The clamshell is a low-impact, bodyweight exercise used in physical therapy and general fitness to improve hip and pelvic strength. Performed while lying on one side, the movement involves rotating the top leg outward, mimicking the opening of a clam. The primary goal is to isolate and strengthen the muscles that govern hip abduction and external rotation.
Primary Muscle Activation: The Gluteus Medius
The muscle most directly activated and strengthened by the clamshell is the gluteus medius, a smaller muscle situated on the outer surface of the hip. This muscle is the main hip abductor, responsible for moving the leg away from the midline of the body. Studies indicate the clamshell is effective for selectively activating the gluteus medius compared to other gluteal movements.
The gluteus medius plays a significant role in stabilizing the pelvis during single-leg activities, such as walking, running, and climbing stairs. When this muscle is weak, it can lead to a drop in the pelvis on the unsupported side, often referred to as Trendelenburg gait. Strengthening the gluteus medius helps mitigate this instability, which is linked to overuse injuries in the knee.
Supporting Muscles and Stabilizers
While the gluteus medius handles the majority of the work, several other muscles assist in the movement or provide necessary stabilization. The gluteus minimus, which lies beneath the medius, works as a hip abductor and assists with stabilization of the hip joint. The upper fibers of the gluteus maximus also contribute to the external rotation and abduction movement.
The tensor fasciae latae (TFL) also participates in hip abduction and rotation, though its activation should remain secondary to the gluteus medius to avoid excessive strain. The deep hip external rotators, such as the piriformis, are engaged during the outward rotation of the thigh bone. Maintaining a stable side-lying posture requires isometric contraction from the abdominal muscles and the quadratus lumborum to prevent the trunk and pelvis from rolling backward.
Technique for Effective Clamshell Execution
Proper technique is necessary to ensure the clamshell movement effectively targets the gluteus medius and avoids compensation from larger muscles. Begin by lying on your side with your hips and shoulders stacked vertically and your knees bent to approximately a 45-degree angle. The ankles and feet should remain stacked and aligned with the hips.
The movement starts by lifting the top knee toward the ceiling while keeping the feet in contact with each other throughout the lift. The most common error is allowing the pelvis to rock or roll backward to gain a greater range of motion, which shifts the work away from the gluteus medius. To prevent this, ensure the top hip bone does not move backward.
The movement should be slow and controlled, particularly during the lowering phase of the top knee. Only open the knee as far as you can without any movement in the pelvis or lower back. Placing a hand on the top hip can help monitor stability.