Stabilizing function of the long head of the biceps in the hanging arm position.

The contribution of the long head of the biceps (LHB) to shoulder stability was studied. Nine fresh-frozen cadaveric shoulders were tested in the hanging arm position. The muscle belly of the long head of the biceps was removed and replaced with a spring device to apply load to the long head of the biceps. An electromagnetic tracking device was used to record the positions of the humeral head (1) without load and with loads of 1.5 kg and 3 kg on the long head of the biceps, (2) with 1.5 kg of force to the proximal humerus in three different directions, and (3) in three different rotations of the arm. Displacement in the anterior and posterior directions was significantly decreased by long head of the biceps loading and was less significant in internal rotation. Inferior displacement in external rotation was significantly decreased by long head of the biceps loading. It is concluded that in the hanging arm position, the long head of the biceps could, if contracted, provide some stabilizing function to the humeral head in all directions, and more importantly, in anterior/posterior directions. Furthermore, the stabilizing function of the LHB is influenced by rotation of the arm.