Download Moment Arms Of The Muscles Crossing The Anatomical Shoulder

MOMENT ARMS OF THE MUSCLES CROSSING THE ANATOMICAL SHOULDER
David C. Ackland and Marcus G. Pandy,
[email protected]
Department of Mechanical Engineering, University of Melbourne, Australia
INTRODUCTION
The moment arm (MA) of a muscle force
represents the mechanical advantage of a
muscle and largely determines its role, for
example, as a stabilizer or a prime mover.
Many biomechanical models rely on accurate
muscle-path and MA data to represent the
anatomical shoulder during movement. To
our knowledge, no study has investigated the
instantaneous MAs of functionally distinct
muscle sub-regions within the rotator cuff,
pectoralis major or latissimus dorsi through
scapula plane abduction (scaption), coronalplane abduction and forward flexion.
Furthermore, muscle MAs have not been
reported over a continuous range of humeral
elevation exceeding 100o, as most studies
ignore scapulothoracic motion.
METHODS AND PROCEDURES
MAs of the musculature spanning the
glenohumeral joint were measured in 8 freshfrozen, entire upper extremities using the
tendon excursion method (An et al., 1984).
Specimens were mounted on a custom
designed dynamic shoulder cadaver testing
apparatus (DSCTA) designed to produce and
quantify 6 degree-of-freedom glenohumeral
joint motion by means of simulated muscle
force and scapula rotation. Tendons of the
following muscles and muscle sub-regions
were exposed by resection: deltoid (anterior,
middle, posterior), subscapularis (inferior,
middle, superior), supraspinatus (anterior,
posterior), infraspinatus (superior, inferior),
latissimus dorsi (superior, middle, inferior),
and pectoralis major (superior, middle,
inferior).
Nylon-lines were sutured to all
tendons and passed through a pulley system to
hanging weights of 10 N; the pulleys were
positioned to reproduce each muscle’s line-ofpull, as determined by visual inspection and
using a computational model (Garner and
Pandy, 2001). Retro-reflective markers were
placed on each hanging weight, and marker
triads placed on the humerus and scapula.
The humerus was passively elevated in the
scapula plane, coronal plane and sagittal plane
to 120o. During elevation, the scapula was
rotated on the DSCTA to simulate
scapulohumeral rhythm as reported by Inman
et al., (1944).
Tendon excursion (vertical trajectory
of hanging weight) and joint angle were
measured from the retro-reflective marker
trajectories using a 6-camera Vicon motion
capture system, and instantaneous muscle
MAs were then computed from the gradient
of the plot of tendon excursion vs. joint angle.
Scapula and humeral coordinate systems were
defined by digitizing bony prominences, as
described in Garner and Pandy (2001).
RESULTS
Significant differences in MAs were reported
across sub-regions of all muscles (p<0.01)
(Table 1). The most effective elevators in
abduction were the anterior and middle
deltoid, while the most effective depressors
were the posterior deltoid and inferior and
middle latissimus dorsi (Figure 1A). In
flexion, the superior pectoralis major was the
most effective elevator (Figure 1B), while
teres major and superior latissimus dorsi had
the largest depressor MAs. Division of multipennate shoulder muscles of broad-origins
into sub-regions highlighted distinct
functional differences across those subregions. Most significantly, we found that the
clavicular fibres of the pectoralis major were
able to exert substantial elevator torque in
flexion, whereas the sternal and lower costal
fibres behaved as stabilizers and flexion
antagonists.
Muscle/muscle subregion
Scaption
CP abduction
Flexion
Ackland et al
(2008)
Ackland et al
(2008)
Ackland et al
(2008)
Superior Subscapularis
AG
AN
AG
AN
AG
AN
9.8
2.2
7.2
-9.5
35.3
-5.4
Middle Subscapularis
1.8
-2.4
1.3
-12.7
24.2
-0.6
Inferior Subscapularis
-1.5
-9.5
-2.2
-16.6
10.4
-3.4
Anterior Supraspinatus
32.4
9.2
23.2
5.6
41.8
0.6
Posterior Supraspinatus
31.9
13.8
26.8
10.4
13.4
2.7
Superior Infraspinatus
22.2
7.1
13.4
5.6
7.1
1.7
Inferior Infraspinatus
12.2
1.9
10.9
3.8
4.2
-6.8
Teres minor
2
-0.8
5.1
-3.3
2.2
-18.7
Teres major
-18.6
-47.3
-12.1
-46.1
-19.7
-54.4
Anterior Deltoid
39.3
2.1
30.2
2
40
11.6
Middle Deltoid
33.1
6.7
29.1
8.3
12.2
0
3
-14.9
2
-15.9
-16.3
-33
-32.9
Posterior Deltoid
Superior Pectoralis major
30.2
3.1
11.2
-1.8
53.7
Middle Pectoralis major
-2.9
-12.7
-17.7
-32.9
15.9
4.4
Inferior Pectoralis major
-12.4
-22.2
-16.2
-33.6
1.9
-9.3
Superior Latissimus dorsi
-7.8
-31.5
-2.1
-29.9
-0.1
-22.1
Middle Latissimus dorsi
-6.4
-21
-10.1
-38.6
-0.6
-15.3
Inferior Latissimus dorsi
-9.9
-28.9
2.6
-38.1
-2.9
-10.8
Table 1. Peak muscle MAs. Positive values
signify agonistic muscle action; negative
values signify antagonistic muscle action.
(AG), maximum elevation agonistic MA;
(AN), maximum elevation antagonist MA.
A 00
20
40
60
80
100
120
Moment arm (mm)
-5
-10
-15
-20
-25
-30
-35
-40
-45
Joint angle (deg)
Superior
Moment arm (mm)
B
DISCUSSION
Middle
Inferior
60
50
40
The MA data presented in this study may
facilitate the design and validation of
biomechanical models of the shoulder
complex. The data presented are valid for an
intact shoulder free of pathology and joint
dysfunction; any condition that is likely to
effect joint congruency may change muscle
moment arm quantities due to eccentric joint
centre of rotation. Demonstrating the torqueproducing potential of the glenohumeral joint
musculature helps to establish a better
understanding of the normal function of the
shoulder joint and its surrounding structures,
and therefore may provide knowledge for the
design and implantation of joint prostheses
and ligament replacements in diseased or
injured shoulders.
SUMMARY
Quantifying the MAs of sub-regions of multipennate shoulder muscles provided
biomechanical evidence for the torqueproducing potential of such muscles across
the glenohumeral joint, as well as the moment
inducing potential of the muscle fibers across
their extensive origins. Such evidence cannot
readily be obtained by approximating broadorigin muscles as single lines-of-force.
Knowledge of MA differences between
muscle sub-regions may assist in identifying
the functional effects of muscle sub-region
tears, assist surgeons in planning tendon
reconstructive surgery, and aid in the
development and validation of computational
models.
30
REFERENCES
20
10
0
-10
0
20
40
60
80
100
120
An et al. (1984) J Biomech Eng 106, 280-2
-20
-30
-40
Joint angle (deg)
Superior
Middle
Inferior
Figure 1. MAs arms of (A) latissimus dorsi,
and (B) pectoralis major. Black lines show
scaption data and grey lines flexion data.
Garner and Pandy (2001) Comp Meth
Biomech Biomed Engin 4, 93-126
Inman et al (1944) JBJS 58, 1-30