Sitting down and squatting are routine activities in daily living that lower the body mass by flexing the trunk and legs, but they obviously require different motor strategies for each goal posture. The former action must transfer the supporting surface onto a seat, whereas the latter must maintain the center of mass within the same base of both feet. By comparing the performance of both maneuvers, the mechanisms involved in initiating the downward-oriented movements and the process of optimizing the performance during their repetitions were studied. Twelve healthy subjects were asked to perform sitting-down and squatting actions immediately when a light cue was given, but at a natural speed. Electromyograms, angular movements of the joints of the right leg, and center of pressure (COP) displacement were recorded before and during each task. The initial mechanisms to initiate the break from the upright posture and the changes of postural adjustments during repetitive movements were analyzed separately. The sitting-down movement was achieved by a stereotyped motor strategy characterized by a gastrocnemius muscle burst coupled with deactivation of the erector spinae muscle. The former produced a transient COP displacement in the forward direction, and simultaneous unlocking of the trunk prevented a fall backward. By contrast, because of the absence of any need to produce momentum in a given direction, a variety of motor strategies were available to initiate squatting. The direction of initial COP displacement to initiate squatting varied with the muscles involved in unlocking the upright posture. During repetition of sitting down, the average COP position of the initial standing posture in the preparatory period was immediately shifted forward after the second trial. Simultaneously, the erector spinae muscle was deactivated earlier in the later trials. These resulted in a decreased momentum in the backward direction while the subjects were transferring themselves onto the seat. In the squatting task, however, these changes could not be identified, except for a slight flexed position of the knee during standing in the first trial. These findings suggest that in the case of transferring the body-mass to another supporting base the central nervous system immediately adjusts the size of the initial impetus to optimize the performance.
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