Complementary recruitment of two fixation chains during cyclic movements of one arm in the horizontal plane

Aims: During voluntary adduction-abduction movements of one arm in the horizontal plane, contraction of Pectoralis Major (PM) and Infraspinatus (IS) muscles discharge a reaction torque (Tr) which would rotate the trunk in the direction opposite to arm acceleration. Rotation is impeded by postural actions (fixation chains) which connect the trunk to a firm support, thus counterbalancing Tr. If only the ground support is available, Tr is contrasted by the hip and leg muscles (e.g. Hamstring, Ham) that exert a torque (Tz) about the body vertical axis. How are postural adjustments organized when a second support is provided? Methods: Standing subjects (n=6), with both shoulders anteflexed, performed cyclic adduction-abductions of the right arm (1.5Hz, paced by a metronome) while grasping a fixed handle with the left hand. EMG was recorded bilaterally from PM, IS and Ham. In this set up, Tr may be contrasted by: 1) the leg fixation chain, exerting Tz on a force platform; and 2) the left arm fixation chain, in which the left PM and IS exert on the handle a latero-lateral force (Fh) recorded by a load cell. Subjects performed 20 trials (15 cycles each). The intensity of the grip concentrically exerted on the handle was stepwise changed in the different trials and kept constant throughout each trial by monitoring the Flexor Digitorum Superficialis EMG. Results: It was found that Tz and Fh (as well as the EMG activity of the corresponding muscles) undergo sinusoidal changes at the same frequency as arm movements, their amplitude being linearly correlated with the grip strength (p < 0.0023 in all subjects). However, as the grip was strengthened amplitude of Fh increased while amplitude of Tz decreased. As expected, the phase of Tz and Fh indicated that both counteracted Tr. Conclusion: Voluntarily strengthening the handle grip complementarily controls the two fixation chains by deviating the postural actions from the leg to the left arm muscles.