CLINICIAN EDUCATION:  Therapeutic Exercise

Does a muscle that tests weak automatically need strengthening?

The functional unit of a muscle, a myofibril, is comprised of 2 primary fibers: actin and myosin. Muscles have optimum lengths determined by the amount of actin and myosin overlap. At optimal length, the actin and myosin are able to couple and create a ratcheting force to pull the fibers closer together and create a concentric contraction (shortening) to move the corresponding joint.

It is well accepted that a muscle that is too long (>4 mm of striation spacing), the muscle well test weak during manual muscle testing (MMT) with a grade of 4/5 or less. Strengthening exercises are required to provide appropriate neural input and muscular strengthening. However, the muscle may be inhibited by a hypertonic (short/tight) antagonist that needs to be appropriately lengthened to allow the agonist to contract properly.

However, a muscle that is too short (2.0 mm or less of striation spacing) will also test weak. In this case, there exists excessive overlap where the actin and myosin are already so contracted that the fibers cannot slide further to create a concentric contraction. In this instance, strengthening the weak muscle will actually make it more hypertonic and, subsequently, weaker if the person is even able to perform the exercise. These shortenings are most likely due to a myofascial adhesion where a more appropriate treatment option would be myofascial release, instrument assisted soft tissue mobilization and/or stretching to restore appropriate muscle length which should restore strength by allowing appropriate sliding of the actin and myosin filaments.