Q: How do you differentiate an extensor carpi radialis longus injury from an extensor carpi radialis brevis injury?

A: Involve the fingers in testing resisted extension of the wrist.

The extensor carpi radialis longus and brevis muscle-tendon units work together to extend the wrist and abduct the hand. To get a sense of this motion, imagine a traffic cop putting his hand out to say, "Stop." The extensor carpi radialis brevis originates on the lateral epicondyle of the humerus and inserts at the base of the third metacarpal bone. The extensor carpi radialis longus lies lateral to the brevis. It originates at the lower third of the supracondylar ridge and inserts at the posterior aspect of the second metacarpal bone. To locate the supracondylar ridge, find the lateral epicondyle of the humerus and slide your finger slowly in a superior direction. You'll feel a knife-like edge on the lower lateral part of the humerus. This sharp section of bone, about an inch in length, is the supracondylar ridge. The longus tendon covers about a third of an inch to a half-inch of this ridge, just above the lateral epicondyle.

Pain felt on resisted extension of the wrist could indicate an injury to either the longus or the brevis structure. To differentiate between the two, you need to perform a variation of this movement that tests specifically for the extensor carpi radialis longus. In the standard version, the client extends the wrist and you place your hand at the back of the client's hand, below the fingers; you then try to pull the hand downward, while the client resists. For the variation, perform the same action but with your hand on the most proximal portion of the second digit. Make sure the elbow is straight as the client resists your pressure.

You can confirm your assessment by noting where the person feels the pain and which structure is tender to the touch (as compared with the same structure on the other side).