Muscle Testing -- EMG

By Warren Hammer, MS, DC, DABCO

Many studies are conducted today to determine which muscles are active during particular motions in sports and other activities. It is interesting to note that just because the origin and insertion of a muscle makes us think that a muscle should work during a particular movement, EMG studies show that it may be inactive during performance of that motion.¹Although EMG studies do not measure the force of a contraction, Basmajian² states that "Almost without exception, investigators report either linear relationships or a more than linear increase of the EMG signal with increasing force." So it is apparent that if muscle activity as measured by EMG is increasing, then the muscle force is also increasing. Important questions to answer about muscles are at what range of motion are they most active and what exercises are most effective for a particular muscle.

Since the subscapularis is an anterior shoulder muscle and an internal rotator, everyone typically strengthens the subscapularis by stressing medial rotation, usually to support anterior shoulder instability. But in a series of EMG studies of the glenohumeral muscles during a baseball rehabilitation program,³ based on the criteria of an exercise being significant if it generated at least 50 percent of its predetermined maximum contraction over three consecutive arcs of motion for that muscle, the subscapularis did not qualify by the internal rotation exercise. The two best exercises for the subscapularis turned out to be elevation of the arm in the scapular plane (30 degrees from the coronal plane) with the arm internally rotated (thumb down), and the military press (holding the weight with the elbow-flexed arm abducted about 30 degrees and lifting the weight straight overhead). While a principle test for testing the strength of a supraspinatus is with the arm elevated 90 degrees in the scapular plane, with the thumb down, the military press exercise is more effective than the scapular plane exercise for strengthening the supraspinatus.

It is often stated that a muscle should be tested in a full range of motion to determine where it shows its weakness, but EMG studies contradict this concept. Muscles do not necessarily work at the same intensity as the range of motion changes. For example, if you were to test the supraspinatus (resisted abduction) in the coronal plane of shoulder elevation, it has been found by EMG studies that you cannot distinguish between the infraspinatus and supraspinatus. EMG studies have shown that there was no significant difference between these two muscles in activity during this motion.⁴

EMG studies have also shown other interesting aspects of muscle activity. While one would think that if an area is injured the muscles protecting the injured area would increase their activity, it appears that these muscles become inhibited. In the elbow, the medial collateral ligament (MCL) protects against increased valgus which occurs especially during the late cocking and acceleration phases of throwing. EMG studies on an elbow with MCL insufficiency⁵ showed that the medial muscles which might protect against valgus, such as the flexor carpi radialis and pronator teres, did not increase their protective activity, while the extensor carpi radialis brevis and longus, which originate on the lateral elbow, increased their activity which might even increase the valgus stress. Is this nature's way of telling us to rest the area? The EMG study is telling us to pay more attention to strengthening the muscles which dynamically protect the deficient area since these muscles are not functioning up to par. Possibly they are being inhibited due to pain. Increased anterior instability in the shoulder and decreased activity of the subscapularis and serratus anterior has also been shown by EMG in throwing athletes.

References

Brunnstrom S: Muscle testing around the shoulder girdle. J Bone Joint Surg, 23:263-272, 1941.
Basmajian, JV, De Luca CJ: Muscles Alive: Their Functions Revealed by Electromyography, ed. 5. Williams & Wilkins, Baltimore, 187:1985.
Moseley JB, Jobe FW, Pink M, Perry J, Tibone J: EMG analysis of the scapular muscles during a shoulder rehabilitation program. Amer J of Sports Med., 20(2):128-134, 1992.
Kronberg M, Nemeth G, Brostrom L: Muscle activity and coordination in the normal shoulder. Clin Orth & Rel Research, 257:76-85, 1990.
Glousman RE, Barron J, Jobe FW, et al: An electromyographic analysis of the elbow in normal and injured pitchers with medial collateral ligament insufficiency. Amer J of Sports Med, 20(3):311-317, 1992.

Warren Hammer, M.S., D.C., DABCO
Norwalk, Connecticut

Editor's Note:

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