The Biomechanics of Amputee Running

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By Robert Gailey, PhD, PT
Robert Gailey, PhD, PT
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. 

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

TTA running absorption phase
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.

TFA amputee running absorption phase
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

TTA running acceleration phase
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.

TFA running acceleration phase
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

TTA running deceleration phase
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.

TFA running deceleration phase
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 <br />Department of Orthopaedics<br />Division of Physical Therapy

References

  1. Buckley JG. Sprint kinematics of athletes with lower-limb amputations. Archives of Physical Medicine and Rehabilitation 1999;80:501-508.

  2. Czerniecki JM, Gitter A. Insights into amputee running: a muscle work analysis. American Journal of Physical Medicine & Rehabilitation 1992;71:209-218.

  3. 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.

  4. 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.