OBJECTIVE: The Protonics brace is a functional resistance brace designed for rehabilitative use in patients with patellofemoral pain syndrome. Our objective was to determine whether the Protonics brace altered quadriceps muscle activity or knee mechanics in healthy subjects. DESIGN AND SETTING: We used a within-subjects design in a laboratory setting. SUBJECTS: Nineteen recreationally active college students (10 females, 9 males; age = 22.6 +/- 2.8 years; height = 172 +/- 9.0 cm, mass = 69.7 +/- 12.5 kg) with no history of patellofemoral pain syndrome. MEASUREMENTS: A standard-length Protonics brace was fit to each subject's leg. Surface electromyography of the vastus medialis obliquus, vastus lateralis, and rectus femoris muscles was recorded during a lateral step-down exercise. Lower extremity kinematics and ground reaction force were assessed during stair descent. Subjects performed both tasks under 4 conditions: no brace and brace with low, medium, and high resistance. Electromyography values were normalized to a maximum voluntary isometric contraction. The brace moment was determined by passive testing in an isokinetic dynamometer. Changes in the subject's muscular knee-extension moment were determined by subtracting the extension moment provided by the brace from the total knee-extension moment calculated from motion and force data using an inverse dynamics approach. RESULTS: Vastus medialis obliquus activity in the brace at the low- and medium-resistance settings was significantly lower than that measured without the brace. Vastus lateralis activity while wearing the brace at medium resistance was significantly less than in the absence of the brace. Regardless of brace setting, vastus medialis obliquus and vastus lateralis activity in the descending phase of the exercise was less than during the ascending phase. A significant interaction was noted between brace setting and phase of the step-down exercise for rectus femoris activity. Significantly less activity was seen in the descending phase than in the ascending phase. Post hoc testing indicated that, in the descending phase, less activity was demonstrated with the brace at the medium and high settings than at the low setting. Muscle activity at the high setting and activity at low resistance were also significantly less than when the brace was not worn in the ascending phase. Knee flexion and extension moment during stair descent were significantly less at the higher resistance settings. CONCLUSIONS: Wearing the Protonics brace at moderate or high resistance during the lateral step-down exercise produced less quadriceps activity compared with not wearing the brace. The knee extensor mechanism was unloaded when the brace was worn during stair descent. These findings indicate that the Protonics brace may unload the quadriceps and therefore decrease the load on the patellofemoral joint.