Background Muscle strengthen exercise of quadriceps femoris is important but risky to the anterior cruciate ligament (ACL) graft, because quadriceps muscle exercise near the knee extension may causes anterior tibial translation. We developed a new exercise method, ‘resisted front bridge exercise with a leg support at proximal tibia’ (RFBP) in a prone position to avoid a risk. Our hypothesis is that RFBP is safe for the ACL graft and effective for muscle strengthen exercise.

Objective To test our hypothesis, we investigated the anterior tibial translation and muscle contraction of quadriceps femoris during RFBP and compared with a front bridge exercise with a leg support at distal tibia (FBD).

Design Quasi-experimental study.

Setting Controlled laboratory research.

Patients Three patients with ACL insufficiency participated in this study approved by the institutional ethical committee.

Interventions Patients lied in a prone position with a leg support at proximal tibia. Leg extension exercise was ordered with a 20% of body-weight load on the back thigh to perform ‘RFBP’.

Main outcome measurements Anterior tibial translation at 15° knee flexion was measured by fluoroscopy according to Franklin's method. Electromyography (EMG) of the vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) were recorded and normalised against the values of maximum voluntary contraction (%MVC) of the involved leg.

Results Anterior tibial translation of the involved knee in RFBP was not statistically significantly different from that of the uninvolved knee, and was smaller than that of the uninvolved knee in FBD in all patients by 4.4 mm in average. The%MVC of VM, VL, and RF muscle activities in the involved knee were 101.8%, 68.2%, and 75.6% during RFBP, respectively.

Conclusion The resisted front bridge exercise with a leg support at proximal tibia is safe and effective for the quadriceps femoris at 15° knee flexion following ACL reconstruction surgery.