Removing a person from the confines of normal gravity should allow them to achieve long and high trajectories during the non-contact phase in running. Yet, in simulated reduced gravity, runners consistently lower their vertical speed at takeoff from the ground-contact phase, producing a flatter gait than in normal gravity. We show that this phenomenon results from a tradeoff of energetic costs associated with ground contact collisions and frequency-based mechanisms (e.g. leg swing work). We asked ten healthy subjects to run on a treadmill in five levels of simulated reduced gravity and measured their center-of-mass vertical speed at takeoff. Vertical takeoff speeds decreased with the square root of gravitational acceleration. These results are consistent with optimization of a work-based model where energy expenditure arises from collisional losses during stance and work done due to swinging the leg. While not exploiting greater takeoff speeds in reduced gravity may be counterintuitive, it is a strategy for minimizing energetic cost to which humans seem extremely sensitive.