A dynamic, in vivo, functional analysis of braces designed for ACL insufficiency has never been reported. In this study, 14 athletes who had arthroscopically proven absent ACLs were evaluated in the Biomechanics Laboratory at the Centinela Hospital Medical Center. None of the ligaments were repaired or reconstructed. Footswitch, high speed photography, and force place data were recorded while the athletes performed running and cutting maneuvers with and without their custom fitted C.Ti. braces (Innovation Sports, Irvine, CA). Cybex testing, KT-1000 evaluation, and radiographs were collected for each subject. Statistical analysis showed that while performing cutting maneuvers, braced limbs generated significantly increased shear forces compared to the same limb unbraced. During straight line running, braced limbs generated significantly less lateral and aft shear forces compared to the same limb unbraced. Running velocity increased while wearing a brace for most athletes, but this was not statistically significant. As expected, the sound limb generated significantly greater shear forces than the unbraced involved limb during most cutting maneuvers. Athletes who did not achieve 80% of the Cybex (Cybex, Division of Lumex, Ronkonkoma, NY) quadriceps torque of the sound limb generated significantly more forces during cutting maneuvers while wearing their braces. The cutting angle, approach time to the cut, and time on the force plate showed no significant differences during brace wear. We conclude that the C.Ti brace allows significantly better running and cutting performances for athletes who have torn ACLs. Athletes who have not achieved 80% of quadricep strength as measured by Cybex testing showed even more improvement while wearing their braces.(ABSTRACT TRUNCATED AT 250 WORDS)