The organization of quick corrections within a two-joint synergy in conditions of unexpected blocking and release of a fast movement

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Within this study, we tested a hypothesis that common organization of central commands to the elbow and the wrist joints within a two-joint synergy is associated with a similar organization of pre-programmed corrections to unexpected changes in the external conditions. The subjects (n = 7) performed series of very fast movements or isometric contractions against a pad at the level of the palm or at the level of the forearm. Some trials within a series of movements were unexpectedly blocked at the initial position leading to an isometric contraction, while some trials within a series of isometric contractions were unexpectedly released leading to a movement. Movement kinematic and electromyographic (EMG) patterns were analyzed. In cases of unexpectedly blocked or unexpectedly released trials, differences in the EMG patterns between perturbed and unperturbed trials were seen at latencies between 50 and 80 ms. Two patterns were observed representing interindividual differences among subjects. One included reciprocal changes in the flexor-extensor pairs controlling both joints. The other pattern included reciprocal changes in the wrist flexor-extensor pair and unidirectional changes in the EMGs of biceps and triceps. The patterns were reproducible within each individual subject across tasks and conditions. In particular, the same pattern in the wrist flexor-extensor pair was seen when the pad was applied to the palm and when it was applied to the forearm, despite the fact that early wrist joint deviations were in opposite directions. It is argued that the observed early EMG changes may be unrelated to local joint kinematics and stretch reflexes, and represent consequences of control patterns for fast corrective movements that are organized with respect to the motion of the endpoint of the limb similarly to the organization of voluntary movements. Within this framework, the organization of joints of a multi-joint limb into a synergy implies a simultaneous, automatic organization of pre-programmed reactions into a similarly organized synergy.