Kinematic synergy adaptation to microgravity during forward trunk movement

The aim of the present investigation was to see whether the kinematic syner gy responsible for equilibrium control during upper trunk movement was pres erved in absence of gravity constraints. In this context, forward trunk mov ements were studied during both straight-and-level flights (earth-normal gr avity condition: normogravity) and periods of weightlessness in parabolic f lights (microgravity). Five standing adult subjects had their feet attached to a platform, their eyes were open, and their hands were clasped behind t heir back. They were instructed to bend the trunk (the head and the trunk t ogether) forward by similar to 35 degrees with respect to the vertical in t he sagittal plane as fast as possible in response to a tone, and then to ho ld the final position for 3 s. The initial and final anteroposterior center of mass (CM) positions (i.e., 200 ms before the onset of the movement and 400 ms after the offset of the movement, respectively), the lime course of the anteroposterior CM shift during the movement, and the electromyographic (EMG) pattern of the main muscles involved in the movement were studied un der both normo- and microgravity. The kinematic synergy was quantified by p erforming a principal components analysis on the hip, knee, and ankle angle changes occurring during the movement. The results indicate that 1) the an teroposterior position of the CM remains minimized during performance of fo rward hunk movement in microgravity, in spite of the absence of equilibrium constraints; 2) the strong joint coupling between hip, knee, and ankle, wh ich characterizes the kinematic synergy in normogravity and which is respon sible for the minimization of the CM shift during movement, is preserved in microgravity. It represents an invariant parameter controlled by the CNS. 3) The EMG pattern underlying the kinematic synergy is deeply reorganized. This is in contrast with the invariance of the kinematic synergy. It is con cluded that during short-term microgravity episodes, the kinematic synergy that minimizes the anteroposterior CM shift is surprisingly preserved due t o fast adaptation of the muscle forces to the new constraint.