Bi-articular, unidirectional arm movements were studied to evaluate the electromyographic (EMG) and neuromuscular force patterns that occur when a limb is unexpectedly perturbed. A series of training trials were continued with a control load spring attached to the apparatus until a pre-specified criterion for learning was attained. The limb was then unexpectedly loaded with one of four test load springs. Examination of the integrated EMG records revealed a coactivation pattern of neuromuscular activity during a major part of the movement. Analysis of applied force data supported the notion of sustained agonist activity. Reciprocal innervation and coactivation patterns were considered in terms of a common motor control system as proposed by the equilibrium point control hypothesis. Manifestation of coactivation rather than reciprocation in the present study was discussed in terms of the establishment of "mental set." (Author)