The Supraspinatus and Infraspinatus Muscles
By Jeffrey R. Cram, PhD
Muscles that lie "deep" to the recording electrodes are sometimes very difficult to record. Recordings from the supraspinatus, which is deep to the fibers of upper trapezius, provides such an example. For this reason, we prefer to call it by its location, the suprascapular fossa placement. The infraspinatus, on the other hand, provides a clear, specific signal which can be easily picked up at the surface. Both muscles are frequently considered in rehabilitation of the shoulder.
Suprascapular Fossa (Supraspinatus) Placement
The supraspinatus muscle participates in the abduction of the arm, and controls the head of the humerus in the glenoid fossa. The fibers of supraspinatus lie deep to middle and upper fibers of trapezius, arise from the supraspinatus fossa and insert on the greater tubercle of the humerus.
To place electrodes at this site, palpate the spine of the scapula, locating its lateral aspect. The electrodes are placed there 2 cm apart, directly above the spine of the scapula, over the suprascapular fossa.
The infraspinatus is involved in lateral rotation of the shoulder joint, along with stabilization of the head of the humerus in the glenoid cavity. The fibers arise from the infraspinatus fossa, below the spine of the scapula and inserts on the greater tubercle of the humerus. It is innervated by the superior cord of the brachial plexus, from the spinal nerves of segments of C4, C5 and C6.
To place electrodes at this site, palpate the spine of the scapula. Two closely spaced electrodes (2 cm apart) are placed parallel to and approximately 4 cm below the spine of the scapula, on the lateral aspect, over the infrascapular fossa of the scapula. Caution should be made to avoid placement over posterior deltoid.
Here, shoulder elevation is seen to be associated with simultaneous recruitment at the upper trapezius and supraspinatus sites, but does not involve the middle deltoid and very little infraspinatus. The recruitment pattern noted during abduction indicates that the deltoid fires first during the initiation of the movement. In both the thumb up and thumb down positions, simultaneous sEMG recruitment is noted at the supraspinatus and upper trapezius sites.
Note that during lateral rotation, the activity at the infraspinatus site is greatest (Fig. 4); while during lateral rotation with extension of the shoulder, the sEMG activity of the posterior deltoid site is greatest. In addition, note that a forward head posture, thoracic kyphosis, along with the downward pull of the rib cage may allow the internal rotators of the shoulder (both scapular and glenohumeral) to shorten, thus altering the normal length tension relationship for the infraspinatus. Also, the joints of the cervical spine related to the muscle, i.e. C4, C5, and C6 may affect the sEMG resting or recruitment patterns, along with the glenohumeral joint, (particularly the anterior glide stability, and the inferior glide capacity), and the acromioclavicular joint. *
* For more information, call (530) 478-9660; or visit Dr. Cram's Web site at www.semg.org.
Dr. Cram is the director of the Sierra Health Institute in Nevada City, CA, where he teaches workshops on sEMG.