The association between transient lateral postural perturbations and upper limbs active movements

BACKGROUND: Although humans use their lower limbs to compensate when postural equilibrium is affected by external motion perturbations, very little is known about how upper limbs (UL) are used. Such an investigation may provide useful insight into how motion affects manual tasks. OBJECTIVE: To identify the compensatory postural strategies in which UL are involved and to investigate the association between UL movements, lateral motion perturbations, and motion induced interruptions (MII). METHODS: Twenty healthy individuals maintained a quiet stance for one hour on a motion platform. Motion stimuli included lateral perturbations in Sway-only and Sway + Pitch motion conditions. A kinematic analysis of the video recordings was conducted. RESULTS: Humans make compensatory movements with their UL when lateral motion perturbations challenge their postural equilibrium. Arm elevation is associated with the lateral linear acceleration (sway), the occurrence and the type of MIIs. We focused on the “both arms” strategy because it has an increased potential for interfering with manual tasks compared to single upper limb movements. When individuals take a step to sustain their balance, they use the both hands strategy more often. However, more than 70% of both arms strategy instances occurred in the absence of an MII. CONCLUSIONS: Manual task interruptions induced by environmental motion may occur even before or without an MII. The limits of the operational envelope of reduced manual performance should be reconsidered and adopted in lieu of the existing rigid-body model.