Athletes who injure the anterior cruciate ligament (ACL) in the knee often have an ACL repair. The surgeon can use a piece of the hamstring tendon to replace the torn ACL. The semitendinosus (ST) tendon is harvested and used as a donor graft.

Studies have shown that the ST does regenerate (grow back). In this study Japanese researchers use MRI and strength testing to see if the athletes regain full function after regeneration.

They found that strength in full knee flexion (squat position) does not always return. It appears that tendons regenerate and reattach in slightly different places. When the tendon reattaches below the knee joint, athletes can perform both shallow and deep knee flexion with strength equal to the other leg.

When the tendon regenerates fully but attaches above the knee joint, then deep knee flexion is compromised. In a few athletes, the tendon did not appear to regenerate at all. They were limited in strength with knee flexion measured at 45- and 90-degrees.

Poor regeneration or altered attachment of the tendon can be a problem for athletes. Those who need greater strength with full knee flexion such as judo athletes, gymnasts, and ballet dancers may be affected most.

The authors conclude there is a need to prevent deficits in knee-flexion torque (force). This is especially important for some athletes. Further studies are needed to find out what will improve tendon regeneration and attachment. It may be that changes in the surgery or the rehab program (or both) could improve the final results.

Reference:

Akie Nishino, et al. Knee-Flexion Torque and Morphology of the Semitendinosus After ACL Reconstruction. In Medicine & Science in Sports & Exercise. Vol. 38. No. 11. Pp. 1895-1900.'