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Electromyographic muscle responses to single acoustic stimuli and repeated acoustic stimuli in supine subjects Nichol, David Daniel
Electromyographical (EMG) motor responses may be elicited by loud acoustic stimuli in humans and vary based on presentation methods and body position. The purpose of this study was to investigate the EMG responses caused by different presentation methods of acoustic stimuli in a supine body position. Participants lay supine and maintained a voluntary plantar flexion contraction during trials. Auditory stimuli were presented from a speaker in front of participants’ face. EMG was recorded from right orbicularis oculi (OOc) and bilaterally from stemocleidomastoid (SCM), medial gastrocnemius, deltoid and soleus muscles. Single acoustic stimuli (SAS) (40 ms, 124 dB tones), were presented to participants with ten minutes between stimuli. Repeated acoustic stimuli (RAS) (40 ms, 124 dB tones), were presented repeatedly at intervals of 3-5 sec. Ten participants in a control condition were exposed to six or more SAS and 210 RAS during testing. Pre-pulse stimuli (40 ms, 85 dB tones) were presented 100 ms before both the RAS and SAS for 8 participants in the experimental condition. These participants were exposed to 3 SAS plus pre-pulse and 3 SAS, then to a total of 200 RAS and 200 RAS plus pre-pulse presented pseudorandomly. Five participants were exposed to 210 RAS stimuli at 85 dB as a follow-up control condition. EMG signals were root mean squared and trigger-averaged to the onset of the acoustic stimulus for the different conditions. Similar responses were rendered from SAS and RAS in voluntarily contracting lower limb muscles. SAS response amplitudes were variable within single muscles across trials. RAS exposures rendered an averaged response in all participants tested which lasted for 500 ms at a 7-8 Hz oscillation in the voluntarily contracting soleus muscles. This response appears to be similar to SAS responses but of smaller amplitude and only visible after the averaging of multiple trials. Responses to the 85 dB RAS stimuli also occurred in voluntarily contracting muscles. Pre-pulses showed inhibition in the OOc muscle in the SAS condition. The observations suggest that in humans, an EMG response may be elicited in contracting lower limb muscles by SAS and RAS and these responses may be related.
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