Sit-to-stand motor strategies investigated in able-bodied young and elderly subjects

For the execution of a certain motor task, a motor strategy is chosen by ea ch individual among those that are consistent with the structural and funct ional constraints of his/her locomotor system, and that tends to maximise t he effectiveness of the motor act. The identification of this strategy allo ws for the assessment of the individual's functional status. This study aim ed at identifying the motor strategies adopted for the execution of the sit -to-stand motor task, at different speeds and initial postures, in a sample of 35 community-dwelling elders and in a sample of 16 young able-bodied in dividuals. This was done using a method, least perceivable to the test subj ect and "economical" for the experimenter, which entailed the recording of external forces only. A musculo-skeletal system model, based on a telescopi c inverted-pendulum (TIP) moved by a linear and two rotational muscle-equiv alent actuators, was then used. Parameters describing the kinematics and dy namics of these actuators were extracted and submitted to statistical analy sis. Different motor strategies were identified in the two age groups, as w ell as associated with both a different initial posture (ankle dorsiflexion angle) and speed of execution of the motor task. In particular, the elder group, as compared with the young group, prior to seat-off tended to flex t he trunk more, thus bringing the CM closer to the base of support, and at a higher velocity, thus gaining a higher momentum. After seat-off, elders ro tated the body forward and, only after having brought their CM over the bas e of support, effectively started elevation. Both global muscular effort an d coordination effort associated with the achievement of balance and raisin g were lower. However, maximal speed was also lower. The above results indi cated that the elders could count on a lower functional reserve than the yo ung individuals and, from the methodological viewpoint, that the TIP approa ch is a good candidate for subject-specific functional evaluation in a clin ical context. (C) 2000 Elsevier Science Ltd. All rights reserved.