Influence of age and feedback modality on the proprioceptive sense of force: insights from motor unit recordings.

The primary purpose of our study was to compare the influence of feedback modality (visual vs auditory) on force-reproduction accuracy in middle-aged and older adults. As a secondary objective, we investigated whether expected differences would be reflected in the neural drive sent to a hand muscle during the task. Participants (n = 42; 40-84 yrs) performed a force-reproduction task with the first dorsal interosseus muscle at two target forces (5% and 20% of maximal voluntary contraction; MVC). Each trial involved a target phase that was guided by visual or auditory feedback and then a reproduction phase without feedback. The neural drive was characterized by measures of force steadiness and motor unit discharge characteristics during the target phase. Force-reproduction accuracy at the lower target force declined with increasing age and with visual feedback compared with auditory feedback. In contrast, there was no evidence of an effect of age or condition on force-reproduction accuracy at the moderate target force (20% MVC). Force steadiness was worse and motor unit coherence in the delta and beta bands was greater when the task was guided by auditory feedback at both target forces. These findings indicate that greater accuracy during the low-force task in the auditory-feedback condition was accompanied by a noisier control signal and differences in motor unit coherence in the delta and beta bands during the target phase.