Background Quadriceps strengthening exercise is recommended for patients with PCL insufficiency (PCLI), but posterior sag may occur under the influence of gravity in general leg extension in the sitting position (LES). We hypothesized that leg extension in the prone position (LEP) would be a safe strengthening exercise because the tibia is repositioned in patients with PCLI.

Objective To compare tibial displacement (step off: SO) between LEP and LES, in order to verify the safety of LEP.

Design Quasi-experimental study.

Setting Controlled laboratory research.

Patients 6 patients with PCL insufficiency (SO 2.9±1.6 mm).

Interventions: LEP and LES were performed for isometricstrengthening with a flexion angle of the knee of 60 degrees and consistent force of knee extension. SO was measured on lateral X-P images of the knee. Differences in tibial movement between LEP and LES were evaluated using a paired t-test (P<0.05).

Main outcome measurement SO measured by digitized XP.

Results At the start position of LES, the tibia of all subjects was displaced posteriorly, and the tibia moved anteriorly 7.1±3.1 mm upon contraction of the quadriceps. In contrast, at the start position of LEP, the tibia was not displaced posteriorly in any subjects (SO=− 2.9±2.7 mm). Furthermore, there was significantly less anterior movement of the tibia during LEP (2.8±3.5 mm) than during LES (P<.05).

Conclusions Since LES produced excessive anterior movement of the tibia upon contraction of the quadriceps, secondary cartilage injury may occur because of increased compression and shear force. In contrast, the repositioning of the tibia in LEP by gravity indicates it could be a safe quadriceps strengthening exercise for PCL insufficiency.