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Half-Hearted Back Muscles

Muscles of the low back fatigue easily, especially in patients with low back pain. What happens to low back muscles as they get tired? Fewer muscles fibers fire, so other muscles must take over. This is different from the way other muscles in the body respond to fatigue.

Electromyographic (EMG) studies of most muscles show that as they become tired, they usually increase their firing activity. Not so with low back muscles. Back muscles work hard until about 55 percent of the muscle is active. You might say the muscle is only "half tired" before it gives up and turns the job over to another muscle.

Muscles in the buttocks and hip work with the low back muscles. A thick, tough fibrous covering holds them together. They share the ability to move the spine into an extended position. Muscles that work together in this way are called synergistic.

Studies of normal human movement using EMG are being done at the Musculoskeletal Research Lab in Syracuse, New York. Researchers say this information can affect how the low back muscles are strengthened. Doing repetitions beyond the 55 percent fatigue level won't increase activity in the muscles of the low back. More studies will help determine better methods to target the low back muscles once they start to fatigue.

References: Brian C. Clark, MS, et al. Derecruitment of the Lumbar Musculature With Fatiguing Trunk Extension Exercise. In Spine. February 1, 2003. Vol. 28. No. 3. Pp. 282-287.

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