Clamshells or clams, as some call them, are one of the most common exercises prescribed by a physical therapist. The exercise targets specific muscles of the hip that tend to be weak in a large patient population. It is clinically relevant for treatment of a variety of pathologies including low back and knee pain, balance deficits, and following knee or hip replacements.
The exercise is performed with the patient on a table or the floor in a side lying position with the legs and feet stacked and heels together. Other than, that, physical therapists have taught patients the exercise with the hips, knees, and pelvis positioned at varying angles based on past clinical experience. A recent article came out in the Journal of Orthopaedic and Sports Physical Therapy discussing the effects of varying hip angles and pelvic positions on muscle recruitment during the clam exercise.
In a previous study, it was determined that the best position for the knees to be in during the exercise was with the knees bent at 90 degrees, so this knee position was utilized for this study. In a current study, it was found that the most important position to facilitate activation of multiple muscle groups of the posterior hip was related to pelvic positioning. Specifically, when performing clams, it is important to make sure that the spine and pelvis are in a neutral position. When the pelvis tilts/rolls backwards, it decreases the activation of the muscles in the back of the hip (the targeted muscles for the exercise.)
Furthermore, it was found that the best angle for the hip to be positioned at for proper muscle recruitment was at a 60 degree angle as compared to 90 degrees or fully extended (0 degrees).
Willcox, EL, and Burden, AM: The influence of varying hip angle and pelvis position on muscle recruitment patterns of the hip abductor muscles during the clam exercise. J Orthop Sports Phys Ther 43:5, 2013.