Effects of chronic lengthening or shortening of muscle: "stretch weakness" and "adaptive shortening"

Postural alignment depends on the resting length of the soft tissues, including muscle, that surround each joint. Abductors and adductors, or flexors and extensors, produce opposing forces and moments. The result of this "tug of war" determines a joint's habitual position or posture. A posture is a kind of immobilization, and animal studies show how muscles' resting lengths and length-tension properties adapt or change when they remain at a fixed length for weeks or months.

If a person's pelvis is not level in the frontal plane, postural compensations in the hip joints and spine are necessary to align the body's center of gravity over the base of support. This posture is associated with changes in the lengths of various muscles. If the pelvis' left side is lower than its right side:
the right hip joint is chronically adducted, causing the right hip's abductor muscles to elongate. the right glenohumeral joint is chronically abducted, causing the right glenohumeral abductors to shorten. the left hip joint is chronically abducted, causing the left hip's abductor muscles to shorten. the left glenohumeral joint is chronically adducted, causing the left glenohumeral abductors muscles to elongate.
the right shoulder is lower than the left, and the right scapula is rotated downward compared to the left scapula. In this position, shoulder elevation produces impingement relatively early in the right shoulder's range of motion.

A muscle that is chronically elongated adds sarcomeres. Over time, its resting length increases (from l0 to l1) and its length-tension curve shifts rightward.
A muscles that is chronically shortened loses sarcomeres. Over time, its resting length decreases (from l0 to l1) and its length-tension curve shifts leftward.

When therapists use a standard muscle test to evaluate the strength of two muscles that have been immobilized in different positions, they detect differences.
In the present example, muscles that are chronically elongated, the right hip abductors and the left shoulder abductors, produce less force (Flong) than their shortened counterparts on the opposite side (Fshort) at the joint angle where therapists perform a standard muscle test. The elongated muscle's relative weakness at this point in the range of motion is called "stretch weakness" (Kendall, McCreary, & Provance, 1993, p.334). Muscles that are chronically shortened, the left hip abductors and the right shoulder abductors, produce more force (Fshort) than their elongated counterparts on the opposite side (Flong) at the joint angle where therapists perform a standard muscle test. The shortened muscle's relative strength at this point in the range of motion is called "adaptive shortening" (Kendall, McCreary, & Provance, 1993, p.334-335).

Research and references that relate to length-associated changes in muscle

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Last updated 11-8-01 ©Dave Thompson PT
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