No abstract

Voluntary arm-raising movement performed during the upright human stance position imposes a perturbation to an already unstable bipedal posture characterised by a high body centre of mass (CoM). Inertial forces due to arm acceleration and displacement of the CoM of the arm which alters the CoM position of the whole body represent the two sources of disequilibrium. A current model of postural control explains equilibrium maintenance through the action of anticipatory postural adjustments (APAs) that would offset any destabilising effect of the voluntary movement. The purpose of this paper was to quantify, using computer simulation, the postural perturbation due to arm raising movement. The model incorporated four links, with shoulder, hip, knee and ankle joints constrained by linear viscoelastic elements. The input of the model was a torque applied at the shoulder joint. The simulation described mechanical consequences of the arm-raising movement for different initial conditions. The variables tested were arm inertia, the presence or not of gravity field, the initial standing position and arm movement direction. Simulations showed that the mechanical effect of arm-raising movement was mainly local, that is to say at the level of trunk and lower limbs and produced a slight forward displacement of the CoM (1.5 mm). Backward arm-raising movement had the same effect on the CoM displacement as the forward arm-raising movement. When the mass of the arm was increased, trunk rotation increased producing a CoM displacement in the opposite direction when compared to arm movement performed without load. Postural disturbance was minimised for an initial standing posture with the CoM vertical projection corresponding to the ankle joint axis of rotation. When the model was reduced to two degrees of freedom (ankle and shoulder joints only) the postural perturbation due to arm-raising movement increased compared to the four-joints model. On the basis of these results the classical assumption that APAs stabilise the CoM is challenged.

The present study explored the effect of different movement orientations on the arm end-effector kinematic features, levels of muscle activity and intermuscular coordination between shoulder and elbow muscles during cyclical movement. Subjects were instructed to trace cyclical lines with their dominant arm along vertical, horizontal, and right (low inertia) or left diagonal (high inertia) orientations. EMG activity from the biceps, triceps and anterior and posterior deltoids were monitored along with the displacements of the end-effector of the arm. The results suggested a differential role for the shoulder versus elbow muscles in the manipulation of the hand end-effector trajectory. The activity in the shoulder flexors was predominantly in anti-phase with that of the shoulder extensors and was therefore presumed to manipulate the global features of the trajectory. Biceps and triceps tended to show less orchestrated activity and were therefore assumed to be responsible for making the fine adjustments and to compensate for intersegmental interactions. The most pronounced differences in kinematics and EMG features among the four principal movement orientations were observed between the two diagonal orientations, which differed profoundly in arm inertial resistance. The findings converged upon the principle of 'inertial anisotropy,' as previously identified for discrete movement, suggesting that the central nervous system did not fully preplan the actual kinematic requirements of cyclical task performance. Moreover, inertial anisotropy was evident in spite of the fact that movement was performed under temporal constraints (metronome pacing) and with availability of a visual template of the task, suggesting that enhancement of the feedback loop did not fully eliminate these effects.

The effect of unilateral tendon vibration on the performance of cyclical bimanual forearm movements was investigated across different cycling frequencies (from 0.67 to 2.53 Hz). The spatiotemporal features of the individual limb motions as well as their coordination were studied. Tendon vibration was found to result in a substantial reduction in the amplitude of the vibrated arm, leaving the nonvibrated arm unaffected. The vibration-induced amplitude reduction decreased from 26% to 11% as cycling frequency increased even though significant reductions were still observed at the highest cycling frequencies. Tendon vibration was also found to result in an increase of the phase lead of the dominant arm with respect to the nondominant arm, but this effect was not modulated by cycling frequency. The data argue in favor of a closed-loop mode of movement control during cyclical high-speed movements. It is suggested that kinesthetic afferent information is processed and used to guide action up to near-maximal movement speeds, reinforcing recent claims with respect to visual information processing.

The evolution of joint dynamics and muscle patterning in the shoulder and elbow was studied for cyclical line drawing tasks at different frequencies, amplitudes, and orientations in the horizontal plane. Three main modes of control were identified: elbow-centered, shoulder-centered, and elbow-shoulder, each referring to the principal joints or joint combinations that were used to achieve the behavioral goals. The contribution of the shoulder joint was most prominent across the majority of movement orientations and largely paralleled changes in the dynamic (inertial) forces in the end effector (shoulder-centered control). The two joints either exchanged roles during the performance of the right diagonal movement (elbow-centered control) or shifted from a single-joint strategy to a dual-joint strategy during the performance of large amplitudes with low or medium cycling frequencies (shoulder-elbow control). These behavioral results support the existence of a modular control mode that allows the central nervous system to effectively tune motor commands to meet a broad variety of orientations, amplitudes, and frequencies. This refers to the emergence of a context-dependent control mode for the shoulder and elbow that optimizes the implementation of the underlying motor goals under a rich combination of spatial and temporal manipulations.