Innovative equipment and exercise modalities for anterior cruciate ligament rehabilitation

Background. A number of research studies provide evidence that, during open kinetic-chain knee-extension exercises, the strain-force on the anterior cruciate ligament (ACL) can be considerably reduced by hamstring co-contraction, as well as by the use of leg extension equipment that implements either an external axial knee compression or a controlled radial displacement of the resistance pad along the lower-leg during the movement. Purpose. The purposes of this study were: 1) to determine the possible increase of hamstrings co-activation caused by a intentional co-contraction effort during open kinetic-chain leg-extension exercises, and to assess whether an intentional hamstring co-contraction can completely suppress the ACL-loading tibiofemoral (TF) shear force during these exercises; 2) to built two leg extension prototypes with knee axial compression and movable resistance pad, respectively, and to assess their effectiveness in limiting the anterior tibial translation. Methods. The electromyographic activity of semitendinosus (ST), semimembranosus (SM), biceps femoris (BF), and quadriceps femoris, and knee kinematics were measured in twenty healthy male subjects during isotonic leg-extension exercises with resistance (R) ranging from 10% to 80% 1RM (Repetition Maximum). The same exercises were also performed with the standard equipment while the subjects attempted to enhance hamstring co-activation through a intentional co-contraction effort, and with two innovative prototypes in absence of intentional co-contraction. The data served as input parameters for a model to calculate the shear and compressive TF forces in leg-extension exercises for any set of co-activation patterns of the different hamstring muscles. X-ray imaging was applied to assess whether the external axial compression has the potential to limit the anterior tibial translation during isometric leg-extension efforts. Results. For R≤40%1RM the peak co-activation levels l obtained with intentional co-contraction were significantly higher (P<0.001) than those l0 obtained without intentional co-contraction. For each hamstring muscle, the level l reached its maximum at R=30%1RM, corresponding to 9.2, 10.5, and 24.5% MVIC (maximum voluntary isometric contraction) for BF, ST, and SM, respectively, whereas the ratios l/l0 reached their maximum at R=20%1RM given approximately by 2, 3, and 4, for BF, SM, and ST respectively. The intentional enhanced co-activation levels l obtained for R≤30%1RM completely suppressed the anterior TF force developed by the quadriceps during the exercise. Hamstring co-activation was not significantly affected by movable pad and axial compression. The external axial compression seems to have an effect in limiting the anterior tibial translation only at high compression levels. Conclusions. In leg-extension exercises with resistances R≤40%1RM co-activation of the BF, SM, and ST can be significantly enhanced (up to 2, 3, and 4 times, respectively) by a intentional hamstring co-contraction effort. The enhanced co-activations levels obtained for R≤30%1RM can completely suppress the anterior TF force developed by the quadriceps during the exercise. Clinical Relevance. This study suggests that leg-extension exercise with intentional hamstring co-contraction may have the potential to be a safe and effective quadriceps strengthening intervention in the early stages of rehabilitation programs for ACL injury or reconstruction recovery. Further studies, including clinical trials, are needed to investigate the relevance of this therapeutic exercise in clinical practice.