An organism's fitness is determined by how it chooses to adapt to effort in response to challenges. Exertion of effort correlates with activity in dorsomedial prefrontal cortex (dmPFC) and noradrenergic pupil dilation, but little is known about the role of these neurophysiological processes for decisions about future efforts, they may provide anticipatory energization to help us accept the challenge or a cost representation that is weighted against the expected rewards. Here, we provide evidence for the former, by measuring pupil and functional magnetic resonance imaging (fMRI) brain responses while 52 human participants (29 females) chose whether to exert efforts to obtain rewards. Both pupil-dilation rate and dmPFC fMRI activity increased with anticipated effort level, and these increases differ depending on the choice outcome: they were stronger when participants chose to accept the challenge compared with when the challenge was declined. Crucially, the choice-dependent modulation of pupil and brain-activity effort representations were stronger in participants whose behavioral choices were more sensitive to effort. Our results identify a process involving the peripheral and central human nervous system that simulates the required energization before overt response, suggesting a role in guiding effort-based decisions.