A common exercise you may see in the clinic is sidelying clam shells. We had previously been hesitant to utilize this exercise ourselves, as we were taught that it preferentially activates the TFL over the Gluteus Medius. With weakness in the Gluteus Medius being a frequent culprit in individuals with knee, hip, and back pathologies, you can understand our avoidance of this exercise.
In JOSPT's recent issue, one of the studies reported on the muscle activation pattern in the hip for various angles of clam exercises. The authors looked at EMG activity of the Gluteus Maximus, Gluteus Medius, and TFL during sidelying clam shell exercises with the hip in 0, 30, and 60 degrees of flexion. Additionally, they studied the effect of complete sidelying and a 35 degree backward rotation (reclined) had on muscle activation. Interestingly, TFL activity was low throughout all positions, while Gluteus Maximus and Gluteus Medius were highest in complete sidelying with 60 degrees of hip flexion. This shows that, with proper positioning, sidelying clam shell exercises can be useful for strengthening the Gluteus Medius (along with the Gluteus Maximums).
That being said, there are two items that come to mind. As much as the Gluts are targeted with sidelying clam shells, do they have increased activation compared to pure sidelying hip abduction for the Gluteus Medius (which is the MMT position)? Obviously, we want to choose the exercises that focus on the muscle the most. This study looked at hip muscle activation in several exercises and found sidelying hip abduction to be the most specific to strengthening of the Gluteus Medius. Additionally, clam shells are sidelying. To what extent do we want to continue sidelying/supine exercises, when they are not as functional as more advanced, standing exercises? That's not to say that clam shells do not serve a purpose for patients at lower functional levels, but are we relying on lower level exercises too much?