Reciprocal recruitment of two cooperative fixation chains during arm movements in the horizontal plane

During voluntary adduction-abduction movements of one arm in the horizontal plane, the prime movers discharge a reaction torque (Treact) which would rotate the trunk in the direction opposite to arm acceleration. Such rotation is impeded by connecting the trunk to a firm support through muscular fixation chains that exert forces counterbalancing Treact. In some occasions Treact is contrasted by one single fixation chain but if a second chain is provided, how would the two interact? The question was investigated on standing subjects (n=7) with arms pointing forward and elbows fully extended. Subjects performed cyclic adduction-abduction movements of the right arm (1.5Hz) while grasping a fixed handle with the left hand. In this set-up, Treact may be contrasted by: 1) a leg fixation chain, exerting a torque on ground (Tg, recorded by a force platform) and 2) a left arm fixation chain, exerting latero-lateral forces on the handle (Fh, recorded by a load cell). Subjects performed 20 movement trials (15 cycles each). The intensity of the grip exerted on the handle was monitored by EMG activity in Flexor and Extensor Digitorum Superficialis muscles (FDS, EDS). In the different trials, voluntary EMG activity in FDS was stepwise changed from 0% to ~60% MVC, keeping it constant throughout each trial. It was found that Tg and Fh underwent sinusoidal changes at the same frequency as arm movements while their phase indicated that both contributed in counteracting Treact. As the grip strength was increased, the amplitude of the handle force Fh increased while the amplitude of the ground torque Tg decreased (both correlations significant in all subjects, P always < 0.03). In conclusions, voluntarily increasing the strength of the handle grip progressively deviates the postural actions from the leg to the left arm fixation chain.