During uphill running limb muscles must perform net mechanical work to increase the body's potential energy, while during level running the net mechanical work required is negligible as long as speed is constant. The increased demands for work as running incline increases might be met by an increase in power output at all joints, or only a subset of joints. We used inverse dynamics to determine which joints modulate net work output in humans running uphill. We measured joint kinematics and ground reaction force during moderate speed running at 0 degrees , 6 degrees and 12 degrees inclines. Muscle force, joint power and work per step were determined at the ankle, knee and hip using inverse dynamics calculations. We found that virtually all of the increase in work output with increasing incline resulted from increases in net work done at the hip (-0.25+/-0.23 J kg(-1), level, vs 0.88+/-0.10 J kg(-1), 12 degrees incline), while the knee and ankle performed similar functions at all inclines. The increase in work output at the hip resulted primarily from a large increase in average net muscle moment during stance (2.07+/-17.84 Nm, level, vs 87.30+/-13.89 Nm, 12 degrees incline); joint excursion increased by only 20% (41.22+/-3.41 degrees , level, vs 49.22+/-2.35 degrees , 12 degrees incline). The increase in hip muscle moment and power was associated with a poorer mechanical advantage for producing force against the ground. The increase in hip moment with running incline allows for the production of the power necessary to lift the body. This power may be developed by hip extensors or by transfer of power from muscles at other joints via biarticular muscles.