The Clamshell Exercise
Purpose: Improve ability of the posterior gluteus medius (and minimus?), glute max, and probably the deep hip rotators to perform hip external rotation. I’m more interested in what is not firing (TFL, rectus femoris, vastus lateralis, biceps femoris).
Position: Sidelying with a slight forward roll. Knees flexed between 45 and 90 deg, hips flexed between 0 and 90 deg. Different angles may be more appropriate for each person. After taking a PRI home study course, I’ve really begun to appreciate the importance of a neutral lumbar spine in the frontal plane. Actively engaging the bottom lateral abdominal wall stabilizes the pelvis quite well (obliques and QL).
Action: Use the posterior hip muscles to pull the femoral head into the acetabulum (ball into the socket) and raise the knee up.
Prescription: I vary this depending on the patient. Sometimes I have people hold for 3-5 breaths. Sometimes 3 sets of 20 reps. Sometimes 1 set up to 50-75 reps (sometimes this is needed to gain the awareness). I have said a few times if you can’t do 50 reps, then you need to do them, but for some people it might be comparing to what the other side can do; I’m looking for symmetry. I rarely use resistance, if it’s too easy, we’ll move on to more functional movements.
Faults: Using hip flexors (TFL, rectus femoris) or hamstrings (biceps femoris). Usually this occurs with rolling the pelvis back.
Manual Assistance. Approximating (centrating) the femoral head in the acetabulum through longitudinal pressure through the knee does this quite well. Palpating the intended muscles helps too.