Article

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 (F(x)). Local reaction forces along the antero-posterior and vertical axes,
at the seat and foot-rests (R(Sx), R(Sz), and R(fx), R(fz), respectively), as
well as global ones (R(x) and R(z)), 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 (pF(x))
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
("BPI(x)") and to the APA periods ("ACPI(x)") were calculated. As ACPI(x) was
very low as compared to BPI(x), it was suggested that the APA action was limited
to the period of the voluntary movement onset.

Langue : ANGLAIS