Article

Voluntary adduction-abduction movements of one arm in the horizontal plane
discharge a reaction torque which would rotate the trunk in the direction
opposite to arm acceleration. Rotation is impeded by muscular fixation chains
that exert forces counterbalancing the reaction torque. We examined how two
different fixation chains cooperate in stabilising the trunk during the above
movements. Standing subjects (n=6), with shoulders ante-flexed, performed cyclic
adductions-abductions of the right arm (1.5 Hz) while grasping a fixed handle
with the left hand. In this set-up, reaction torque is contrasted by: (1) a leg
fixation chain, exerting on the ground a torque around the vertical axis (Tz),
recorded by a force platform; and (2) a left arm fixation chain, exerting on the
handle a force in the medial-lateral direction (Fh), recorded by a load cell.
Subjects performed 20 trials (15 cycles each). It was found that Tz and Fh
underwent sinusoidal changes at the same frequency as arm movements and
contributed in counteracting the reaction torque. The intensity of the handle
grip, monitored by EMG activity in the left Flexor Digitorum Superficialis, was
changed from trial to trial and kept constant during each trial. As grip strength
increased, Fh amplitude increased linearly while amplitude of Tz linearly
decreased. In conclusion, voluntarily strengthening the handle grip progressively
deviates the postural actions from the legs to the left arm.
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Langue : ANGLAIS