (3,6,8).…”

“…Acute WBV has been found to cause both improvements and decrements in performance (3,8). These conflicting observations appear to be the result of the different frequencies and amplitudes of the vibration utilized, the mode of the exercise prescribed, the outcome variables measured, and the post-treatment time point when the measurement were taken (25).…”

Side-Alternating Vibration Training for Balance and Ankle Muscle Strength in Untrained Women

“…This greater muscle activation could be due to an augmented excitatory input from the muscle spindle. 8 However, researchers 27,28 have demonstrated that reflex activity either remains unchanged or decreases after a bout of whole-body vibration. Although spinal reflex excitability decreased, the changes in homosynaptic depression (presynaptic mechanism that regulates the function of the Ia afferent-motoneuron synapse) point to a particular mechanism that may directly improve neuromuscular function after whole-body vibration.…”

“…This decrease is highly variable and may be associated with individual differences other than sex, training specificity, or conditioning. 28 Postural sway in the M/L direction is considered a better predictor of the risk of falling than A/P sway, even for individuals who have no recent history of falling. 29 Our results showed a greater sway decrease in the M/L than A/P direction (approximately 50% versus 25%) after SAV training.…”