Brachioradialis Muscle Function During Forearm Rotation from EMG, Anatomy, and Biomechanical Modeling
+1Bader, JS; 1Boland, MR; 1Spigelman, T;1Uhl, T;1Pienkowski, D
+1University of Kentucky, Lexington, KY
Senior author: joseph.bader@uky.edu
RESULTS:
Muscle length data (Fig.1) shows that the BRAR is shortest at neutral
and longest as the forearm reaches its maximum position in either
direction. EMG data shows that as the arm pronates from full
supination, EMG activity decreases from its maximum to its minimum
around neutral and then slightly increases toward full pronation. As the
arm supinates from full pronation, the EMG signal increases. The
BRAR effect on calculated DRUJ loading is shown (Fig. 2).
% of Maximum Muscle
Activity/EMG Activity
95
90
85
80
75
70
% Maximum Muscle Length
% Maximum EMG During Weighted Pronation
% Maximum EMG During Weighted Supination
65
N
10
S
20
S
30
S
40
S
50
S
60
S
70
S
80
S
90
S
80
P
70
P
60
P
50
P
40
P
30
P
20
P
10
P
60
Forearm Rotation (Degrees)
Force (N)
Figure 1: Comparison of BRAR muscle length to BRAR EMG activity
during prono-supination.
Average DRUJ Transverse Force
240
210
BRAR Removed
BRAR Intact
180
N
10
S
20
S
30
S
40
S
50
S
60
S
70
S
80
S
90
S
P
P
20
10
P
P
30
40
P
P
50
60
80
70
P
P
150
Average DRUJ Shear Force
180
BRAR Removed
BRAR Intact
120
60
N
10
S
20
S
30
S
40
S
50
S
60
S
70
S
80
S
90
S
P
P
10
20
P
P
30
40
P
P
60
70
50
P
0
P
Force (N)
Forearm Rotation (Degrees)
80
METHODS:
This study had two components: one cadaveric, the other in vivo.
Regarding the first component, the origin and insertion of 15 muscles
were marked on the upper extremity of nine fresh cadaver specimens.
The positions of these muscles were digitally measured at various stages
of pronation (P) and supination (S) by using an electromagnetic 3D
tracking sensor (Motion Star, Ascension Technologies, Burlington, VT).
Data were collected for the muscles at 10° increments of simulated
forearm pronosupination (PS) throughout the entire range of motion.
Coordinates of the origins and insertions were recorded to determine the
length of the muscles of interest. For the purposes of this study, the
assumption was made that the muscles formed a straight line from the
origins to the insertions. The lengths of the muscles were normalized as
a percentage of the longest length observed during rotation. The muscle
data was also used to create a vector based model of the forces occurring
at the distal radioulnar joint (DRUJ) (1). This model was based on
maximum muscle forces from physiological cross sectional area data.
Joint reaction forces were calculated about the transverse axis (radioulnar axis) and the shear axis (palmar-dorsal axis). The resultant force
acting at the DRUJ was determined from the resultant of the shear and
transverse forces. To determine the role that brachioradialis (BRAR)
had on DRUJ loading, the muscle was removed from the model. The
forces at each position were averaged for each of the nine specimens
before and after BRAR removal.
For the in vivo component of this IRB approved study, subject
consent was first obtained. Then, a fine-wire electrode was placed in the
BRAR of twelve subjects, approximately 5cm below the lateral
epicondyle. In addition, an electromagnetic tracking sensor was attached
to each subject’s arm so that the angle of forearm rotation could be
measured in conjunction with the EMG signal. The subjects were asked
to grab a handle with an 18N load attached to it. They were then was
asked to dynamically move their hand from pronation to neutral and
back, and then from supination to neutral and back. Each task was
performed five times. To make the EMG data comparable to the
cadaveric data, the EMG data was divided into 10° arcs based on the
angle provided by the electromagnetic tracking system. The data were
also grouped by whether the forearm was rotating in the direction of
pronation or the direction of supination. All EMG values were then
normalized as a percentage of the maximum signal observed for each
individual subject. The values contained within each arc for a specific
subject were then averaged. The final value for each 10° increment was
obtained from the average of all subjects. The normalized muscle length
and EMG data were then compared.
Muscle Length and EMG Comparison
100
Forearm Rotation (Degrees)
Average Resultant Force
Force (N)
INTRODUCTION:
Quantification of muscle function is essential for the solution of
musculoskeletal challenges. Accurate muscle placement data is needed
to understand the role of individual muscles and their contribution to
overall biomechanical function. The most commonly used methods of
determining muscle function involve electromyography (EMG),
however this only elucidates muscle activity and is thus insufficient. To
improve our understanding of forearm motion as well as forces across
the distal radio ulnar joint, this study incorporated EMG data with
muscle length analysis and biomechanical modeling to clarify the
function of the Brachioradialis (BRAR) muscle during forearm pronosupination.
260
235
210
BRAR Removed
BRAR Intact
185
160
80
P
70
P
P
P
P
P
P
P
60 50 40 30 20 10
N
10
S
20
S
30
S
40
S
50
S
60
S
70
S
80
S
90
S
Forearm Rotation (Degrees)
Figure 2: Effect of BRAR on DRUJ joint reaction forces along the
transverse axis (top), shear axis (middle), and resultant forces (bottom).
DISCUSSION:
The results of this study show that the BRAR muscle has important
roles in forearm motion and it also has a key role in governing the
amplitude of forces across the DRUJ. As clearly shown by the EMG
data, the BRAR muscle initiates pronation through a concentric
contraction. Second, the BRAR muscle decreases transverse forces for
most of the range of PS, while acting to compress the joint at full
pronation or supination. Third, along the dorsal palmar axis, the BRAR
works to reduce the amplitude of DRUJ loads except near mid-rotation.
The supination curve increases from neutral to full supination where the
muscle is at its longest. This would be consistent with an eccentric
contraction which would help slow the resulting supinating motion.
When the forearm is in a neutral position and supporting an object of
significant weight, the BRAR protects the DRUJ by lifting the radius up
off the ulna, thereby protecting the ulna from a significant bending
moment. In terminal PS, firing of the BRAR dynamically assists in
generating stability from translational motion at the DRUJ.
This research shows that the BRAR appears to function more with
pronator muscle actions than with supinator muscle actions. These
findings are novel and will help shed light on the potential
biomechanical consequences of muscle loss in injury or tendon transfer
procedures.
REFERENCES: (1) Bader, JS et al., Combined ORS (2007), Paper
#268.
Poster No. 1850 • 55th Annual Meeting of the Orthopaedic Research Society