Vol. 16 Issue 10
sEMG Case Study
What Does the Quadratus Lumborum Do?
Therapists, without the guidance of sEMG, make many assumptions about which muscles are performing any given exercise. Therapists rely on anatomy textbooks for those "actions" attributed to any given muscle. My experience over 15 years of doing sEMG is that we have much to learn about functional activity that goes beyond the biomechanical origin, insertion and line of pull of any given muscle.
When an exercise is performed, therapists generally have to rely on the fact that the prime movers are responsible for the successful completion of the task. While palpation may be helpful, it does not provide discriminatory distinction of active muscles in many cases. Since a muscle does not fire in its entirety, dysfunction may be present in isolated motor units, making the identification even more difficult.
Anatomy texts typically identify the quadratus lumborum (QL) as a muscle that is a prime mover in lateral trunk flexion and hip-hiking, as an assist with trunk extension when acting bilaterally, and stabilizer of the lower ribs against the diaphragm. Richardson et al describes the QL function as a stabilizer, the medial fibers acting with the local system (controlling stiffness, intervertebral alignment and lumbar posture) while the lateral fibers of the QL function act as part of the global system (moving the spine and transferring load directly between the thoracic cage and pelvis).1 Travell and Simons cite a study that makes the stabilizing function of the QL poignantly clear: complete bilateral paralysis of the QL makes walking impossible, even with braces.2
In addition, they cite rotation of the lumbar spine as a function of the QL on the contralateral side to the direction of rotation. They consider this muscle to be the most frequent cause of low back pain of muscular origin, an observation with which I agree strongly. Its unique dynamics and structure may be responsible for my observation that in the first hours post low back strain, myofascial trigger points are often limited to the QL, only to spread to include secondary muscles as compensatory postural adjustments are made to pain and activation of trigger points in the areas of secondary overload.
Observation of QL activity is difficult with sEMG, as the muscle lies deep and access is limited to the lateral fibers. Isolation of any data sample to only QL motor units is unlikely. Perhaps, due to the many functions and the multiple directional pull of the various components of the QL, particular care must be taken in the examination of active trigger points and the frequent task-specific inhibition that accompanies them.
A patient presenting with chronic low back pain is asked to side bend laterally, sitting in a chair with the pelvis stabilized. The contralateral QL is expected to function as an eccentric "guy wire" to side bend away from the active QL, and then function concentrically to return the trunk to the upright position. Amplitude is greater in a concentric contraction than in an eccentric one, but one should see activity in both directions of movement. In this patient, the eccentric amplitude of 100 uVs is exceeded by the concentric amplitude of 200 uVs, suggesting this muscle is working well.
However, inhibition is apparent when the patient is prone and performs prone extension of the trunk, an activity expected to include activation of the QL bilaterally. Instead, they are silent on both sides. These findings reinforce the need to examine multiple motor unit systems within any muscle in multiple tasks and to seek to determine the pattern of activation vs. inhibition (or hyperactivity) that one observes. It is in the pattern of muscle use (or lack thereof) that one can determine the perpetuating factors in task-specific inhibition, unlocking the key to what is needed for successful treatment.
1. Richardson, C., et al. (1999). Therapeutic Exercise for Spinal Segmental Stabilization in Low Back Pain. New York: Churchill Livingstone.
2. Travell, J., & Simons, D. (1992). Myofascial Pain and Dysfunction. The Trigger Point Manual. Vol. 2. Baltimore: Williams & Wilkins.
Barbara J. Headley has a clinical practice in Boulder, CO, and is active in lecturing after years of research with sEMG. You can learn more about her products and courses by calling 720-564-0212 or visiting her Website at barbaraheadley.com