Which Tendon Pulls the Knee Medially
No single tendon "pulls" the knee medially in the sense of creating medial displacement—the medial hamstring tendons (semitendinosus and gracilis) resist valgus (lateral) forces and thereby stabilize the medial side of the knee, particularly in extension and early flexion.
Anatomical and Biomechanical Clarification
The question likely refers to which structures provide medial stability or resist lateral displacement of the knee:
- The semitendinosus and gracilis tendons are the primary dynamic medial stabilizers that resist valgus stress (which would push the knee laterally) 1.
- These medial hamstring tendons contribute significantly to valgus stability at 0° and 30° of knee flexion 1.
- Combined unloading of both the semitendinosus (ST) and gracilis (GRA) increases valgus laxity in intact knees and even more so when medial ligamentous structures are injured 1.
Clinical Evidence for Medial Hamstring Function
- The semitendinosus provides greater individual contribution to medial stability than the gracilis—isolated ST unloading increases valgus laxity at 0° flexion in both intact and MCL-injured knees, while isolated gracilis unloading has no significant effect 1.
- The semitendinosus tendon has an accessory insertion that overlaps and runs parallel to the anterior band of the medial collateral ligament (MCL), reinforcing medial stability 2.
- When the posterior oblique ligament (POL) is damaged, the stabilizing role of the medial hamstrings becomes even more critical 1.
Surgical Implications
- Harvesting medial hamstring tendons for graft material compromises medial knee stability, particularly in patients with pre-existing medial-sided ligament injuries 1.
- Semitendinosus tenodesis techniques have been developed specifically to reconstruct both the MCL and POL by looping the tendon to reinforce medial structures 2.
- Gracilis tenodesis with two functional bundles can address anteromedial knee instability by reconstructing both deep and superficial MCL components 3.
Common Pitfall
Do not confuse dynamic tendon stabilization with passive ligamentous restraint—while the MCL, deep MCL, and POL are the primary static medial stabilizers 4, the medial hamstrings provide crucial dynamic support that becomes increasingly important when static restraints are compromised 1.