Ten subjects completed a series of goal-directed arm flexion movements unexpectedly perturbed by three different types of mechanical load. Examination of electromyograph (EMG) waveforms and kinematic information collected during randomly distributed test trials facilitated investigation into the interaction between loading conditions and the response-associated EMG innervation patterns. Results of the EMG waveform analysis revealed that inertial and spring loads produced cocontraction and triphasic activation patterns, respectively. Unexpected application of a stretched-spring load, which produced a change in initial torque values without changing the rate of loading, also resulted in the use of a triphasic activation pattern. These different EMG patterns were observed while movement displacement for all three loads fell within the limits of the target area.