ARTICLE extensor carpi radialis brevis and extensor carpi ulnaris are members of the wrist extensor group . These muscles attach into the lateral epicondyle via what is known as the common extensor tendon because the proximal tendons of these muscles blend into each other . A better name for this group would be the common extensor belly / tendon , because as with the musculature of golfer ’ s elbow , the bellies of the muscles of the common extensor tendon also blend together before their tendons do .
Figure 4 . The muscles of tennis elbow common extensor belly / tendon group . ( Permission Dr . Joe Muscolino . The Muscular System Manual , 4th edition . Elsevier , 2015 )
As the name common extensor implies , these muscles are involved with extension . The two extensor carpi muscles cross the wrist joint posteriorly , so they extend the hand at the wrist joint . The extensor digitorum crosses the wrist joint posteriorly and the MCP and proximal and distal IP joints of fingers 2-5 posteriorly , so it extends the hand and fingers . The extensor digiti minimi crosses the wrist and the MCP and IP joints of the little finger , so it extends those joints .
Only at the elbow joint does some of the mirror symmetry between the common flexor and common extensor groups disappear . The extensor carpi radialis brevis of the common extensor group actually flexes the elbow joint because it crosses it anteriorly ( Figure 5 ). All other wrist extensor muscles of the tennis elbow group cross the elbow joint posteriorly , so they extend the elbow joint as they extend the wrist and finger joints that they cross . All of these joint actions place a stress on the musculature of this group , as well as its attachment onto the lateral epicondyle of the humerus .
As with golfer ’ s elbow , even though tennis elbow was originally described for its periosteal inflammation of the lateral epicondylar humeral attachment . Before the humeral inflammation occurred , tennis elbow involved common extensor tendinopathy ; and before the tendinopathy occurred , it began as an overuse and tightness of the myofascial tissue of the muscles involved .
In addition to the stress of the joint motions just described , tennis elbow has another cause . Whenever the fingers flex to grip an object , the finger flexor muscles ( flexor digitorum superficialis and flexor digitorum profundus ) would also cause the hand to flex at the wrist joint . To keep the wrist joint in a neutral anatomic position so that grip strength is optimal , the wrist joint must be stabilized . This is primarily accomplished with isometric contraction of the extensor carpi radialis brevis of the tennis elbow group ( Figure 6 ). For this reason , in addition to extension motions of the wrist and hand , tennis elbow is also caused by finger flexion whenever we grip an object . This is another asymmetry between golfer ’ s and tennis elbow . Whereas golfer ’ s elbow is caused solely by flexion , tennis elbow is often thought of as being caused solely extension ; but is also caused activities of life that involve finger flexion .
Figure 5 . The extensor carpi radialis brevis of the common extensor muscle group actually crosses the elbow joint anteriorly , so it flexes the forearm at the elbow joint . Lateral view of the right upper extremity . ( Permission Dr . Joe Muscolino . The Muscular System Manual , 4th edition . Elsevier , 2015 ).
JATMS | Winter 2021 | 87