Why? Look at a deep squat. As you descend past parallel, your hamstrings actually slacken because they attach above the knee and below the hip. When the knee bends, the hamstring tension drops. However, the Adductor Magnus doesn't have this problem.
In deadlifts and RDLs (Romanian Deadlifts), the hamstring portion of the AM acts as a synergist. If you feel a deep stretch on the inside of your lower thigh during an RDL, that isn't your hamstring—that is your Adductor Magnus begging for attention. The "Mystery Groin Pull" Most athletes fear the "groin pull." But did you know that most proximal (high) groin pulls are actually tears of the rectus abdominis or the pectineus? Distal (low) groin pain near the knee is often the Adductor Magnus. adductor magnus muscle
To train the hamstring portion (hip extension), you need hip extension with a wide stance. To train the adductor portion (leg adduction), you need resistance pulling you outward. When the knee bends, the hamstring tension drops