The intentional on-line adaptive capabilities of human movements during continuous variation in gravitational force were investigated. Subjects performed rhythmic forearm movements in the gravitational plane during parabolic flight maneuvers that induced a continuous change from 1.8 G to 0 G over a period of 2.3 s. During the initial plateau of hypergravity, subjects produced movements at two frequencies, with and without space constraints. Afterward, they were faced with the drop in gravity, during which they were instructed either to let the movement evolve freely while maintaining the initial frequency (time-constrained task) or to intentionally maintain the frequency, amplitude, and forearm center of oscillation (time/space-constrained task). The results showed (a) a reduced angle for the forearm center of oscillation and maintenance of movement amplitude in the time-constrained task, (b) a change from an in-phase to an out-of-phase biceps/triceps activation pattern regardless of the task, and (c) an earlier occurrence of this change in the time/space-constrained task, impeding the spontaneous forearm rise. These results are discussed in the perspective of the λ model.
Mireille Bonnard, Jean Pailhous and Frédéric Danion
Sheng Li, Frederic Danion, Mark L. Latash, Zong-Ming Li and Vladimir M. Zatsiorsky
One purpose of the present study was to compare indices of finger coordination during force production by the fingers of the right hand and of the left hand. The other purpose was to study the relation between the phenomena of force deficit during multifinger one-hand tasks and of bilateral force deficit during two-hand tasks. Thirteen healthy right-handed subjects performed maximal voluntary force production tasks with different finger combinations involving fingers of one hand or of both hands together. Fingers of the left hand demonstrated lower peak forces, higher indices of finger enslaving, and similar indices of force deficit. Significant bilateral effects during force production by fingers of both hands acting in parallel were seen only during tasks involving different fingers or finger groups in the two hands (asymmetrical tasks). The bilateral deficit effects were more pronounced in the hand whose fingers generated higher forces. These findings suggest a generalization of an earlier introduced principle of minimization of secondary moments. They also may be interpreted as suggesting that bilateral force deficit is task-specific and may reflect certain optimization principles.