Co-contraction
PD
We extracted the mean EMG signal for PD and PM during a period 100 ms before movement onset to examine whether participants used muscle co-contraction to modulate the limb intrinsic properties and counter the force field (Mean and SEM of normalized EMG of the posterior deltoid and the pectoralis major averaged across all participants for the right arm (orange line) and the left arm (green line). The mean EMG for both muscle groups was computed in a 100-ms window before movement onset (the gray region in the plot). In experiment 1, we extracted these values for the first 10 and last 10 force field trials (In experiment 2, we computed these same values for the first ten baseline trials and the last ten null field trials (Altogether, these results suggest that the changes in performance in both experiment 1 and experiment 2 were not related to co-contraction modulating the limb intrinsic properties.