Motor output oscillations with magnification of visual feedback in older adults.

Magnification of task visual feedback increases force variability in older adults. Although the increased force variability with magnified visual feedback in older adults relates to the amplification of oscillations in force below 0.5Hz, the related frequency modulation in muscle activity remains unknown. The purpose of this study, therefore, was to characterize the oscillations in muscle activity that contribute to the amplification of force variability with magnified visual feedback in older adults. Fifteen older adults (76.7±6.4years, 7 females) performed isometric contractions at 15% of maximal voluntary contraction (MVC) with ankle dorsiflexion with low-gain (0.05°) or high-gain visual feedback (1.2°). The standard deviation (SD) of force increased significantly (55%) from low- to high-gain visual feedback condition (P<0.0001), without changing the mean force (P>0.5). The increase in force variability was related to greater power in force oscillations from 0 to 0.5Hz (R2=0.37). The increase in force oscillations was associated with greater power in EMG burst oscillations from 0.5 to 1.0Hz (R2=0.50). In conclusion, these findings suggest that magnification of visual feedback alters the modulation of the motor neuron pool in older adults and exacerbates force variability by increasing the oscillations in force below 0.5Hz.