HUMAN EQUILIBRIUM ON UNSTABLE SUPPORT - THE IMPORTANCE OF FEET-SUPPORT INTERACTION

Healthy humans maintained equilibrium on rocking supports (seesaw) of different curvatures and heights. We recorded platform tilt, horizonta l displacements of the upper body, ankle joint angle and activity of a nkle joint muscles. Subjects maintained balance by making seesaw rotat ions placing the support under the body's centre-of-gravity. Forward d isplacement was balanced by compensatory plantariflexion: thus the rel ation between muscle activity and ankle joint angle differed from that on a rigid floor. Mechanical analysis of stability showed that standi ng on low seesaws requires ankle torque increase during forward body s hift (as on a rigid floor) and torque decrease on high seesaws (when t he seesaw height exceeded its radius). In the latter case, balancing w as impossible with eyes closed. The results suggest that directionally specific torque changes in response to centre-of-gravity shifts provi de important information for maintenance of orthograde posture. (C) 19 97 Elsevier Science Ireland Ltd.