This article was downloaded by: [Seo, Na Jin]
On: 24 November 2009
Access details: Access Details: [subscription number 917110973]
Publisher Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 3741 Mortimer Street, London W1T 3JH, UK
Ergonomics
Publication details, including instructions for authors and subscription information:
http://www.informaworld.com/smpp/title~content=t713701117
Biomechanical analysis for handle stability during maximum push and
pull exertions
Na Jin Seo a; Thomas J. Armstrong b
a
Department of Industrial Engineering, University of Wisconsin-Milwaukee, Milwaukee, WI, USA b
Department of Industrial and Operations Engineering, University of Michigan, Ann Arbor, MI, USA
Online publication date: 24 November 2009
To cite this Article Seo, Na Jin and Armstrong, Thomas J.(2009) 'Biomechanical analysis for handle stability during
maximum push and pull exertions', Ergonomics, 52: 12, 1568 — 1575
To link to this Article: DOI: 10.1080/00140130903287999
URL: http://dx.doi.org/10.1080/00140130903287999
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf
This article may be used for research, teaching and private study purposes. Any substantial or
systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or
distribution in any form to anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation that the contents
will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses
should be independently verified with primary sources. The publisher shall not be liable for any loss,
actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly
or indirectly in connection with or arising out of the use of this material.
Ergonomics
Vol. 52, No. 12, December 2009, 1568–1575
Biomechanical analysis for handle stability during maximum push and pull exertions
Na Jin Seoa* and Thomas J. Armstrongb
a
Downloaded By: [Seo, Na Jin] At: 15:41 24 November 2009
Department of Industrial Engineering, University of Wisconsin-Milwaukee, 3200 N. Cramer St., Milwaukee,
WI 53211, USA; bDepartment of Industrial and Operations Engineering, University of Michigan, 1205 Beal Ave,
Ann Arbor, MI 48109, USA
This study investigated the effect of handle stability on maximum push/pull force. It was hypothesised that
people apply force in directions deviated from the pure push/pull direction to generate a moment that assists
producing greater push/pull force when the handle position is fixed (stable) compared to when it is not fixed
(unstable). Eight healthy subjects performed maximum push and pull exertions on a stable and an unstable
handle in a seated posture, while maximum push/pull force, vertical force and lateral force were recorded. For
the unstable handle, vertical and lateral forces were not different from zero during push and pull. For the stable
handle, subjects intuitively applied significant downward force during push and significant upward force during
pull exertions. As predicted from biomechanical analysis, this downward and upward force was found to be
significantly associated with increased push and pull force, respectively, for the stable handle compared to the
unstable handle.
Keywords: stability; handle; push; pull; grip
1.
Introduction
1.1. Significance
The purpose of this paper is to investigate the effect of
handle stability on maximum push/pull force using
biomechanical analysis. Push/pull activities are frequently performed to move an object from one
location to another, to join it to another part, to
support the body or to propel a wheelchair. With an
increasing number of lifting tasks replaced by pushing
and pulling tasks in workplaces, pushing and pulling
tasks contribute to 20% of all industrial back injuries
in the USA, Canada and the UK (Hoozemans et al.
1998). Repeated forceful push/pull exertions lead to
fatigue and musculoskeletal disorders including cumulative trauma disorders for workers installing hoses
during automotive assembly (Ebersole and Armstrong
2004) and manual wheelchair users (Richter et al. 2006,
Dubowsky et al. 2008). In addition, an individual’s
limited push/pull capability can pose safety risks in
situations such as climbing a ladder (Christensen and
Cooper 2005).
Thus, an understanding of push/pull force exertions in relation to handle features is important for
analysing causes of hand injury as well as for designing
grip objects to prevent overexertion and repetitive
*Corresponding author. Email: seon@uwm.edu
ISSN 0014-0139 print/ISSN 1366-5847 online
Ó 2009 Taylor & Francis
DOI: 10.1080/00140130903287999
http://www.informaworld.com
stress injury. Many studies have been undertaken to
examine handle design factors associated with
individuals’ push/pull forces. These factors include
handle shape (Fothergill et al. 1992), handle
orientation (Okunribido and Haslegrave 2008), hand–
handle friction (Seo et al. 2008b), location of the
handle from the body (Davis and Stubbs 1977, Grieve
and Pheasant 1981, Fothergill et al. 1992, Kumar 1995,
Kumar et al. 1995, Das and Wang 2004, Chow and
Dickerson 2009), obstruction around the handle
(Grieshaber 2007), push method such as
simultaneously applying torque during push (Seo et al.
2008b) and handle stability (Bober et al. 1982,
Kornecki et al. 2001, Fischer et al. 2009). The present
study focuses on handle stability.
1.2.
Handle stability
Previous studies focused on the effect of handle
stability on upper extremity muscle activities during
push (Bober et al. 1982, Kornecki et al. 2001, Fischer
et al. 2009). The present study examined how people
apply forces in three dimensions during push and pull
on stable and unstable handles. Specifically, the
present paper proposes a viewpoint that people exploit
the handle stability and apply forces not only in push/
1569
Ergonomics
pull directions but also in other directions to reduce
external moments at the joints of the arm. This
argument is derived from biomechanical analysis as
shown below and is applicable to both push and pull
exertions.
Figure 1 shows a typical push/pull posture in two
dimensions. Push and pull forces are defined as the
force in the anterior–posterior horizontal axis (z-axis)
in this paper, as opposed to the resultant force (vector
sum of all 3-D forces). Exertion of push/pull force (Fz)
and vertical force (Fy) produces reaction forces to the
hand in the opposite directions with the same amounts
of forces (Fy,reaction, Fz,reaction). These reaction forces,
in turn, produce an external moment at the wrist joint
about the x-axis (Mexternal):
Downloaded By: [Seo, Na Jin] At: 15:41 24 November 2009
Mexternal ¼ Fz;reaction dy þ Fy; reaction dz
¼ Fz dy Fy dz
ð1Þ
where dy is the perpendicular distance between Fz and
the wrist joint and dz is the perpendicular distance
between Fy and the wrist joint. This external moment
should be resisted by the internal moment generated by
muscles crossing the wrist (Minternal) in an isometric
condition:
Minternal þ Mexternal ¼ 0
pull exertions, the thenar area of the hand makes a firm
contact with the handle, resulting in the forearm and
wrist pronation, in which case, x-axis moments are
generated by the wrist flexor and extensor muscles.
Even when the forearm was half-pronated at 458, most
wrist extensor muscles are still located above the
horizontal plane through the wrist joint and most wrist
flexor muscles are below the horizontal plane passing
through the wrist joint (Brand and Hollister 1993,
Gonzalez et al. 1997).
From Equations (1), (2) and (3), maximum push/
pull force can be expressed as follows.
jMax pushforcej ¼ ðFz Þmax
Mwristextensionstrength Fy dz
¼
dy
ð4Þ
Max pull force ¼ Fz;max
Mwristflexionstrength þ Fy dz
¼
dy
ð5Þ
If a handle position is constrained to prevent any
movement and rotations (i.e. stable handle, shown in
Figure 1 and Figure 2a), then by pulling up or pressing
ð2Þ
Wrist internal moment about the x-axis is bounded
by wrist extension strength (Mwrist extension strength) and
wrist flexion strength (Mwrist flexion strength):
Mwristflexionstrengh Minternal Mwristextensionstrength
ð3Þ
Wrist flexion/extension strength as opposed to
wrist adduction/abduction strength was considered as
being responsible for generating internal moments
about the x-axis for the following reason. During push/
Figure 1. A typical posture for push/pull exertions.
Reaction forces (Fz, reaction and Fy, reaction) from push/pull
force (Fz) and vertical force (Fy) exertions generate an
external moment about the wrist joint (Mexternal).
Figure 2. Subjects applied maximum push/pull forces
for the stable (a) vs. unstable (b, c) handles. The unstable
handle was simulated using universal joints, which
prevented subjects from applying forces in the x- and ydirections during push (b) and pull (c). All three-direction
forces were measured using a load cell.
1570
N.J. Seo and T.J. Armstrong
down the handle, it is possible to adjust the vertical
force (Fy) to increase push/pull force for a given wrist
strength (Equations (4) and (5)). Specifically,
maximum push force can increase by simultaneously
applying downward force (negative Fy) as described in
Equation (4). Likewise, maximum pull force can
increase by simultaneously applying upward force
(positive Fy), as described in Equation (5).
If a handle position is not constrained (i.e. unstable
handle, shown in Figure 2b,c), the handle can move
up, down or to the side as a person applies upward
force (positive Fy), downward force (negative Fy) or
forces to the side (+Fx), respectively. To maintain the
handle’s position during push/pull exertions, the
person should minimise forces in these directions,
resulting in the following:
Downloaded By: [Seo, Na Jin] At: 15:41 24 November 2009
Fx 0 and Fy 0 for an unstable handle
ð6Þ
Based on Equations (4) and (6), it can be seen that
inability to apply forces in directions other than the
push/pull direction (especially in the vertical direction)
will result in reduced push force. Likewise, inability to
apply forces in other directions (especially in the
vertical direction) will result in reduced pull force
based on Equations (5) and (6).
The same biomechanical analysis can be performed
about the elbow and shoulder joints. Similar conclusions can be obtained. Application of vertical force can
increase maximum push/pull force limited by the
elbow and shoulder flexion/extension strength. In the
posture examined in this study (Figure 2), it was
postulated that maximum push/pull force is limited by
the wrist strength rather than by the elbow or shoulder
strength, given the joint strengths for the wrist (Seo
et al. 2008a), elbow (Holzbaur et al. 2007) and
shoulder (Murray et al. 1985) and the moment arms
for each joint (vertical distance, dy, from the handle to
each joint) (Chengalur et al. 2004, Choi et al. 2007).
Thus, the present paper focused on the wrist strength
and how maximum push/pull force is related to wrist
strength as described in Equations (4) and (5).
However, it is to be noted that the elbow or the
shoulder could be the limiting joint depending on the
posture used during push and pull exertions (Daams
1992, Al-Eisawi et al. 1994).
In summary, biomechanical analysis suggests that
additional push/pull force can be exerted without
exceeding the strength capacity of the wrist by
applying Fy for a stable handle. Therefore, it was
hypothesised that: 1) people will apply downward force
during push and upward force during pull on a
stable handle; 2) this downward or upward force is
related to increased maximum push or pull force for a
stable handle compared to an unstable handle,
respectively.
2.
Methods
2.1. Procedure
A 22 factorial experiment was conducted to test this
hypothesis. The independent variables were handle
stability (stable vs. unstable as shown in Figure 2a–c)
and direction of force exertions (push vs. pull). The
dependent variables were maximum push/pull force,
vertical force and lateral force. The stable handle was
simulated by fixing the handle to the force transducer
such that the handle was immobilised (Figure 2a). The
unstable handle was simulated using two universal
joints at the attachment of the handle to the transducer
(Figure 2b,c). Under this arrangement, the handle was
free to move in all three axes within the spherical space
limited by the lengths of the linkage consisting of the
universal joints. This restricted subjects from pressing
down or pulling up the handle or pushing/pulling the
handle to the side.
Note that, previously, handle stability has
been examined only for push, with the handle
instability provided in front of the hand via joints
(‘inverted pendulum’ type of instability) (Bober et al.
1982, Kornecki et al. 2001, Fischer et al. 2009).
For this type of unstable handle, the handle can
buckle at the joint unless push force is in line with the
joint. To prevent buckling, greater muscle efforts
were observed for the upper extremity muscles. The
present study investigated unstable handles by
introducing the handle instability via joints behind the
hand for push and in front of the hand for pull
(‘regular pendulum’ type of instability; see Figure 2).
Forces applied in directions other than the push or
pull direction on this type of unstable handle may
result in changes in the handle location, but not
buckling.
Subjects washed their hands with soap, rinsed with
water and dried with paper towels 10 min prior to
testing, to eliminate artefacts due to contaminants
(Comaish and Bottoms 1971, Buchholz et al. 1988).
Subjects were seated on a chair that supported the
back and feet to minimise the effect of balance/slip
(Figure 2a). The handle height was adjusted to each
subject’s elbow height when the arm was resting
vertically on the side of the body. The horizontal
distance from the handle to the subject was adjusted to
allow subjects to maintain an extended elbow posture,
as shown in Figure 2a.
Subjects were instructed to grasp a cylindrical
handle with the right hand in a power grip and perform
maximum push/pull exertions for 5 s. The cylindrical
handle’s long axis was parallel to the push/pull
1571
Ergonomics
Downloaded By: [Seo, Na Jin] At: 15:41 24 November 2009
direction. No instructions regarding vertical or lateral
force (Fy, Fx) were given to subjects. Each condition
was tested twice. Conditions were randomly presented
to subjects. A 2-min break was given between
consecutive trials.
The cylindrical handle had a smooth aluminium
surface and its diameter was 38.1 mm. All forces (Fx,
Fy, Fz) were measured using a six-axis load cell. Data
were collected at 5 Hz. All data were averaged over 2 s
during maximum push or pull exertions.
2.2. Subjects
Eight healthy subjects (four male, four female, average
age 26.3 + 4.5 years ranging from 23 to 37) participated in the experiment. Their grip strength ranged
from 8th to 77th percentile for males, and 8th to 73rd
percentile for females. The protocol for the experiments was approved by the University of Michigan
Institutional Review Board. Subjects gave written
informed consent prior to testing.
were performed, a p-value of 0.025 was considered
significant after Bonferroni correction.
In addition to the two main hypothesis tests,
prediction of maximum push and pull forces was
attempted using Equations (4) and (5). The predicted
maximum push force was calculated by plugging the
wrist extension strength, dy, dz, and Fy, into Equation
(4). Wrist extension strength measured during
maximum grip (Seo et al. 2008a), 6.0 N m, was used
as an input in this calculation. The moment arms
values of 59 and 75 mm for dy and dz, respectively,
were adopted from a previous study (Choi et al. 2007).
Vertical force (Fy) was set to zero for the unstable
handle. For the stable handle, vertical force measured
using the load cell was used as an input in this
calculation. Likewise, maximum pull force was
predicted based on Equation (5) using wrist flexion
strength of 12.2 N m (Seo et al. 2008a), the same
moment arm values (dy, dz) and zero and measured
vertical force for the unstable and stable handles,
respectively.
2.3.
3.
Data analysis
First, repeated measures ANOVA was performed to
determine if vertical force (Fy) was significantly
affected by the handle stability, force direction (push
vs. pull) and the interaction between the two. A
significant main effect of handle stability would mean
that vertical force changes depending on whether the
handle is stable or unstable. A significant interaction
effect between the handle stability and force direction
would mean that the change in the vertical force differs
depending on whether it is a push exertion or a pull
exertion (e.g. the vertical force increases to the upward
direction for pull, whereas it increases to the downward direction for push). The same analysis was
performed for lateral force (Fx).
To double check if the vertical force direction is
consistent with the hypothesis, one-sample t-tests were
performed to compare vertical forces to zero for each
group (push on a stable handle, pull on a stable handle,
push or pull on an unstable handle). More specifically,
it was tested to see if vertical force for pushing on the
stable handle is less than zero (negative Fy), if vertical
force for pulling on the stable handle is greater than
zero (positive Fy) and if vertical force for the unstable
handle was different from zero.
Then, another repeated measures ANOVA was
performed to determine if the magnitude of push/pull
force (jFzj) was significantly affected by the amount of
vertical force (jFyj) and force direction (push vs. pull).
If jFzj changes significantly with jFyj, it would mean
that push/pull force is significantly affected by vertical
force generation. Since two major statistical analyses
Results
Mean maximum push/pull force, vertical force and
lateral force measured for the stable and unstable
handles are summarised in Table 1. Mean maximum
pull force was 53% greater than push in absolute
values (jFzj) (Table 1). The magnitude of maximum
push/pull force (jFzj) for the stable handle was, on
average, 38% greater than that for the unstable handle
(Figure 3, Table 1).
Repeated measures ANOVA showed that vertical
force (Fy) was significantly affected by handle stability
and the interaction between handle stability and force
direction (pull vs. push) (p 5 0.025 for all). The onesample t-tests showed the following: vertical force
was not different from zero for push and pull on the
unstable handle (Fy 0; p ¼ 0.463); vertical force
was less than zero for push on the stable handle (i.e.
downward force; p 5 0.025); vertical force was greater
Table 1. Mean + SE lateral (Fx), vertical (Fy) and
push/pull force (Fz) during maximum push/pull exertions
for the stable and unstable handles (eight subjects’ data
pooled).
Stability
Push
Pull
Stable
Unstable
Stable
Unstable
Fx (N)
71
11
716
0
+
+
+
+
13
4
9
1
Fy (N)
758
74
36
8
+
+
+
+
14
5
14
3
Fz (N)
7177
7109
243
195
+
+
+
+
Note: See Figure 2 for illustration of handle stability conditions
and force directions.
29
14
36
25
Downloaded By: [Seo, Na Jin] At: 15:41 24 November 2009
1572
N.J. Seo and T.J. Armstrong
than zero for pull on the stable handle (i.e. upward
force; p 5 0.025) (see Figure 3); lateral force (Fx) was
not significantly affected by stability, direction nor
their interaction.
Repeated measures ANOVA for the push/pull
force showed that the magnitude of push/pull force
(jFzj) was significantly affected by the magnitude of
vertical force (jFyj) and force direction (push vs. pull)
(p 5 0.025 for both). More specifically, jFzj increased
with increasing jFyj. These relationships are depicted in
Figure 3. Vertical force (Fy) is close to zero for push/
pull on the unstable handle. Significant upward force
(positive Fy) and downward force (negative Fy) were
observed for pull and push, respectively, on the stable
handle. These upward and downward forces were
associated with increased magnitude of pull and push
forces (jFzj), respectively, as shown in Figure 3.
The measured push/pull forces for the stable and
unstable handles are compared with the predicted
forces in Figure 4. Prediction showed the same trend
with the measured forces that the push/pull forces were
greater for the stable than for the unstable handle. The
predicted push/pull forces for the stable and unstable
handles were within 1 SE from the mean measured
forces for the corresponding conditions.
Figure 3. Mean + SE maximum pull/push force (positive
Fz/ negative Fz) and upward/downward forces (positive Fy/
negative Fy) for the stable and unstable handles (eight
subjects’ data pooled).
Figure 4. Comparison between measured and predicted
push/pull forces (Fz) for the stable and unstable handles.
Predicted push/pull forces were calculated using wrist
extension/flexion strength and measured vertical force.
4.
4.1.
Discussion
Handle stability
The present paper proposes a viewpoint that during
push/pull exertions, people apply forces not only in the
push/pull direction but also in other directions to
increase their push/pull force if forces applied in other
directions can reduce external moments applied at the
joints of the arm. On the other hand, inability to
generate forces in other directions due to handle
instability may limit individuals’ push/pull capability.
More specifically, in the posture examined in this
study, it was hypothesised, based on Equations (4) and
(5), that: 1) people will apply downward force during
push and apply upward force during pull on a stable
handle: 2) this downward or upward force is related to
increased maximum push or pull force for a stable
handle compared to an unstable handle, respectively.
Both hypotheses were supported by the empirical
data obtained in this study (Table 1, Figure 3). When
the handle position was unstable, subjects could apply
little forces in the medial–lateral and vertical directions
during push or pull. When the handle position was
fixed, allowing subjects to exert forces in all directions
(i.e. stable handle), the subjects indeed applied
downward and upward forces during push and pull,
respectively, although no instruction regarding the
vertical and medial–lateral force was given to the
subjects.
These downward and upward forces could generate
a moment about the x-axis at the wrist in the opposite
direction from the moment generated by the reaction
force from push and pull. As a result, the total external
moment at the wrist joint could be reduced, decreasing
the required muscle efforts to counterbalance the
external moment or affording to produce greater push/
pull forces (as reflected in Equations (4) and (5)).
Empirically, these downward and upward forces were
indeed associated with increased push and pull forces
(z-direction forces), respectively (Figure 3). Push/pull
forces were greater when the subjects pulled up or
pushed down (for the stable handle) than when they
could not (for the unstable handle). In addition,
predictions performed using Equations (4) and (5)
agreed favourably with measured push/pull forces
(Figure 4).
This study provides a biomechanical basis for
explaining decreased push/pull force for unstable
handles. Previously, the effect of handle stability has
been examined as a motor control issue by categorising
stabilising muscles and directional force generating
muscles (Bober et al. 1982, Kornecki 1995, Kornecki
et al. 2001). The findings of the present study suggest
that the reason people could use less muscle effort
for stable handles than for unstable handles in the
1573
Downloaded By: [Seo, Na Jin] At: 15:41 24 November 2009
Ergonomics
previous studies (Bober et al. 1982, Kornecki 1995,
Kornecki et al. 2001, Fischer et al. 2009) may be
because they reduced external moments at the joints,
consequently reducing required joint stabilisation
efforts by applying forces in directions deviating from
the pure push direction. It agrees with the previous
studies that a stable handle provides persons with
leverage. The leverage appears to be the ability to
apply force in other directions in an effort to lessen
external moments about the wrist joint that will assist
push/pull force exertions.
Subjects intuitively knew that applying force in
deviated directions could improve their push and pull
force magnitudes, which is consistent with a previous
study by Grieve and Pheasant (1981). The present study
differs from the previous study in that the present paper
predicted which direction force would be beneficial from
joint biomechanical analysis in Equations (4) and (5),
whereas the previous study empirically measured
individuals’ maximum force capability in all directions
in the sagittal plane (Pheasant and Grieve 1981). In
addition, this paper assumes that maximum push/pull
force is achieved when force is applied in the direction
that has the greatest push/pull-direction component
force, as opposed to considering component forces in all
possible directions as in Grieve and Pheasant (1981).
The present study is in line with the discussion in the
previous study (Pheasant and Grieve 1981) that the
reason force capacity is bigger in one direction vs. other
directions may be related to the external moment at the
body joints that muscles would have to work to resist in
order to maintain the posture.
The finding in the present study is sensitive to the
posture adopted during push/pull exertions. If different
postures are used during push/pull exertions on the
stable handle, people may apply forces in different
directions (Grieve and Pheasant 1981). For example, if
a push/pull exertion is performed for a handle at
shoulder height, with the upper arm abducted 908 and
flexed 458 and with the elbow flexed at 908, the
biomechanical analysis shows that push and pull force
may increase by applying force in the negative and
positive x-direction, respectively, rather than applying
force in the y-direction. Thus, the specific finding in
this paper that vertical force was related to maximum
push/pull force may be applicable only to the
particular posture examined in this study. Separate
analyses may be required for different postures used
during push/pull exertions.
4.2. Comparison with previous studies
Maximum push/pull forces measured in this study
were not significantly different from those previously
reported for the stable handle (Grieshaber 2007,
Okunribido and Haslegrave 2008) and for an
unstable handle (Seo et al. 2008b) under the same
handle orientation. Consistent with previous studies
(Davis and Stubbs 1977, Keyserling et al. 1980, Kumar
1995, Kumar et al. 1995, Das and Wang 2004), average
maximum pull force was 53% greater than push force
(Table 1). Equations (4) and (5) suggest that pull force
is related to wrist flexion strength and push force is
related to wrist extension strength. Therefore, greater
wrist flexion strength than extension strength may be
responsible for greater maximum pull force than push
observed for the unstable handle in the posture
examined in the present study. For the stable handle,
with forces in other directions (for example, in the
y-direction) reducing external moments for the wrist,
the push strength may have been limited by other joint
strength such as the elbow and shoulder strength. For
pull, forces in the z- and y-directions could have
resulted in the arm under the tensile load, in which case
the pull strength is limited by the torso extension or
whole-body pull strength.
4.3.
Future studies
Maximum push/pull force is limited by the weakest
link in the chain, which often includes the hand and the
wrist. The present study focused on the wrist
moment; however, push/pull force can also be limited
by normal force that the hand could apply to the
handle surface by gripping, as previously modelled
(Seo et al. 2008b). Force generation deviated from the
pure push/pull direction may not only affect wrist
external joint moments, but also increase normal force
on the handle surface. Thus, vertical force observed in
this study may have contributed to increased push/pull
force, not only in terms of wrist joint moment but also
by increasing normal force. Future studies may
measure normal force on the handle surface and
gauge the effect of increase in normal force on push/
pull force for the stable handle compared to the
unstable handle.
5. Conclusions
This study investigated the effect of handle stability on
maximum push/pull force using biomechanical
analysis and empirical data. The unstable handle
prevented subjects from applying force in directions
other than the push/pull direction. When the stable
handle was presented, the subjects intuitively applied
downward force during maximum push exertions and
upward force during maximum pull exertions for the
stable handle. As predicted from the biomechanical
analysis (Equations (4) and (5)), this downward and
upward force was significantly associated with
1574
N.J. Seo and T.J. Armstrong
increased maximum push and pull force, respectively,
for the stable handle compared to the unstable handle.
It appears that deviation of the force exertion direction
from the pure push–pull direction could reduce the
external moment applied to the joints of the arm,
resulting in increased force capability. In other words,
inability to generate forces in other directions due to
handle instability resulted in decreased maximum
push/pull force.
The present study demonstrated that joint biomechanics can be used to analyse and predict the effect of
handle stability on push/pull exertions, which has
previously been examined in a motor control paradigm. The finding can be applied to the design of a
workstation that requires high force exertions to
enhance individuals’ push/pull capabilities and reduce
fatigue and musculoskeletal disorders.
Downloaded By: [Seo, Na Jin] At: 15:41 24 November 2009
Acknowledgements
The authors would like to thank Justin G. Young and
Kathryn L. Dannecker for their assistance in acquisition of
data. This project was funded by a NIOSH pilot grant.
Conflict of interest
The authors declare no conflict of interest.
References
Al-Eisawi, K.W., Kerk, C.J., and Congleton, J.J., 1994.
Limitations of wrist strength to manual exertion capability
in 2D biomechanical modeling. Santa Monica, CA:
Human Factors and Ergonomics Society.
Bober, T., et al., 1982. Biomechanical analysis of human arm
stabilization during force production. Journal of Biomechanics, 15, 825–830.
Brand, P.W. and Hollister, A., 1993. Clinical mechanics of the
hand. Maryland Heights, MO: Mosby Elsevier Health
Science.
Buchholz, B., Frederick, L.J., and Armstrong, T.J., 1988. An
investigation of human palmar skin friction and the
effects of materials, pinch force and moisture. Ergonomics, 31, 317–325.
Chengalur, S.N., Rodgers, S.H., and Bernard, T.E., 2004.
Kodak’s ergonomic design for people at work. New York:
John Wiley and Sons, Inc.
Choi, J., Grieshaber, D.C., and Armstrong, T.J., 2007.
Estimation of grasp envelope using a 3-dimensional
kinematic model of the hand. In: Human factors
and ergonomics society annual meeting proceedings,
Baltimore, MD. Santa Monica, CA: Human Factors
and Ergonomics Society, 889–893.
Chow, A.Y. and Dickerson, C.R., 2009. Shoulder strength of
females while sitting and standing as a function of hand
location and force direction. Applied Ergonomics, 40,
303–308.
Christensen, T. and Cooper, N., 2005. Attacking ladder fallsone rung at a time. Occupational Hazards, 67, 39.
Comaish, S. and Bottoms, E., 1971. The skin and friction:
deviations from Amonton’s laws, and the effects of
hydration and lubrication. British Journal of Dermatology, 84, 37–43.
Daams, B.J., 1992. Comparison of moments exerted round
one or more joints in the arm. Contemporary ergonomics,
1992: proceedings of the Ergonomics Society’s 1992
annual conference. Volume 36. Birmingham, UK:
Ergonomics Society, 104–109.
Das, B. and Wang, Y., 2004. Isometric pull-push
strengths in workspace: 1. Strength profiles. International
Journal of Occupational Safety and Ergonomics, 10,
43–58.
Davis, P.R. and Stubbs, D.A., 1977. Safe levels of manual
forces for young males (2). Applied Ergonomics, 8,
219–228.
Dubowsky, S.R., et al., 2008. Validation of a musculoskeletal
model of wheelchair propulsion and its application to
minimizing shoulder joint forces. Journal of
Biomechanics, 41, 2981–2988.
Ebersole, M.L. and Armstrong, T.J., 2004. An
analysis of task-based worker self-assessments of force.
In: Human Factors and Ergonomics Society 48th
annual meeting, New Orleans, Santa Monica, CA:
HFES.
Fischer, S.L., Wells, R.P., and Dickerson, C.R., 2009. The
effect of added degrees of freedom and handle type on
upper limb muscle activity during simulated hand tool
use. Ergonomics, 52, 25–35.
Fothergill, D.M., Grieve, D.W., and Pheasant, S.T., 1992.
The influence of some handle designs and handle height
on the strength of the horizontal pulling action.
Ergonomics, 35, 203–212.
Gonzalez, R.V., Buchanan, T.S., and Delp, S.L., 1997. How
muscle architecture and moment arms affect wrist
flexion-extension moments. Journal of Biomechanics, 30,
705–712.
Grieshaber, D.C., 2007. Use of a biomechanical model of the
hand to evaluate physical task requirements. Ann Arbor:
Department of Industrial and Operations Engineering,
University of Michigan.
Grieve, D.W. and Pheasant, S.T., 1981. Naturally preferred
directions for the exertion of maximal manual forces.
Ergonomics, 24, 685–693.
Holzbaur, K.R., et al., 2007. Moment-generating capacity of
upper limb muscles in healthy adults. Journal of
Biomechanics, 40, 2442–2449.
Hoozemans, M.J., et al., 1998. Pushing and pulling in
relation to musculoskeletal disorders: a review of risk
factors. Ergonomics, 41, 757–781.
Keyserling, W.M., Herrin, G.D., and Chaffin, D.B., 1980.
Isometric strength testing as a means of controlling
medical incidents on strenuous jobs. Journal of
Occupational Medicine, 22, 332–336.
Kornecki, S., 1995. Biomechanical consequences of hand’s
action on unstable handle. International Journal of
Occupational Safety and Ergonomics, 1, 199–207.
Kornecki, S., Kebel, A., and Siemienski, A., 2001. Muscular
co-operation during joint stabilisation, as reflected by
EMG. European Journal of Applied Physiology, 84,
453–461.
Kumar, S., 1995. Upper body push-pull strength of normal
young adults in sagittal plane at three heights.
International Journal of Industrial Ergonomics, 15,
427–436.
Kumar, S., Narayan, Y., and Bacchus, C., 1995.
Symmetric and asymmetric two-handed pull-push
strength of young adults. Human Factors, 37,
854–865.
Ergonomics
Downloaded By: [Seo, Na Jin] At: 15:41 24 November 2009
Murray, M.P., et al., 1985. Shoulder motion and
muscle strength of normal men and women in two age
groups. Clinical Orthopaedics and Related Research,
268–273.
Okunribido, O.O. and Haslegrave, C.M., 2008. Ready steady
push – a study of the role of arm posture in manual
exertions. Ergonomics, 51, 192–216.
Pheasant, S.T. and Grieve, D.W., 1981. The principal
features of maximal exertion in the sagittal plane.
Ergonomics, 24, 327–338.
View publication stats
1575
Richter, W.M., et al., 2006. Reduced finger and wrist flexor
activity during propulsion with a new flexible hand rim.
Archives of Physical Medicine and Rehabilitation, 87,
1643–1647.
Seo, N.J., et al., 2008a. Wrist strength is dependent on
simultaneous power grip intensity. Ergonomics, 51,
1594–1605.
Seo, N.J., et al., 2008b. The effect of handle friction and
inward or outward torque on maximum axial push force.
Human Factors, 50, 227–236.