Electromyographic responses to a mechanical perturbation applied during impending arm movements in different directions: one-joint and two-joint conditions

Directional tuning is a common finding for many physiological features of a rm movements and related neuronal activity. We investigated whether the ele ctromyographic response to a brief (30 ms) torque perturbation prior to vol untary movement depends on the direction of the impending movement. Pointin g movements with the elbow joint alone and those involving both the shoulde r and elbow joints were studied in separate experiments. Target direction w as varied between flexion and extension for the one-joint experiments and a mong four spatial directions for the two-joint experiments. Movement trials in which a perturbation stretched the flexor muscles just prior to the poi nting movement were randomly interspersed among unperturbed movement trials in each experiment. A small pre-load ensured some background activity of t he flexor muscles. Results were remarkably similar for the one- and two-joi nt conditions. The short-latency reflex response of the stretched muscles ( in a 30-60 ms window after perturbation onset) was not modulated with direc tion of target-reaching movement in a statistically significant manner, whi ch confirms earlier findings for one-joint movements and extends these to t he two-joint condition. Beyond the short-latency window, the perturbation p rovoked earlier onsets of target-reaching muscle activities for the agonist muscles, whether or not the muscle had been stretched by the perturbation. The onset of the braking activity of the antagonist muscles also occurred earlier in the presence of the brief perturbation prior to movement, irresp ective of whether the muscle had been stretched or not. The magnitude of ta rget-reaching muscle activity, in general, was greater for the perturbed tr ials, though not consistently for all muscles or all directions. These resu lts suggest that, when movement is about to be initiated, in either single- or multi-joint conditions, the long-latency effects of the stretch strongl y depend on the intended direction of movement. The dependence is such that the response serves to hasten and augment the intended movement, but not n ecessarily to oppose the perturbation.