Cooperation of two fixation chains in stabilising the trunk during horizontal arm movements

Aim: During voluntary adduction-abduction movements of one arm in the horizontal plane, the prime movers discharge a reaction torque (TR) which would rotate the trunk in the direction opposite to arm acceleration. Rotation is impeded by connecting the trunk to a firm support through muscular fixation chains that exert forces counterbalancing TR. In some occasions TR is contrasted by one single fixation chain but what happens if a second chain is provided? Methods: Seven standing subjects performed cyclic adduction-abductions of the right arm (1.5Hz) while grasping a fixed handle with the left hand. In this set-up, TR may be contrasted by: 1) a leg fixation chain, exerting a torque on ground (TG) and 2) a left arm fixation chain, exerting latero-lateral forces on the handle (FH). Subjects performed 20 movement trials (15 cycles each). The intensity of the grip exerted on the handle, estimated by EMG activity in Flexor Digitorum Superficialis muscle, was stepwise changed in the different trials from 0% to ~60% MVC, but kept constant throughout each trial. Results: TG and FH were found to undergo sinusoidal changes at the same frequency as arm movements while their phase indicated that both counteracted TR. As the grip strength was increased, the amplitude of the handle force FH increased while the amplitude of the ground torque TG decreased. Conclusions: voluntarily increasing the strength of the handle grip progressively deviates the postural actions from the leg to the left arm fixation chain.