These experiments examine how human subjects use information from a target to trigger a response and to specify its trajectory. We first determined if response initiation is predicated on the prior specification of response amplitude by examining the latencies and trajectories of impulses of isometric elbow flexion aimed to one of three visual targets. We varied target predictability (simple versus choice), the urgency with which the response was required, and the level of practice. With practice, subjects could respond to unpredictable targets with the same latency as to predictable ones; the range of response amplitudes was, however, always constricted. This central tendency bias disappeared when subjects were allowed long latencies to respond to the target, suggesting that with urgency, subjects can respond before specification is complete. To determine the time course of specification, the subjects were trained to initiate force impulses in synchrony with the last of a series of predictable tones. They also attempted to match the amplitude of their force impulses to one of three unpredictable visual targets presented at randomly varying times (50–400 ms) prior to the synchronizing tone. At the shortest stimulus-response intervals, before target information could be processed, the amplitudes of responses to all targets were clustered around that of the middle-sized target. Then, as the stimuli-response interval increased, response amplitudes gradually converged upon their specific targets. Specification started at stimulus-response intervals of about 100 ms and extended until about 350 ms. We conclude that (1) subjects prepare an adaptive or default response prior to the occurrence of the target; (2) the specification of response amplitude is a gradual process by which the dimensions of the default are adjusted according to target information, and (3) amplitude specification begins earlier and terminates later than the time needed to initiate the motor response or reaction tim