Reciprocal aiming precision and central adaptations as a function of mechanical constraints.

The present study investigated the influence of mechanical constraints (load and movement velocity) on the movement accuracy during a reciprocal aiming precision task. Seven participants had to point rhythmically and alternatively (with flexion-extension of the right elbow) a cursor at two targets as accurately as possible. Two loads (applied to the limb effectors; 500 and 2500g), two movement frequencies (1.25 and 1.75Hz) as well as two target sizes (1 and 5cm) were manipulated. Surface EMG activity of both biceps brachii and triceps brachii was recorded. Attentional demands, reflecting the central cost associated with the performance of aiming movements was assessed using a dual-task paradigm (via a probe reaction time task - RT). While the results demonstrated a significant degradation of pointing accuracy with mechanical loading (mean absolute error - AE for 500g load: 0.32mm±0.64; mean AE for 2500g load: 0.51±0.74mm), no significant effect of movement frequency was found. For the two mechanical constraints, the mental effort to meet the task demands remained the same (mean RT(-500g): 370±123ms; mean RT(-2500g): 395±119ms). Electromyographic activity of both biceps brachii and triceps brachii muscles evidenced neural adaptations to changes in mechanical constraints. Put together, the present findings suggest that the cause of the observed loss of movement accuracy may probably result from more peripheral alterations such as an impairment of the afferent information processing.