Are noisy tendon vibration derived reflex responses sensitive to postural task characteristics?

This study used a noisy achilles tendon vibration (NTV) protocol combined with coherence analysis as a non-invasive, quantitative method for characterizing reflex adaptability during standing tasks. NTV was used to examine the role of somatosensory reflexes when posture and vision were constrained by task instructions. We applied vibration for 120 s and recorded surface electromyography (EMG) from the soleus and medial gastrocnemius during various standing conditions: natural (relaxed) stance, focus on minimizing sway, and holding a handrail each performed with eyes open and eyes closed. The amplitude of the EMG-vibration temporal response (cumulant density) did not differ across task or visual condition. In contrast, vibration-EMG coherence exhibited task- and vision-dependent modulation. Relative to relaxed standing, coherence was reduced when focusing on minimizing postural sway across multiple frequencies and differed during supported standing at both discrete and broadband frequency ranges. Additionally, when asked to stand as still as possible or stand supported with eyes closed, coherence increased compared to eyes open while vision had no effect on relaxed standing. These findings suggest that while the overall amplitude of the somatosensory reflex response was preserved across postural and visual conditions, the frequency-specific coupling between tendon vibration and muscle activity is sensitive to postural constraints. These results highlight the potential utility of NTV for assessing somatosensory reflex function in populations with balance deficits, including individuals with impaired vision and/or those requiring postural support to stand during testing.