Unilateral, transtibial amputees must rely on the remaining muscles in
their leg and their prosthesis to provide the necessary functions for walking,
such as supporting the body, forward motion, leg-swing initiation and balance,
and studies have shown that amputees have altered walking mechanics and
experience a range of deficits while walking.
“People with below-knee amputations have many alterations in their
gait, such as asymmetry in their kinematics and joint loads, reduced preferred
walking speed, a greater risk of falling while walking and a greater metabolic
cost of walking,” Anne Silverman, PhD, assistant professor in the
Department of Mechanical Engineering at Colorado School of Mines, told
O&P Business News. “Previous studies have shown that amputees
compensate as a result of their amputation, but how individual muscles and the
prosthesis are contributing to altered gait patterns is unclear.”
Silverman and colleagues decided to investigate how the individual
muscles in unilateral, transtibial amputees function during walking using a
computational modeling study.
“The purpose of this study was to determine how the prosthesis and
individual muscles work in synergy to provide body support, forward propulsion,
leg-swing initiation and mediolateral balance during amputee walking using
forward dynamics simulation techniques,” she said. “Using a
musculoskeletal modeling and simulation approach, we can isolate the effects of
individual muscles and the prosthesis on walking mechanics.”
Computational modeling and simulation
The researchers performed gait analyses using motion capture techniques
on 14 unilateral, transtibial amputees and 10 non-amputees. They also measured
ground reaction forces (GRFs) with force plates embedded in a walkway as the
subjects walked over ground. They then implemented an optimization framework to
drive a computational model to match the kinematics and GRFs of the subjects,
which resulted in 3-D forward dynamics simulations of amputee and non-amputee
walking.
Anne Silverman
“The computational model is a detailed representation of the body,
including the muscle paths and inertial properties of body segments,”
Silverman said. “And once we have a computational simulation of this model
that reproduces the motion we have seen in the lab experimentally, we are able
to investigate many quantities, such as muscle forces and muscle contributions
to accelerations of the body.”
The researchers found that, while walking, the remaining muscles in the
residual leg of the amputee functioned similarly to the intact leg and
non-amputee model, but the levels of muscle contribution varied.
“We saw similar functions from a variety of muscles, but different
levels of contribution. The vasti muscles in the quadriceps, for example, are
responsible for supporting and braking the body center-of-mass early in the
gait cycle in non-amputee walking,” Silverman said. “The vasti
muscles in the amputee simulation also performed these functions, but the
muscle output was reduced compared to the non-amputee simulation.”
The researchers also found that the passive prosthesis provided
necessary body support and lateral acceleration, mimicking much of the same
function as the soleus muscle. Also like the soleus, the prosthesis transferred
energy from the leg to the trunk. However, the prosthesis did not replace one
of the key functions of the gastrocnemius muscle, which normally provides
leg-swing initiation.
“The gastrocnemius muscle generates energy to the leg for
swing-initiation. This function was missing from our amputee simulation,”
Silverman said. “We had less energy overall delivered to the leg because
the prosthesis couldn’t provide that function, which explains many of the
differences that we saw in the overall gait pattern of the amputees.”
According to the researchers, there was also a reduced contribution from
the rectus femoris to braking the body center-of-mass in the amputee
simulation, similar to the results for the vasti. These reduced contributions
are consistent with previous studies that have shown atrophy in the residual
leg thigh muscles, leading to a smaller force output from these muscles.
Potential limitations
One of the limitations of the study is the simulation of the group
average data. To further understand the functional deficits in unilateral
amputees, the researchers plan to examine individual subjects in future
studies.
“We analyzed group average data, which is a great first step, but
I’d like to apply these methods to individual subjects in the future.
There is a great range of functional mobility across individuals, as well as a
variety of muscle coordination strategies that may be used by different
individuals,” Silverman said.
Despite the limitations, Silverman hopes that these results can be
applied to the modification of rehabilitative approaches and future devices.
“I hope these results can help with the development of targeted
rehabilitation protocols, which may be designed to strengthen and train the
muscles that can to deliver more energy to the leg for swing initiation.
Similarly, perhaps prostheses that provide this function can be designed to
help these patients,” Silverman said. “Given that we have a better
understanding of the deficits in amputee gait, we are that much closer
to overcoming them.” — by Megan Gilbride
For more information:
Silverman, AK. Neptune, RR. Muscle and prosthesis contributions to
amputee walking mechanics: A modeling study. J Biomech.
45(13):2271-2278.
Disclosure: The authors have no relevant financial disclosures.
