Since the etiology of tennis elbow (lateral epicondylitis) is poorly understood, we studied the anatomical changes in the extensor carpl radialis brevis (ECRB) muscle during elbow joint rotation. Specifically, we measured ECRB sarcomere length, using an intraoperative laser diffraction procedure that measures muscle sarcomere length with an accuracy of +/- 0.05 micron. We found an unexpected biphasic response in ECRB sarcomere length as the elbow was rotated from full extension to full flexion. The initial sarcomere length of 3.49 microns, with the elbow extended, was gradually changed to 3.68 microns, 3.34 microns, 3.81 microns, and 3.45 microns with progressive elbow flexion. Based on the very nonlinear mechanical properties of skeletal muscle, this "double lengthening" of the ECRB during progressive flexion would impose intense eccentric contractions on the muscle itself. Given that eccentric contractions cause muscle injury and subsequent inflammation, these findings may provide insights into the etiology of lateral epicondylitis.