Disruption of the anterior cruciate ligament (ACL), a primary stabilizer of the knee, can produce disability. The purpose of our work has been to study the normal ACL in humans, in the presence of normal muscle function and body weight, and develop clinical criteria for reconstruction, establish a basis for rehabilitation programs, and evaluate how knee braces protect this important ligament. The strain behavior of the ACL has been measured by arthroscopic implantation of the Differential Variable Reluctance Transducer while subjects are under local anesthesia. Movement of the knee from a flexed to an extended position, either passively or through contraction of the leg muscles, produces an increase in ACL strain values. Isolated contraction of the dominant quadriceps with the knee between 50° and extension creates substantial increases in strain. In contrast, isolated contraction of the hamstrings at any knee position does not increase strain. With the knee un-weighted, the protective strain shielding effect of a functional knee brace decreases as the magnitude of anterior shear load applied to the tibia increases. A different behavior occurs during weight bearing, the strain shielding effect of the brace remains constant as the magnitude of anterior load increases. Our approach is novel in that it can be used to measure on important portion of the ACLs strain distribution while clinically relevant loads are applied to the knee, subjects perform rehabilitation exercises, or in the presence of different orthoses such as functional knee braces.