This work investigates the potential of using an air-muscle actuated orthosis controlled by an electromyographic (EMG) signal to reliably supplement the grasping force of the hand, thereby allowing the user to reduce the muscle activation required for a power-grasping task. In particular, the study reported herein tested the hypotheses that subjects could stably handle objects and learn to reduce both their grip force and muscle activation levels with force supplementation. In this study, a surface-mounted EMG sensor on the flexor digitorum provides the input to a proportional-integral-derivative controller governing the force generated by the orthosis. Although this approach presumes that the human motor system will stably adapt to the orthotic system, we designed the system to operate in an intuitive and predictable manner. Nine subjects performed a sequence of unassisted and assisted lifts of a weighted and instrumented cylinder. When using the orthotic system to lift the cylinder, subjects reliably reduced their mean grip force and mean percent maximum voluntary contraction (%MVC) (p<0.01). The grip force applied to the cylinder was reduced for seven of the nine subjects (p<0.01) and the %MVC was reduced for five of the nine subjects (p<0.01). None of the subjects exhibited any instability or reported any difficulties when using the orthosis. On average, the subjects reduced their %MVC and grasp force by 31% and 56% respectively.