University of Nevada, Reno
Department of Physics
College of Science
1664 N. Virginia Street
University of Nevada, Reno I 0220
Reno, Nevada 89557-0220
Office: (775) 784-6792
Fax: (775) 784-1398
http:/ / www.physics.unr.edu
December 5, 2014
I have attached my thoughts and ideas about the dominant forces present during swing-through
perambulation using a traditional axillary crutch and the recently developed Hope Crutch®. The
content of my report does not represent the views or opinions of the University ofNevada, Reno.
I am happy to perform this analysis for you as part of the university's land grant mission,
supporting innovation and economic development across the state of Nevada.
I include the executive summary of my evaluation in the body of this letter, and attach a more
detailed explanation for your information. The context of my analysis was to evaluate the forces
required for a stable equilibrium condition just as the body passes through the crutch during
The executive summary of my analysis is as follows.
• There are significant differences in the forces employed in the safe use of the axillary
crutch and the Hope Crutch®.
• During swing-through motion, there is a time when the contact force between the hand
and the crutch tends to rotate the crutch forward, out from under the armpit, and out of
the control of the user.
• Without a compensating force, this would present an unstable condition, and in the
extreme case could lead to a fall.
• In the case of the axillary crutch, the compensating force required to prevent this
potentially unsafe rotation of the crutch must be provided by "locking" the wrist, and/or
"squeezing" the top of the axillary crutch between the ribcage and inner-surface of the
upper-arm. This will result in wrist, forearm, and upper body fatigue.
• With the Hope Crutch® the compensating force is provided by the contact force between
the top of the crutch and the body behind the shoulders.
• The compensating force and therefore the associated stability are inherent to the design of
the Hope Crutch®, without additional expenditure of effort by the user.
• The Hope Crutch® design improves safety, stability, and energy efficiency.
• In my opinion, there are two significant advantages associated with the Hope Crutch®
design. First, the Hope Crutch® design adds stability and security absent in the axillary
crutch. Second, it avoids many of the risks for vascular damage. It achieves this primarily
by removing the axilla contact present that may be present in the use of axillary crutches.
Please do not hesitate to contact me if you have questions or need additional interpretation of my
Paul Neill, PhD
Comparison of Forces Required for Stable Use of
the Axillary Crutch and the Hope Crutch®
Paul Neill, December 2014
For all crutch designs that suspend the body during ambulation the shoulder joint will be the
pivot point for pendular (swing-through) motion. It might seem that the physiological impact of
the pendular motion, i.e. the body weight pivoting around the shoulder, should be similar for the
axillary crutch and Hope Crutch®. However this is not the case, and a rough analysis of the
forces involved indicates significant differences.
In the use of the axillary crutch, figure 1, the body weight is supported on the hands. As the body
swings through between the crutches the contact force between the hand and the crutch can be
represented by the vertical and horizontal components, F1 and F2 respectively. These tend to
rotate the crutch forward out from under the armpit, and out of the control of the user. Clearly,
without a compensating force, this would present an unstable condition and in the extreme case
could lead to a fall and/or re-injury.
The user has two means to compensate. The first is to prevent rotation by "locking" the wrist.
This would require significant effort by the hand, and lower-arm muscles. The second, and
perhaps more obvious means to provide the required compensating force, is to "squeeze" the top
of the axillary crutch between the ribcage and inner-surface of the upper-arm. The resulting
contact force may provide a large enough friction force, F3, to facilitate controlled swingthrough
motion. However, the user must exert a sufficiently large force to prevent the axillary
crutch from sliding out from between the inner-arm and the outer ribcage. This is an inefficient
process since the friction forces depend on the coefficient of friction (i.e. how "slick" or "sticky"
the contact is between the crutch and the clothing and the clothing and the body). In addition, it
is likely that there will be some relative motion between the crutch and the body leading to the
generation of heat, and tissue irritation. Therefore, we conclude that use of the axillary crutch is
not inherently secure, and that it is likely to cause skin irritation and upper-body fatigue.
The Hope Crutch®, figure 2, is designed to be in contact with the body behind the shoulders.
This contact force, F3, provides stability that is inherent to the Hope Crutch® design without
additional expenditure of effort by the user. This force not only facilitates a measure of control of
the swing-through motion, but also eliminates the possibility of the crutch rotating forward out of
control of the user. In addition, the magnitude of the force is minimized due to the additional
length of the Hope Crutch®, and therefore eliminates the potential physiological challenges
associated with larger contact forces with the body. The crutch design improves security and is
In my opinion, there are two significant advantages associated with the Hope Crutch® design.
First, the Hope Crutch® design adds stability and security absent in the axillary crutch. Second,
it avoids many of the risks for vascular damage. It achieves this by removing the axilla contact
present in the axillary crutches.