Simulating mechanical consequences of voluntary movement upon whole-body equilibrium: the arm-raising paradigm revisited

Voluntary arm-raising movement performed during the upright human stance po sition imposes a perturbation to an already unstable bipedal posture charac terised by a high body centre of mass (CoM). Inertial forces due to arm acc eleration and displacement of the CoM of the arm which alters the CoM posit ion of the whole body represent the two sources of disequilibrium. A curren t model of postural control explains equilibrium maintenance through the ac tion of anticipatory postural adjustments (APAs) that would offset any dest abilising effect of the voluntary movement. The purpose of this paper was t o quantify, using computer simulation, the postural perturbation due to arm raising movement. The model incorporated four links, with shoulder, hip, k nee and ankle joints constrained by linear viscoelastic elements. The input of the model was a torque applied at the shoulder joint. The simulation de scribed mechanical consequences of the arm-raising movement for different i nitial conditions. The variables tested were arm inertia, the presence or n ot of gravity field, the initial standing position and arm movement directi on. Simulations showed that the mechanical effect of arm-raising movement w as mainly local, that is to say at the level of trunk and lower limbs and p roduced a slight forward displacement of the CoM (1.5 mm). Backward arm-rai sing movement had the same effect on the CoM displacement as the forward ar m-raising movement. When the mass of the arm was increased, trunk rotation increased producing a CoM displacement in the opposite direction when compa red to arm movement performed without load. Postural disturbance was minimi sed for an initial standing posture with the CoM vertical projection corres ponding to the ankle joint axis of rotation. When the model was reduced to two degrees of freedom (ankle and shoulder joints only) the postural pertur bation due to arm-raising movement increased compared to the four-joints mo del. On the basis of these results the classical assumption that APAs stabi lise the CoM is challenged.