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

Abstract

Voluntary arm-raising movement performed during the upright human stance position imposes a perturbation to an already unstable bipedal posture characterised by a high body centre of mass (CoM). Inertial forces due to arm acceleration and displacement of the CoM of the arm which alters the CoM position of the whole body represent the two sources of disequilibrium. A current model of postural control explains equilibrium maintenance through the action of anticipatory postural adjustments (APAs) that would offset any destabilising effect of the voluntary movement. The purpose of this paper was to quantify, using computer simulation, the postural perturbation due to arm raising movement. The model incorporated four links, with shoulder, hip, knee and ankle joints constrained by linear viscoelastic elements. The input of the model was a torque applied at the shoulder joint. The simulation described mechanical consequences of the arm-raising movement for different initial conditions. The variables tested were arm inertia, the presence or not of gravity field, the initial standing position and arm movement direction. Simulations showed that the mechanical effect of arm-raising movement was mainly local, that is to say at the level of trunk and lower limbs and produced a slight forward displacement of the CoM (1.5 mm). Backward arm-raising movement had the same effect on the CoM displacement as the forward arm-raising movement. When the mass of the arm was increased, trunk rotation increased producing a CoM displacement in the opposite direction when compared to arm movement performed without load. Postural disturbance was minimised for an initial standing posture with the CoM vertical projection corresponding to the ankle joint axis of rotation. When the model was reduced to two degrees of freedom (ankle and shoulder joints only) the postural perturbation due to arm-raising movement increased compared to the four-joints model. On the basis of these results the classical assumption that APAs stabilise the CoM is challenged.

Modular Control of Pointing beyond Arm's Length

“…Passive mechanical effects might additionally help in joint coupling. Indeed, previous studies showed that the mechanical consequences of forward arm reaching from an upright stance position consisted of passive forward trunk bending and knee flexion (Ramos and Stark, 1990;Eng et al, 1992;Pozzo et al, 2001), similar to these observed during the present WBP task. Coordinative structures of redundant degrees of freedom can also be accounted for by attractor dynamics, operating on an internal model of the underlying kinematic system (Mussa Ivaldi et al, 1988;Mohan and Morasso, 2006).…”

Postural Adjustments in Catching: On the Interplay Between Segment Stabilization and Equilibrium Control

“…Nevertheless, also in the RAISE task, where considerably less elbow flexion took place than during catching, no consistent APA before movement onset was observed ( Figure 5, right panel), which seems in conflict with some suggestions made in the literature. Taken together, our findings indicate that the proposed backward shift of COP before movement onset is not a ubiquitous indicator of APA, and that if APA is considered present its functional role for whole body equilibrium control should be questioned (Pozzo et al, 2001;Patla et al, 2002). The initial motion of whole body COM might be a passive consequence of reaction forces due to counter-rotating segments rather than an active control of postural equilibrium, with antagonistic coactivations for segment stabilization.…”

“…While early postural responses have most frequently been associated with an inverted pendulum acting at the level of the ankle, which generates anticipatory forward acceleration of the whole body COM to compensate for the ensuing inertial forces from prime mover motion, a comparison of model-based and experimental data has led to the suggestion that the initial postural motion of whole body COM is in fact passive and hence that APA are better explained as a side effect of local segment stabilization (Pozzo, Ouamer, & Gentil, 2001;Patla, Ishac, & Winter, 2002). Segment stabilization is needed to counter reactive torques from the arm acting on the shoulder, which in turn affect the hip, knee and ankle joints.…”

“…Putting aside recent debate on the functional role of observations identified as APA (Pozzo et al, 2001;Patla et al, 2002), their presence and subsequent SPA have been well described in reactive arm raising tasks. This has provided a useful baseline for understanding postural adjustments in arm raising tasks, but as alluded to above there are important differences compared with one-handed catching, which could influence the mechanisms involved in maintaining postural balance.…”

Variant and invariant features characterizing natural and reverse whole-body pointing movements

“…In their work, Berret et al (2009) attributed the strong joint coordination during unconstrained F movements to multiple factors, such as passive mechanical effects (Ramos and Stark 1990;Eng et al 1992;Pozzo et al 2001) or attractor dynamics (Mussa Ivaldi et al 1998;Mohan and Morasso 2006). Another hypothesis is that joint coupling may be the results of the combination of a low number of muscle synergies.…”