The Ups and Downs of the Clamshell Exercise

Background

Clamshells are one of the most prescribed exercises for individuals with knee pain. This exercise is intended to target the hip abductors, specifically the gluteus medius muscle. EMG studies have found good activation of both the gluteus medius and gluteus maximus muscles during the clamshell. Additionally, the clamshell has been shown to preferentially activate the gluteus medius while minimizing activity in the tensor fascia latae (TFL).

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Gluteus Medius over TFL
From a clinical movement impairment syndrome perspective, the TFL is often found to be dominant and hypertonic. Many patients with hip stability issues preferentially engage the TFL instead of stabilizing the hip using the Glut Med. Since the TFL continues onto the Iliotibial Band (ITB), symptoms often present along the lateral leg when the TFL is hypertonic. As Shirley Sahrmann writes, "the attachment of the TFL-ITB to the patella and the lateral aspect of the tibia contribute to impairments of the patella and to lateral rotation of the tibia." 
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Muscle Function
The TFL is a hip flexor, abductor and medial rotator. 
The Glut Med is a hip extensor, abductor, and external rotator. Additionally it stabilizes the pelvis in the frontal plane. 

Common Compensations

While the clamshell is intended to have good activation of the gluteal muscles, the exercise will only be as effective as the person instructing the movement. In regards to movement patterns, the body always takes the path of least resistance (i.e. the repetitive, facilitated movement pattern). Individuals who will benefit from performing the clamshell exercise likely have difficulty engaging the gluteus medius. Proper cueing is necessary to avoid compensatory motions. 

Two Most Common Compensations:
1) Rolling the Pelvis Backward. To avoid activating the posterior gluteus medius (PGM), clients will often rock their lumbar spine and pelvis backward. Since the PGM is weak or under-facilitated, other local muscles will try to complete the movement. (Video: 1:51)
​2) Lumbar Side Bending. This compensation is common across the population, but especially prominent in individuals with larger hip bones.  When the PGM is not activating, trunk muscles (such as the obliques and quadratus lumborum) will attempt to compensate for the poor movement. To minimize lumbar compensation, always make sure the lumbar spine is positioned in neutral. Putting a pillow underneath the abdomen can be beneficial in clients with larger hips. (Video: 2:35)

Move from The Movement Corner

How to Prescribe

The video below contains a 3-part progression for performing the clamshell exercise. When the client can perform 20-30 repetitions without compensation, I progress to the next level of the exercise. 

Author: Jim Heafner PT, DPT, OCS

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Citations:
​Sahrmann, S. (2002). Diagnosis and treatment of movement impairment syndromes. St. Louis: Mosby.