Contribution of seat and foot reaction forces to anticipatory postural adjustments (APAs) in sitting isometric ramp pushes

The aim of this paper was to examine the role of the upper and lower body on the dynamic phenomena, which precede the voluntary movement (anticipatory postural adjustments: APAs), and the way in which they contribute to postural control. In this view, sitting subjects were asked to perform horizontal two-handed ramp pushes as quickly as possible. A dynamometric bar was used to provide the push force (Fx). Local reaction forces along the antero-posterior and vertical axes, at the seat and foot-rests (RSx, RSz, and Rfx, Rfz, respectively), as well as global ones (Rx and Rz), were measured. Two postural conditions were considered: full (100 BP) and one-third ischio-femoral contact (30 BP). Anticipatory postural adjustments durations (dAPAs) were measured between the onset of global or local (that is, at the seat and foot level) reaction forces, and the onset of push force increase.Firstly, the dAPAs were longer at the foot than at the seat level, that is, the APA sequence starts at the foot level: it is suggested that a “posturo-focal” sequence is followed, whose progression order is precisely dependent on the postural conditions. Moreover, the APA peak amplitudes (pAPA), measured at the seat contact were significantly greater than the corresponding ones measured at the foot contact: the upper body dynamics are larger than the lower body dynamics.Secondly, a greater peak push force (pFx) entailed significant dAPA increases, in preference to pAPA increases. As APAs are dynamic phenomena, they can perturb balance, suggesting that, in order to avoid unnecessary perturbation, APAs are increased in terms of duration rather than amplitude.Lastly, the impulses corresponding to the push force increase (“BPIx”) and to the APA periods (“ACPIx”) were calculated. As ACPIx was very low as compared to BPIx, it was suggested that the APA action was limited to the period of the voluntary movement onset.