Visually guided targeting enhances bilateral force variability in healthy older adults

Potential Motor Benefits of Visual Feedback of Error Reduction for Older Adults

This study investigated how visual feedback of virtual error reduction (ER) modified the visuomotor performance of older adults with limited attentional capacity. Error structures of young and older adults during birhythmic force tracking were contrasted when the visualized error size was exact or half of the actual size. As compared with full-size error feedback, ER feedback improved the force tracking symmetry of older adults, but undermined that of young adults. Extended Poincaré analysis revealed that young adults presented greater short-term error variability (mean value of κ-lagged SD1 of the error signal) with ER feedback, which led to a smaller mean value of κ-lagged SD1 of the error signal for older adults. The ER-related task improvement of the older adults was negatively correlated with the size of the tracking errors with real error feedback and positively correlated with ER-related increases in force spectral symmetry and decreases in the mean value of κ-lagged SD1 of the error signal. ER feedback could advance visuomotor tasks for older adults who perform worse with full-size visual feedback by the enhancement of self-efficacy and stabilization of negative internal feedback.

Effects of mechanical assistance on muscle activity and motor performance during isometric elbow flexion

Mechanical assistance on joint movement is generally beneficial; however, its effects on cooperative performance and muscle activity needs to be further explored. This study examined how motor performance and muscle activity are altered when mechanical assistance is provided during isometric force control of ramp-down and hold phases. Thirteen right-handed participants (age: 24.7 ± 1.8 years) performed trajectory tracking tasks. Participants were asked to maintain the reference magnitude of 47 N (REF) during isometric elbow flexion. The force was released to a step-down magnitude of either 75% REF or 50% REF and maintained, with and without mechanical assistance. The ramp-down durations of force release were set to 0.5, 2.5, or 5.0 s. Throughout the experiment, we measured the following: (1) the force output using load cells to compute force variability and overshoot ratio; (2) peak perturbation on the elbow movement using an accelerometer; (3) the surface electromyography (sEMG) from biceps brachii and triceps brachii muscles; and (4) EMG oscillation from the biceps brachii muscle in the bandwidth of 15-45 Hz. Our results indicated that mechanical assistance, which involved greater peak perturbation, demonstrated lower force variability than non-assistance (p < 0.01), while EMG oscillation in the biceps brachii muscle from 15 to 45 Hz was increased (p < 0.05). These findings imply that if assistive force is provided during isometric force control, the central nervous system actively tries to stabilize motor performance by controlling specific motor unit activity in the agonist muscle.