Effects of Fatigue on Forearm Supination
Brandon T. Brown, MS1, Christopher C. Schmidt, MD2, Tyler J. Madonna, BS1, Zubair Sarmast, MD2, Mark Carl Miller, PhD1
1
University of Pittsburgh, Pittsburgh, PA, 2University of Pittsburgh Medical Center, Pittsburgh, PA
INTRODUCTION:
Supination strength of patients with and without distal biceps ruptures
are greater in pronated than in either neutral and/or supinated forearm
positions; these studies reported relative supination weakness from
neutral-to-supinated position (1, 2, 3). However, the clinical impact of this
isometric strength information is not well understood, because arc of
rotation and power were not simultaneously measured. The goal of this
paper is to develop and test a novel device that measures the arc of rotation
and power, while keeping supination torque constant, during a timed task.
Our hypothesis is that with the passage of time the rotation arc will
decrease in magnitude, shift from an equal supination-to-pronated to
mainly a pronated position, and the power will decrease.
METHODS:
A novel torque dynamometer was developed and validated that
measures supination arc and power, while keeping resisted supination
torque constant.
After IRB approval, ten individuals with no prior history of elbow
injury participated in this study. With the arm positioned at 0˚ of shoulder
abduction, 90º of elbow flexion and at a neutral forearm position, subjects
grasped the handle of the new torque dynamometer and were instructed
not to release this handle until testing was complete. The dynamometer
provided a torque resistive to supination, set to 25% of the average
maximum isometric supination torque for a neutral forearm arm position
taken from a previous study (2.5 Nm) (1). No resistance was applied in
the pronation direction. Subjects were asked to move a visual wheel (task)
for 5 minutes. The dynamometer also recorded both the torque applied
and the arc of forearm rotation. After testing with one arm, subjects
continued with the other arm. The dominance was randomized. The
average maximum supination and pronation angles, the arc center, and
power were extracted from the data at 5 time points during the test. A twoway mixed ANOVA was used to determine the effects of arm dominance
and time on all four variables of interest (α=0.05).
RESULTS:
The average age of the subjects was 49 ± 10 years (range: 36-61), there
were 8 right hand and 2 left hand dominant individuals. The forearm arc
of rotation decreased significantly with time (p=0.001). Both the average
maximal supination (red) and pronated (blue) positions decreased with
time (Fig. 1) (p<0.001). The arc (yellow) center significantly (p=0.020)
shifted 10˚ into the pronation range. The non-dominant arms (dotted lines)
had a shorter forearm arc of rotation, which decreased with time
(p=0.042). Arm dominance and time did not interact (p>0.138).
Forearm power output decreased with time (p<0.001) (Fig. 2) and both
time and arm dominance had significant effects on power (p<0.008).
Power changed less after 48 seconds such that the remaining time points
were not significantly different (p > 0.126). Dominant arms (solid)
significantly produced more power than Non-Dominant (dotted) arms
(p=0.008). There was no interaction between time and arm dominance
(p=0.997).
Figure 2: Forearm power production vs time: Blue solid line
indicates dominant arms and red dotted line indicates nondominant arms.
DISCUSSION:
To our knowledge, this is the first study to show that the arc of forearm
rotation significantly decreases with fatigue and shifts into a pronated
position. This shift may occur to take advantage of the brachioradialis
muscle, which is only a supinator in a pronated forearm (4). The subject
population represents the average population for individuals suffering
from distal biceps ruptures (1). It would be interesting to test biceps
deficient and repair arms to understand if a change in forearm arc occurs
as an adaptation. We would predict based on this studies findings that a
biceps deficient arm’s arc would dramatically switch into a pronated
position. This hypothesis is currently being tested.
The power significantly declined within the first 48 seconds indicating
early fatigue. This may suggest that forearm supination is not designed to
be an endurance activity. Non-dominant arms were shown to be more
susceptible to fatigue, meaning supination endurance is side specific and
dominance should be considered in supination fatigue strength testing.
The novel torque dynamometer is cost-effective and transportable
device that measures kinematics and kinetics. The current testing
population will be expanded and used as a control to understand the
physical loss of distal biceps rupture and impact of treatment. The data
highlights the fact that forearm arm position, arm dominance, and fatigue
are factors to consider when performing strength evaluations. Such
information should lead to an improved understanding of forearm
function, which could lead to improved treatments for injuries.
SIGNIFICANCE:
Forearm fatigue leads to reduction in the supination rotation arc and a
shift of this arc to a more pronated position. This data creates a control
database that can be used to understand injury states, i.e., distal biceps
rupture.
Figure 1: Forearm motion vs time: Supination is defined as a
positive angle. Solid lines indicate dominant arms, dotted lines
indicate non-dominant arms. Maximum supination is shown in
red, maximum pronation is blue, the arc center is yellow.
References: 1) Schmidt CC et al, JSES 2014;23:68-7 2) Schmidt CC et
al, JSES 2012; 2012; 21: 1623-31 3) Matsuoka J et al, JHS Am
2006;31:801-5 4) Murray WM et al, J Biomech 1995;28:513-25
ORS 2016 Annual Meeting Poster No. 1516