The Impact of Imperceptible Vibratory Noise on the Spinal Motor Reflex

Abstract

Imperceptible noise stimulation has been shown to increase agility, stability and decrease muscle tremor. Previous research has shown that this effect is mediated via decreases in motor unit discharge variability and decreased motor unit synchronization. However, whether these motor unit effects arise from a spinal or cortical mechanisms is unknown and may change how these technologies are implemented to enhance human performance. PURPOSE: Determine the effect of imperceptible vibratory noise on the spinal motor reflex (Hoffman Reflex). METHODS: 12 males participated in the collection of a Hoffman reflex (H-reflex) recruitment curve and underwent either a randomized STIM (noise applied) trial or SHAM (control). The H-reflex recruitment curve was obtained from the median nerve of the subject’s dominant arm. The intensity that elicited the onset of the M-wave was used to standardize the H-reflex stimulation intensity. The STIM trial was performed by introducing a random imperceptible vibratory noise 3 seconds before the collection of the H-reflex. The test was repeated for a total of 10 stimulations with six seconds of rest between stimulations. The resulting H-reflex amplitudes were then normalized to the maximal M-wave (Mmax) found during the H-reflex recruitment curve. Data were assessed with a generalized estimating equation, clustering for multiple observations. RESULTS: The H-reflex was 19.1% (SE±2.42) of Mmax in the STIM trials and 17.4% (SE±2.66) in the SHAM trials, showing a significant increase of 1.73% with STIM (p = 0.0016). CONCLUSIONS: Subjects showed an increase in spinal excitability while undergoing STIM. The results demonstrate that the spinal reflex plays a role in the motor adaptation response to imperceptible vibration. This increase in spinal excitability suggests that the performance benefits of imperceptible noise stimulation may have a rapid onset, on the order of 10-20 milliseconds, in contrast to cortical mechanisms which are greater than 100 milliseconds. Determining which motor centers mediate the behavioral response to noise stimulation, and to what degree, will help define the optimal parameters for the application of noise stimulation.