The purpose of this research was to determine if bracing altered muscle firing amplitude, duration, or timing, creating improved dynamic stability. We hypothesized that a derotational knee brace improved the stability of an ACL deficient knee by augmenting limb proprioception, causing hamstring muscles to increase in activity and/or to contract earlier during a side-step cut. Ten subjects with documented unilateral isolated ACL deficient knees and five normal controls participated. A strap dominant brace (Lenox Hill, Lenox Hill Brace, Inc., Long Island City, NY) and a shell dominant brace (CTi, Innovation Sports, Irvine, CA) were selected for study. Using footswitches and dynamic EMG, we tested each subject during performance of a side-step cutting maneuver. Subjects completed 15 trials: 5 without bracing, 5 with the strap dominant brace, and 5 with the shell dominant brace. Normals cut 10 times each on their dominant limb. In swing phase, subjects had 38% more and 32% higher lateral hamstring EMG activity than normals; instance phase, subjects had less quadriceps and gastrocnemius activity but more medial hamstring activity. When braced during stance phase, the ACL deficient legs demonstrated a further reduction of 18% in quadriceps total activity and 14% in peak activity compared to the unbraced situation. The hamstrings showed a concomitant decrease of 18% in total activity. No timing differences were noted between the braced and unbraced conditions during swing or stance phase. Clinically, the increase in hamstring activity and de crease in quadriceps activity found during the subjects’ stance phase could have a protective effect on the unstable knee due to the increased activity of muscles that work synergistically with the ACL to control ante rior tibial translation. The reduced activity of antagonist muscles could enhance this effect. Bracing did not alter the relative EMG activity nor did it change firing patterns compared to the unbraced situation. All muscles showed a similar reduction in activity, suggesting that these braces did not have a proprioceptive influence.