Four muscles form the rotator cuff group -- supraspinaturs, infraspinatus, teres minor and subscapluaris, according to the 1976 manual "Physical Examination of the Spine & Extremities" by Stanley Hoppenfeld. All four of the rotator cuff tendons insert onto the greater tuberosity of the humerus. Collectively, these muscles assist in stabilization of the humeral head in the glenoid fossa of the shoulder joint. Applying rotator cuff strength tests in the overall shoulder assessment gives the evaluator an indication of a possible underlying issue.
Anatomy
An easy way to remember the rotator cuff group is by its acronym "sit" -- supraspinatus, infraspinatus and teres minor. The name is given by the location of the cuff muscles' placement on the shoulder blade. Passive extension of the arm makes the tendons more prominent, Hoppenfeld says. The fourth muscle, supscapularis, is located on the front of the shoulder blade and can be difficult to find.
Significance
Strength tests are performed to measure the ability of that muscle when applied to a resistive force, says the 2006 manual "Examination of the Shoulder: The Complete Guide," by Edward G. McFarland. Poor indicators of strength can allude to weak or torn muscle or tendon. Performing muscle testing offers another clue for finding the source of pain or complication.
Grading Strength
Muscle tests range from levels 0 to 5, with 5 normal and 0 weak, McFarland says. This scale is the most frequently used strength testing system. Level 5 strength indicates the shoulder has full range of motion and strength and no discomfort. Level 4 means good, with full range of motion and some strength with little to no discomfort. Level 3 is fair, with full range of motion when minimal pressure is applied and increased level of discomfort. Level 2 is poor; minimal range of motion and little to no strength with pain. Level 1 is minimal contraction of the muscle but no range of motion. Level 0 is no muscle contraction or movement.
Strength Tests
There are three common muscle tests for the rotator cuff group: internal rotation, external rotation and abduction. Resistance applied in external rotation isolates the teres minor and infraspinatus. Opposite, internal rotation tests the strength of the supscapularis. Abduction tests the ability of the supraspinatus, commonly torn the most out of the rotator cuff group, Hoppenfeld says.
Considerations
Although the muscle strength grading system is a useful tool, there are validity issues, McFarland says. First is the ability to feel small differences in strength in a Level 5 versus a Level 4. The second is inconsistent grading when being evaluated by different people.
References
- "Physical Examination of the Spine & Extremities"; Stanley Hoppenfeld; 1976
- "Examination of the Shoulder: The Complete Guide"; Edward G. McFarland; 2006
