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Abstract: Action observation is increasingly considered as a rehabilitation tool as it can increase the cortical excitability of muscles involved in the observed movements and therefore produce effector-specific motor facilitation. In order to investigate the action observation mechanisms, simple single joint intransitive movements have commonly been used. Still, how the observation of everyday movements which often are the prime target of rehabilitation affects the observer cortical excitability remains unclear. Using transcranial magnetic stimulations, we aimed at verifying if the observation of everyday movements made by the proximal or distal upper-limb produces effector-specific motor facilitation in proximal (arm) and distal (hand) muscles of healthy subjects. Results suggest that, similar to simple intransitive movements, observation of more complex everyday movements involving mainly the proximal or distal part of upper limb induces different patterns of motor facilitation across upper limb muscles (P=0.02). However, we observed large inter-individual variability in the strength of the effector-specific motor facilitation induced by action observation. Yet, subjects had similar types of response (strong or weak effector-specific effects) when watching proximal or distal movements indicating that the facilitation pattern was highly consistent within subjects (r=0.83–0.88, P<0.001). This suggests that some individuals are better than other at precisely mapping the observed movements on their motor repertoire and that this type of response holds for various types of everyday actions. [ABSTRACT FROM AUTHOR]

Abstract: Recent evidence shows that the primary motor cortex continues to send motor commands when amputees execute phantom movements. These commands are retargeted toward the remaining stump muscles as a result of motor system reorganization. As amputation-induced reorganization in the primary motor cortex has been associated with phantom limb pain we hypothesized that the motor control of the phantom limb would differ between amputees with and without phantom limb pain. Eight above-elbow amputees with or without pain were included in the study. They were asked to produce cyclic movements with their phantom limb (hand, wrist, and elbow movements) while simultaneously reproducing the same movement with the intact limb. The time needed to complete a movement cycle and its amplitude were derived from the kinematics of the intact limb. Electromyographic (EMG) activity from different stump muscles and from the homologous muscles on the intact side was recorded. Different EMG patterns were recorded in the stump muscles depending on the movement produced, showing that different phantom movements are associated with distinct motor commands. Phantom limb pain was associated with some aspects of phantom limb motor control. The time needed to complete a full cycle of a phantom movement was systematically shorter in subjects without phantom limb pain. Also, the amount of EMG modulation recorded in a stump muscle during a phantom hand movement was positively correlated with the intensity of phantom limb pain. Since phantom hand movement–related EMG patterns in above-elbow stump muscles can be considered as a marker of motor system reorganization, this result indirectly supports the hypothesis that amputation-induced plasticity is associated with phantom limb pain severity. The discordance between the (amputated) hand motor command and the feedback from above-elbow muscles might partially explain why subjects exhibiting large EMG modulation during phantom hand movement have more phantom limb pain. [Copyright &y& Elsevier]