Brachioradialis Forearm Muscle: Elbow Flexor or Forearm Rotator?
Debate has been ongoing since as far back as 1756 about the role of the brachioradialis muscle in movement. Some say that it just flexes (bends) the elbow. Others insist it helps rotate the forearm to turn the hand palm up (supination) or palm down (pronation). The results of this study may put the arguments to rest.
The brachioradialis muscle goes from the lateral (outside) part of the lower humerus (upper arm) down to the wrist. At the wrist it attaches to the lateral aspect of the radius (one of the two bones in the forearm).
This study was done in a musculoskeletal laboratory at the University of Kentucky. Ten normal, healthy volunteers were recruited. None of the subjects had any prior health problems or forearm injuries of any kind. No one had a previous history of surgery or known arthritis of the elbow. Everyone was willing to have needle electromyography (EMG) testing done.
Once the fine-wire needle electrodes were implanted, testing was done. Each person performed a series of elbow movements while hooked up the EMG. The EMG was connected to a computer software program. In addition to measuring electrical impulses of the muscles, the data collected was able to show the percentage of maximal muscle activation and time of maximal activation during each task.
Testing was done with elbow flexion and rotation. Flexion was done with the forearm in one of three different starting positions (pronated, supinated, and neutral). Neutral means the hand was in a thumbs up position with the elbow bent to 90 degrees at the subject's side.
Rotation was measured in two phases: supination to neutral and pronation to neutral. All motions were tested and measured with four different loads. The various loads were designed to mimic daily activities (e.g., lifting a gallon jug of milk or a bag of groceries). Heavier loads could be used during elbow flexion compared to forearm rotation.
The subjects went through five repetitions for each position and load. A metronome (device used to set a beat) was used to help each person keep a steady pace throughout all the movements. The speed was set so that movement occurred at 130 degrees of flexion per second. Frequent, regular rest breaks were built into the test protocol to avoid over fatiguing the muscles.
The scientists set out to test four things: 1) difference between pronation, supination, and neutral forearm positions during elbow flexion, 2) difference in EMG amplitudes during different phases of muscle contraction during elbow flexion, 3) when the brachioradialis is activated the most, and 4) determine the primary (main) role of the brachioradialis and any secondary role present.
Here's what they found out. The main role of the brachioradialis muscle is to flex the elbow (starting from an extended position). This movement is called concentric elbow flexion. In addition, the brachioradialis does indeed, work as a forearm rotator.
In particular, it assists in concentric forearm pronation. It also has an eccentric role moving the forearm from a neutral position back to a supinated (palm up) position. Moving to a palm up position is an eccentric movement. This means the brachioradialis works to slow down (control) the movement. These results actually confirm the first hypothesis about the function of the brachioradialis first suggested by a man named W. Cheseden in 1756.
Why does it even matter what the brachioradialis do? There have been an increasing number of brachioradialis injuries lately. Rock climbers seem to be at greatest risk. But anyone doing any lifting with the elbow flexed and rotated can be affected. Less often, there are cases where injuries of other muscles result in weak elbow flexion. Surgeons looking for a muscle they can transfer to do the work of the main elbow flexor (biceps muscle) may want to consider using the brachioradialis.
Studies like this help surgeons understand what functions might be affected if the brachioradialis is used to replace another muscle/tendon unit. Analyzing the patient's activities and needs for movement, strength, and function can help guide the decision in tendon transfer procedures.
References: Michael R. Boland, MBChB, et al. The Function of Brachioradialis. In The Journal of Hand Surgery. December 2008. Vol. 33-A. No. 10. Pp. 1853-1859.Back