THE FUNCTION OF THE BRACHIORADIALIS DURING FOREARM ROTATION

Abstract

The objectives of this study were to elucidate the function of Brachioradialis during forearm rotation to determine whether it is a neutralizing muscle and a protector of hyper-rotation by eccentric contraction.

The distance from the brachioradialis (BRAR) origin to insertion was measured on 10 left fresh frozen cadaveric arms using an electromagnetic tracking system. This was done in 10¢^aincrements over the full range of forearm rotation. In addition, fine-wire electrodes were placed in the BRAR of twelve living subjects. EMG data was collected as the subject rotated the forearm in both a pronating and a supinating direction.

The muscle length data shows that length is shortest at neutral and greatest closer to full rotation in either direction. When rotating from full pronation to neutral the EMG data show a steady increase while the muscle length decreases indicating a concentric contraction. When rotating from neutral to full pronation the muscle length increased and with load the EMG level increased indicating an eccentric contraction. During rotation from full supination to neutral, the EMG activity increased slightly with the muscle length, indicating a concentric contraction. When rotating from neutral to full supination, the EMG level remained variable while the muscle length increased indicating an eccentric contraction or a passive stretch.

EMG activity can occur during isometric, eccentric, or concentric contractions, the accompanying muscle length data is useful for establishing the direction of the activity. We conclude BRAR is a neutralizing muscle as it has a linear relationship with EMG activity when returning the forearm to neutral. It also acts eccentrically slowing extreme pronation and thus it has a dynamic effect on DRUJ stability.

This knowledge will assist surgeons in Tendon Transfer surgery and injury to the Brachioradialis muscle.