Anterior cruciate ligament strain behavior during rehabilitation exercises in vivo.

Before studying the biomechanical effects of rehabilitation exercises on the reconstructed knee, it is important to understand their effects on the normal anterior cruciate ligament. The objective of this investigation was to measure the strain behavior of this ligament during rehabilitation activities in vivo. Participants were patient volunteers with normal anterior cruciate ligaments instrumented with the Hall effect transducer. At 10 degrees and 20 degrees of flexion, ligament strain values for active extension of the knee with a weight of 45 N applied to a subject's lower leg were significantly greater than active motion without the weight. Isometric quadriceps muscle contraction at 15 degrees and 30 degrees also produced a significant increase in ligament strain, while at 60 degrees and 90 degrees of knee flexion there was no change in ligament strain relative to relaxed muscle condition. Simultaneous quadriceps and hamstrings muscles contraction at 15 degrees produced a significant increase in ligament strain compared with the relaxed state but did not strain the ligament at 30 degrees, 60 degrees, and 90 degrees of flexion. Isometric contraction of hamstrings muscles did not produce change in ligament strain at any flexion angle. Exercises that produce low or unstrained ligament values, and would not endanger a properly implanted graft, are either dominated by the hamstrings muscle (isometric hamstring), involve quadriceps muscle activity with the knee flexed at 60 degrees or greater (isometric quadriceps, simultaneous quadriceps and hamstrings contraction), or involve active knee motion between 35 degrees and 90 degrees of flexion.