1. Experiments were performed to characterize the trajectories, net muscle torques, and electromyogram (EMG) patterns when subjects performed voluntary elbow flexions against different compliant loads. Subjects made movements in a single-joint manipulandum with different loads generated by a torque motor. Some series of movements were performed under entirely known and predictable load conditions. Other series were performed with the same known loads, interspersed, just before movement onset with occasional, unpredictable changes in the magnitude of the load. 2. To move a larger load, subjects increase the impulse (torque-time integral) by prolonging the duration of the accelerating torque while keeping its rate of rise constant. Subjects modulate torque most for inertial loads, less for viscous loads, and least for elastic loads, and modulation is greater under predictable than unpredictable load conditions. 3. Even when the loads are predictable, subjects move large inertial and viscous, but not elastic, loads more slowly than small. Unpredictable changes in load have a larger effect on movement kinematics than do known changes of the same magnitude. 4. Subjects prolong the duration and increase the area of the agonist EMG burst but do not change its rate of rise to move larger, predictable loads. Subjects change the area of the antagonist burst according to the torque requirements of the load, increasing it only for increases in inertial loads. These effects are usually greater for predictable than unpredictable loads but in either case, are highly variable across subjects. 5. Predictable loads that slow the movements delay the onset of the antagonist burst. When changes in load are unpredictable, only inertial changes affect antagonist latency. 6. The initial change in muscle force when there is an unexpected change in the external load is due to the viscous properties of muscle tissue. Electromyographic evidence of reflex changes in muscle activation follow this intrinsic mechanical response by 50-70 ms. Elastic neuromuscular properties may also be important but only late in the movement as the final position is approached. 7. We propose that the central command for a voluntary movement should be described by three elements. The first element (alpha) specifies the muscle activation pattern expected to generate dynamic forces adequate and appropriate to produce a satisfactory trajectory. This feed-forward control program uses simple rules, based on an internal model of task dynamics constructed from prior experience. The second element (lambda) is a kinematic plan or reference trajectory utilizing the negative feedback of reflex action to partially compensate for errors in alpha or for unexpected perturbations during the movement. It defines the locus of a moving, instantaneous equilibrium position of the limb, a "template" for the intended trajectory. As movements become slower and require smaller dynamic (velocity and acceleration dependent) forces, lambda will become the dominant control signal. It is also used for correction and updating of the internal model used to generate alpha. The third element (gamma) modulates volitional set, the degree and manner in which multiple reflex mechanisms can contribute to the muscle activation patterns if the actual trajectory deviates from the planned one. Reflex mechanisms work in parallel with intrinsic muscle compliance to provide partial adaptation of neuromuscular system dynamics to external load dynamics. These controlled compliant mechanisms maintain the stability of the motor system, without which both posture and movement would not be possible.