Intermanual transfer of force control is modulated by asymmetry of muscular strength

Interlateral transfer of learning is conceptualized as an index of the degree to which learning takes place at a lower level of motor control, with strong dependence on the effector system, or at a higher effector-independent level in the movement organization hierarchy. In this study, the locus of motor learning was investigated by increasing lateral asymmetry of force between the wrist flexor muscles, and comparing the amount of interlateral transfer of force control in relation to a condition of symmetric force. To perform this contrast, the participants were assigned to one of three groups: symmetric force (SM), who were left with original asymmetries of muscular strength; asymmetric force (AS), who had unilateral training for increment of maximum strength for the wrist flexor muscles; or a control condition (CO). The learning task consisted of launching a small cart across a metallic trackway with the preferred hand, aiming at making the cart achieve an instantaneous velocity of 70 cm/s. This action was practiced for 300 trials by the SM and AS group, while the CO group had active rest. The groups, then, were submitted to a transfer task requiring a mirrored action with the contralateral hand. The results indicated that the SM group achieved significantly higher interlateral transfer of learning as compared to the AS group, which presented response variability similar to the CO group. Analysis of directional trend of error revealed that the AS group presented a significant target overshoot as compared with the symmetric force groups. These findings suggest that an absolute force is learnt at a higher level in the action hierarchy, and that decline in interlateral transfer of learning in the asymmetric force condition was motivated by a resetting in the interplay between higher and lower levels of movement control.