In everyday life we must constantly balance our intentions to act in a certain way with reactions that are imposed upon us by the outside world. Recent neuroimaging studies have examined these classes of movement separately but despite the fundamental requirement for us to efficiently organize our internally generated and externally triggered actions, few studies have examined the relationship between these two classes of movement. We measured EMG activity in the right first dorsal interosseous while subjects performed right index finger key presses either in an internally generated condition or an externally triggered condition. In addition, in an attempt to probe the relationship between the processing underlying these two types of action, we examined the effect on reaction time (RT) and EMG activity in a third "truncation" condition in which subjects were forced to switch from an intentional (internally generated) mode of response production to an externally triggered mode. Results indicated significantly greater muscle activation for actions that were internally generated as compared to externally triggered. Truncation caused responses to be delayed by, on average, 54.7 ms as compared with simple externally triggered responses, suggesting that the motor system cannot take advantage of preexisting levels of preparation when switching between internally generated and externally triggered actions. Interestingly, the unique EMG signatures of internally generated and externally triggered actions were preserved in truncation. Thus, subjects switched between the two types of action rather than simply modifying an ongoing action. The results provide peripheral physiological support for previous neuroimaging work suggesting that internally generated actions are preceded by greater levels of preparation than externally triggered actions. The present findings also raise the interesting possibility that the motor system processes these two classes of action separately even though the motor output required is the same.