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Muscle reflex responses to acoustic stimuli Luxon, Sarah Margaret


Loud acoustic stimuli (>115 dB) are known to evoke electromyographical (EMG) responses in human musculature that differ with body position, presentation rate, and stimulus duration. Long duration acoustic tones (40 ms) with an inter-stimulus interval of 3 – 5 s evoke small amplitude reflex responses in tonically contracted limb musculature, whereas short duration acoustic tones (0.1 – 20 ms) with an inter-stimulus interval of 0.2 – 1 s can evoke EMG responses in limb muscles that are posturally engaged. Therefore the purpose of this study was to investigate the similarities and differences of the EMG responses evoked with repeated short and long duration acoustic tones in tonically contracted axial and limb musculature of supine participants. Methods: Twenty subjects (aged 19 – 30) were exposed to 256 presentations of air conducted (AC) acoustic stimuli that were 7 and 40 ms in duration (500 Hz; 118 dB SPL). Two blocks of 128 AC stimuli at each stimulus duration, and one block of no stimuli were presented randomly and binaurally through calibrated headphones. Surface EMG was sampled from the right sternocleidomastoid (SCM), biceps brachii (BB), and soleus (SOL) while participants maintained low level contractions in each muscle. Results: Repeated 7 and 40 ms AC stimuli evoked a myogenic potential in the tonically contracted SCM, BB, and SOL in at least 80%, 75%, and 75% of participants respectively. Significant effects of stimulus duration were observed in the SCM and SOL, where significant peaks occurred 5.4 and 6.7 ms earlier in the SCM, and 9.3 ms earlier in the SOL with a shorter stimulus. No significant effects were observed in the BB. Conclusion: We have shown that repeated short duration acoustic stimuli presented at a short inter-stimulus interval can evoke reflex responses in tonically contracted limb muscles which has not been shown before. These observations suggest that the EMG responses observed here may differ from those that are influenced by postural engagement.

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