This study evaluated the effect of the lacertus fibrosus on the biceps brachii moment arms to determine the lacertus importance in forearm strength. The work proceeded from the null hypothesis that the lacertus fibrosus has no effect on flexion/extension and supination/pronation.

Ten fresh-frozen cadaveric specimens were mounted in an elbow simulator to measure tendon excursion, elbow flexion/extension (FE) and forearm pronation/supination (PS) angles. FE moment arms were calculated throughout FE excursion at multiple, constant PS angles and PS moment arms were measured throughout PS excursion at multiple, constant FE angles. Tests included the lacertus intact, the superficial lacertus transected, and the entire lacertus transected along with any connections to surrounding tissue.

The intact lacertus increased the FE moment arm of the biceps with the elbow in less than 40° flexion and decreased the FE moment arm with the elbow in greater than 80° flexion, where the decrease could exceed 50%, depending on the flexion angle. The intact lacertus decreased the PS supination moment arm at all flexion angles;​ the intact lacertus even caused the biceps to act as a pronator at full elbow extension.

The intact lacertus assists in early elbow flexion and the lacertus hinders elbow flexion at greater than 80° flexion. The lacertus hinders the biceps function of supination, especially in a supinated position. Injury of the lacertus without direct biceps tendon damage can be tested best at the ends of the flexion ROM and at higher supination angles. Lacertus syndrome may be more quickly diagnosed with provocative tests that isolate the lacertus function.