A recent article on an EMG study of muscle activity in the normal shoulder has changed some of my thoughts about muscle testing.1 I had always heard that muscles should be tested throughout their range of motion since the pain may only occur at a particular range. For example, a supraspinatus muscle may not be painful when tested at zero degrees coronal abduction and may exhibit pain at 90 degrees abduction. This concept allows the clinician to be more specific especially for an individual who experiences pain only at a specific range of testing. The problem with this theory is that as the range of motion changes, synergistic muscles kick in and may be more active than the muscle tested in its neutral position.
According to Kronberg, et al.,1 during coronal abduction of the shoulder, "there was no significant difference between the supra- and infraspinatus." From 0-75 degrees abduction, the supraspinatus is working only slightly harder than the infraspinatus, and at 90 degrees coronal abduction to 180 degrees, the infraspinatus is more active than the supraspinatus. At 150 degrees coronal abduction, the infraspinatus is much more active than the supraspinatus. It is thought that at 150 degrees coronal abduction, the stability function of pressing the humeral head into the glenoid fossa by the infraspinatus comes into play. Of course, at 90-150 degrees coronal abduction, the middle and anterior deltoid are also extremely active. It is reported that specificity for the supraspinatus with minimal activity of the middle deltoid may occur if the test occurs with the arm abducted 90 degrees, forward flexed 30 degrees with the arm extended, and the hand internally rotated with the thumb down.2,3
In testing shoulder external rotation, it must be realized that the supraspinatus is active throughout external rotation because it lies behind the longitudinal axis of motion.1 But at least in the following ranges of testing external rotation, the infraspinatus is significantly stronger than the supraspinatus, in the neutral position (arm at the side) and at 90 degrees abduction. While testing external rotation in the neutral position and at 90 degrees, the infraspinatus will exhibit a marked difference over the accompanying supraspinatus if the infraspinatus is tested with the forearm additionally laterally rotated between 20 to 60 degrees.
It appears that testing the infraspinatus after the supraspinatus might help incriminate the supraspinatus if the infraspinatus testing pain is substantially less.
Of course, after functional testing leads us to the area of pain, we should also palpate for the most tender area. We should realize that a double lesion may also be present. Sometimes frictioning one of the tendons to an anesthetic state may help determine which tendon is involved on repeat muscle testing.
- Kronbert, M.; Nemeth, G.; Brostrom, Lars-Ake. "Muscle activity and coordination in the normal shoulder." Clin Orth & Rel Res 1990; 257:76-85.
- Hammer, W.I. Functional Soft Tissue Examination and Treatment by Manual Methods: The Extremities. Gaithersburg, Md., Aspen 1991.
- Yocum, L.A. "Assessing the shoulder: history, physical examination, differential diagnosis, and special test used." Clin Sports Med 1983; 2:285.
Warren I. Hammer, M.S., D.C., D.A.B.C.O.
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