Angle-Specific Analysis of Isokinetic Quadriceps and Hamstring Strength at 6 and 12 Months After Unilateral ACL Reconstruction.

Background: Quadriceps and hamstring strength deficits are related to the increased risk of reinjury after anterior cruciate ligament reconstruction (ACLR).

Hypothesis: Knee angle-specific quadriceps and hamstring strength differences would be observed in patients with ACLR 6 and 12 months after surgery.

Study design: Case-series.

Level of evidence: Level 4.

Methods: A total of 23 postprimary unilateral ACLR patients followed-up at 6 and 12 months postoperatively and 25 controls were included. Isokinetic knee extension and flexion strength were evaluated at 60 deg/s from 20° to 90°. Statistical parametric mapping were performed to explore the angle-specific strength and the limb symmetry index (LSI).

Results: At 6 months postoperatively, the reconstructed leg demonstrated lower knee extension and flexion strength than the contralateral (20°-77°, 24°-90°) (P < 0.01) and control legs (22°-90°, 40°-82°) (P < 0.01). From 6 months to 12 months, knee extension (60°-90°) and flexion (20°-79°) strength improved in the reconstructed leg (P < 0.05), while LSI remained unchanged (P > 0.02). At 12 months, knee extension strength differences persisted in the reconstructed leg compared with the contralateral (20°-81°) and controls (25°-63°) (P < 0.01). ACLR patients had lower LSI of knee extension strength at 6 (20°-59°) and 12 (24°-57°) months postoperatively than the controls (P < 0.02).

Conclusion: The reconstructed leg exhibited differences in knee extension strength compared with the contralateral and control legs. Although bilateral knee extension strength increased from 6 to 12 months postoperatively, LSI did not show improvement during this period.

Clinical relevance: Quadriceps restoration was observed only in knee flexion angles greater than 60° compared with controls. Future studies should investigate whether knee extension strength, especially in lower flexion angles, can be enhanced through rehabilitation programs. Furthermore, assessing the impact of this improvement on long-term outcomes and reinjury risk in ACLR patients is warranted.