Leaf spring exercise: A safe quadriceps strengthening exercise after anterior cruciate ligament reconstruction

Background: Leg extensions should be avoided in the early stages after anterior cruciate ligament reconstruction because the force exerted by the quadriceps muscle leads to anterior tibial displacement. To allow for safe quadriceps training in the knee extension range during this period, we devised the leaf spring exercise, which involves placing subjects in the prone position with their knee slightly flexed and instructing them to perform maximum isometric quadriceps contractions while supporting the proximal region of the lower leg's anterior surface and immobilizing the femur's posterior surface to prevent lifting. The current study aimed to examine the safety of Leaf spring exercise by determining the femur-tibia relationship using ultrasound imaging. Methods: This controlled laboratory study included patients with unilateral anterior cruciate ligament -deficient knees (8 men and 8 women; age, 24.2 +/- 8.3 years) who were instructed to perform Leaf spring exercise of both lower limbs. We measured the femur-tibia-step-off, which indicates the distance between the last point of the medial and lateral condyles of the femur and posterior margin of the tibial plateau, as a parameter to evaluate anterior tibial displacement via ultrasound diagnostic device. Further, peak torque of the quadriceps muscle was calculated using force measurement device. Findings: No difference in anterior tibial displacement and peak torque was observed between the uninjured and injured sides during Leaf spring exercise. Interpretation: Leaf spring exercise may add some strain on the reconstructed anterior cruciate ligament; hence, it can be considered a safe quadriceps exercise in the knee extension range.