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| Robert Gailey, PhD, PT |
The biomechanics of amputee running is an
interesting area that is useful in clinical application. With
prosthetic developments such as the Flex-Sprint and C-Sprint
designs permitting amputees of all levels of athletic ability to
participate in recreational jogs or even competitive track, today’s
prosthetist and physical therapist must maintain a certain level of
knowledge in this area.
Understanding what occurs during running will assist greatly
with prosthetic socket fabrication, component selection, and the
design of an appropriate training program that will assist the
amputee in attaining their athletic goals.Â
This article will examine the fundamentals of amputee running.
Many of the principles discussed apply to most sports requiring
running or agility and speed-related movements, such as basketball
or tennis.
The running cycle is divided into a stance and swing phase.
During stance phase, the period from initial contact to mid-stance
is referred to as the “absorption phase,” where forces decelerate
as the runner contacts the ground. From mid-stance to toe-off is
known as the “propulsion phase,” where the body generates the
acceleratory forces that are carried over as the limb enters the
swing phase. From mid-swing to terminal swing, the limb begins to
decelerate as it returns to the absorption phase.
The beginning and end of each swing phase has a period of
double-float, where neither limb is in contact with the ground. As
a result, the stance phase accounts for less than 50 percent of the
running gait cycle. As speed increases, the percentage of stance
phase decreases.
Absorption Phase
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| TTA running absorption phase |
The initial contact to mid-stance phase is
regarded as the absorption period. In this period, the lower limb
acts as a shock absorber for the body, reducing the considerable
ground reaction forces passing through the limb, which can be two
to three times greater than body weight.
As the foot strikes the ground, a backward force is generated by
the strong contraction of the hip extensor muscles, while the hip
abductors provide the necessary pelvic stability. Muscular
stabilization, coupled with joint motion, creates a biomechanical
spring that reduces the effects of the ground reaction forces.
When amputees run, there is an absence of an impact ground
reaction force peak for the prosthetic limb. This reduction in
ground reaction force suggests that amputees both absorb and
generate less energy with their prosthetic limb. The reduction in
energy generated with the prosthetic limb could be the result of a
more passive use of the limb, the absorption of forces by the soft
tissue encapsulated within the socket, or the presence of an
isometric contraction by the muscles.
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| TFA amputee running absorption phase |
As the transtibial amputee (TTA) strikes the
ground with the prosthetic limb, a backward force is instantly
created by the prosthetic-side hip musculature. This generates two
to three times more work than the sound limb, partly to help move
the body over the stationary foot, and partly to compensate for the
loss of active plantarflexion at the ankle.
Probably the most notable difference between novice and
well-trained TTA runners is that during initial contact, knee
flexion is often absent in the novice runner. However, with proper
training, strength, and adequate residual limb length, comparable
knee flexion can be achieved with the prosthetic limb.
Length of the residual limb and the amount of muscle mass
retained play a significant role in determining the transfemoral
amputee’s (TFA) running potential. This has become very apparent in
recent years as knee disarticulation amputee runners appear to be
extremely successful in competition. The additional power
potentially available to knee disarticulation runners should not
overshadow the need for athletic ability and training, which also
play a very important role.
Acceleration Phase
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| TTA running acceleration phase |
From mid-stance to terminal stance and through
initial swing is referred to as the “acceleration phase” of the
running cycle, in which the body moves from stance phase energy
absorption to acceleration. At this point, the majority of the
forward propulsion of the body comes from the contralateral swing
limb and the arms.
The well-trained TTA can achieve flexion-extension patterns
similar to non-amputee runners during stance. Contraction of the
quadriceps, coupled with the calf muscles, creates adequate knee
stability. The use of the Flex-Foot “J” shape design, which permits
controlled dorsiflexion, is considered by many to assist
significantly with knee flexion control. In fact, the Flex-Foot has
been found to provide a more normal pattern of hip and knee
extensor muscle work throughout the stance phase.
The TFA’s hip remains in a neutral position and is related to
the extended prosthetic knee. To continue the advancement over the
prosthetic stance limb, the hamstrings and gluteus maximus promote
rapid hip extension. The amount of ankle dorsiflexion present is a
direct result of the prosthetic foot design and alignment. Again,
to date the Flex-Sprint design has delivered the maximum mechanical
energy return for TFA runners.
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| TFA running acceleration phase |
As the hip reaches maximum extension, all
movements are passive during terminal stance except for the hip
adductors, which contract for pelvic stabilization. The peak
plantarflexion is the result of the rapid movement of the tibia
over the foot, creating a rigid lever in the foot to release the
elastic energy. During running, over half the elastic energy is
stored in two springs, the Achilles tendon and the arch of the
foot.
The “elastic energy” found in the anatomical foot has been
replicated to varying degrees in prosthetic feet. Dynamic feet have
been found to generate two to three times greater elastic energy
than SACH feet. Czernieki, Gitter and Munro (1991), defined spring
efficiency as “the amount of energy generated, divided by the
amount of energy absorbed.” The spring efficiency of the SACH foot
was found to be 31 percent, the Seattle foot had 52 percent, and
the Flex-Foot had an impressive 82 percent. In comparison, the
human foot has 241 percent spring efficiency, with the addition of
the concentric plantarflexion contraction.
At terminal stance, the transtibial amputee runner’s total
muscle work on the prosthetic side is half that measured in the
intact limb and in non-amputee runners. This is not too surprising,
considering the absence of the plantarflexors. To compensate, there
appears to be approximately a 75 percent increase in energy
transfer from the amputee’s intact swing phase leg.
The hip flexion is generated by a powerful contraction of the
hip flexors. Stability and line of progression of the limb are
maintained by stabilizing contractions of the hip abductor and
adductor muscles. The mechanical work of the hip, or the energy
generated by the intact hip flexors, was found to be more than
twice the magnitude of that of non-amputee runners, with the
prosthetic hip being somewhat greater than normal, but not as great
as the intact side.
Deceleration Phase
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| TTA running deceleration phase |
As the foot prepares to strike the ground, the
muscles are preparing to accelerate the body forward, while also
absorbing the ground reactive forces. The hip extensors work
eccentrically to decelerate the thigh and leg during late swing,
and extend the hip prior to and immediately upon initial contact.
The hip abductors and adductors contract to stabilize the pelvis as
the initial contact is approached.
Transtibial amputee runners tend to have lower peak flexion and
extension angular velocities, as well as maximal hip and knee
flexion angles. Premature extension of the knee
during swing is also commonly observed. Socket design and
suspension requirements have been identified as probable causes for
the reduction in peak knee flexion, which in turn limits hip
flexion. Creating a transtibial socket that provides both stance
phase stability and swing phase mobility has been a perplexing
task.
The TTA will also contract the muscles of the lower limb in an
identical pattern to the non-amputee during terminal swing. The
knee should be slightly flexed and, as stated earlier, there will
be a reduction in forces as the limb prepares to strike the
ground.
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| TFA running deceleration phase |
The TFA must land on an extended knee with the
prosthetic limb. Initiating a backward force prior to contact will
not only accelerate the body forward, but will simultaneously
ensure that the knee will remain in extension. Many transfemoral
amputee runners also adopt an extended trunk posture as they
descend to the ground, although this is unnecessary.
Trunk and Arm Swing
For the amputee, arm swing is extremely important, yet often
difficult to master. A concentrated effort must be made to maintain
a symmetrical arm swing, especially as speed increases when the
legs have a tendency to lose symmetry of movement.
Transfemoral amputees have a tendency to demonstrate increased
abduction of the prosthetic-side arm, especially when the
prosthetic lower limb is abducted. This adverse position of both
the leg and the arm creates opposing forces that tend to impede
forward momentum and increase the metabolic requirement. Poor
medial/lateral socket stability will also require additional effort
by the prosthetic-side arm and facilitate unwanted trunk
movement.
This overview of the biomechanics of amputee running should help
in socket fabrication and component selection, as well as in
planning an appropriate training program. In turn, amputees will be
better able to optimize their performance in order to achieve their
athletic goals.
University of Miami School of Medicine
Department of Orthopaedics
Division of Physical Therapy
References
- Buckley JG. Sprint kinematics of athletes with lower-limb amputations. Archives of Physical Medicine and Rehabilitation 1999;80:501-508.
- Czerniecki JM, Gitter A. Insights into amputee running: a muscle work analysis. American Journal of Physical Medicine & Rehabilitation 1992;71:209-218.
- Czerniecki JM, Gitter AJ, Beck JC. Energy transfer mechanisms as a compensatory strategy in below knee amputee runners. Journal of Biomechanics 1996;29(6):717-722.
- Czerniecki JM, Gitter A, Munro C. Joint moment and muscle power output characteristics of below knee amputees during running: the influence of energy storing prosthetic feet. Journal of Biomechanics 1991;24(1):63-75.
