1. Renee Kitto Port Macquarie Base Hospital
Biomechanics of BKA
Renee Kitto
Port Macquarie Base Hospital

2. Contents Normal gait Prerequisites of Normal Gait
Gait characteristics of BKA
Other causes of gait abnormalities

3. Normal gait Analysing pathological gait, compare to normal
Identifying gait deviation, you can work towards normal gait
Normal gait is the most energy efficient
In analysing pathological gait, normal function is the model against which it is judged.
The purpose of identifying gait deviation and its cause is to improve the gait pattern towards normal.
The nearer to normal the gait pattern becomes, the less energy is consumed and therefore a great advantage to the BKA
Uni-lateral BKA require up to 9% more oxygen consumption than normal.
So before we can identify gait deviations from the norm I though it would be a good idea to re-cap Normal gait.

4. Stance Phase
Begins at heel strike on one leg and ends at toe off on the same leg
Initial contact (heel Strike)
Loading response (0-10%)
Mid-stance (10-30%)
Terminal stance (push off) (30-50%)
Pre swing (toe off) (50-60%)
Heel strike is a momentary event as the foot strikes the ground and it initiates the period of stance. The leg is optimally positioned to initiate progression and knee stability. The ankle is in neutral DF, knee extended and the hip flexed at approx 30 degrees.The foot is held in the neutral position by the ankle dorsiflexors. Knee extension is passive and the hip extensors (hamstrings and glut max) decelerate the thigh at t his point.
Loading response is the initial period of double support, during which weight is transferred onto the stance limb. This drives the forefoot towards the floor resulting in PF and knee flexion. The knee flexion provides shock absorption and is controlled by eccentric contraction of the quads while maintaining knee stability. PF is controlled by eccentric contraction of DF lowering foot to the ground. Hip extension is initiated by concentric contraction of the hip extensors.
Mid stance progression is dependent on the pivotal action of the ankle rocker to advance the limb over the supporting foot. Eccentric contraction of the PF advances the tibia over the foot achieving approx 5 degrees of DF by the end of mid stance, Progression is assisted by momentum of the contralateral swinging limb.The ground reaction vector is moved ant to the ankle and knee joint therefore assisting progressive knee extension. By end of mid stance, the ground reaction vector is posterior to the hip joint necessitating minimal muscle activity to extend the hip from 30 to 10 degrees of hip flexion. As the opposite foot leaves the ground, the hip abductors contract to maintain a stable pelvis.
Terminal stance the body is advanced ahead of the stationary foot achieving max DF and the heel raises. Passive hip and knee ext allows forward progression of the trunk. There is a powerful contraction of the PF which assist in accelerating the body forwards. Gastrocs also acts as a knee flexor and initiates the second wave of knee flexion, which begins at the end of terminal stance.
Pre-swing is the final phase of stance. Body weight is unloaded and transferred to the contralaterl limb. The critical action during this period is the rapid initiation of knee flexion contributing to limb advancement in swing. Peak PF of approx 20 degrees is reached at the end of this phase but PF activity is rapidly decreasing as the limb is being unloaded.

5. Swing Phase
Begins where stance ends and is the period between toe off on one leg and heel strike on the same leg
Initial swing (60-73%)
Mid-swing (73-87%)
Terminal-swing (87-100%)
1)Momentum generated in pre-swing continues in initial swing and is facilitated by hip and knee flexor activity. In order to assure foot clearance, the knee reaches its peak flexion of approx60degrees and the ankle rapidly begins to dorsiflex.
2) Mid-swing continues the task of limb advancement and foot clearance. Hip flex and knee extension are essentially passive. The hip continues to flex and the knee begins to extend assisted by gravity once the knee flexors are relaxed. The ankle continues to DF to neutral.
3) Terminal swing is the transition between swing and stance and muscle activity is intense. Limb advance ment is completed by active knee extension to neutral. Eccentric contraction of glut max and the hamstrings decelerate the hip and knee in readiness for heel strike and neutral ankle DF is maintained.

6. Prerequisites of normal gait
Stability in Stance
Clearance in swing
Pre-position of the foot in terminal swing
Adequate step length
Energy conservation
1) Stability in stance requires trunk stability and adequate central balance. The COM must remain within the BOS while standing, and move forward from one BOS to the next while walking
2)Clearance in swing requires stability of the stance foot, appropriate positioning and power of the ankle, hip and knee on the stance side and adequate D/F knee and hip flexion on the swing side.
3)An adequate step length requires a stable and appropriately positioned stance limb, while the swing limb needs adequate hip flexion, relatively complete knee extension and neutral D?F.
4) To conserve energy, the body uses several biomechanical mechanisms including joint stability provided by the ground reaction force in conjunction with ligaments wherever possible. COM excursion is minimised.
Interestingly Unilateral BKA require up to 9% more mean oxygen consumption than normal.

7. Gait characteristics BKA gait is asymmetrical
BKA perform an asymmetrical gait and it is to be expected. One of the main reasons that is out of our control is the result of the inherent variability and inability of foot prosthesis to meet the kinematic demand to produce a powerful PF moment. Also the loss of ankle proprioception to help provide stability in stance. It is important to try and get as close to normal gait as we can because it is the most energy efficient and therefore most beneficial for the patient however symmetrical gait is not going to happen with BKA. Therefore it is important to look at the adaptions and compensations that BKA make in order to either a) help correct them closer to the norm or b) use them to our advantage to help assist in energy conservation.
So what are these adaptions and compensations or changes in biomechanics that BKA perform.

8. Gait Characteristics Temporal and distance factors
Stance phase shorter on prosthetic side
Step length of the prosthetic side longer and faster
Self selected walking velocity is lower
Decreased Cadence
Average stride length is shorter
1) The stance time on the prosthetic side is reported to be less than the unaffected limb and therefore the swing phase in the prosthetic side lasts longer. One study showed that single limb support was 37% of the gait cycle for the affected limb and 43% for the unaffected limb. This is common among pathological gait patterns and a cause of this may be a lack of trust of the affected side for weight bearing
2) Obviously then the step length of the prosthetic side is longer but interestingly it is accomplished in less time therefore the acceleration of the involved limb is probably greater than the acceleration of the uninvolved limb during the swing phase of the gait cycle
3) Self selected speed of walking among amputees is lower than mean normal and even after an isometric exercise program it was shown that amputees have lower than normal walking velocity
4) Mean cadence for amputees is slightly less than common values for normal
5) The average stride length is shorter than the mean normal values

9. Gait Characteristics Joint Angles
Decreased knee flexion (prosthetic side) during early stance
Decreased knee flexion (prosthetic side) during late stance
Larger relative knee angle range on the prosthetic side compared to the unaffected side
Greater than normal positions of maximum hip flexion (prosthetic side)
Increased knee flexion (unaffected side) during early stance
1) The knee flexion of the amputee is less than the mean normal amount during early stance. This occurs because the prosthetic foot does not produce the controlled P/F obtained naturally by eccentric contraction of the D/F
2) During late stance, knee flexion is also less than mean normal values. Usually knee motion coordinates with foot motion. Unlike the anatomical foot, which P/F at toe-off, the prosthetic ankle cannot move when weight has been transferred to the toe section. If the amputee were to allow the knee to flex to the mean normal extent, the torso would lower excessively, producing an inefficient, abnormal gait.
3) The prosthetic side knee relative angle range is considerably larger than that of the unaffected side.
4) Greater than mean normal positions of maximum hip flexion (by about 10%) occurred on the prosthetic side during the gait cycle. The increased flexion may be attributed to the tendency for the amputees to increase their step length on the prosthetic side. It may also be related to concurrent differences in knee angles. For instance if the knee was flexed more than mean normal in mid stance phase as it was shown in one study, the hip would be flexed more to keep the posture upright. The increased hip flexion could also be the result of a slight forward bending posture. Because BKA are less secure on their artificial limb, they tend to lean forward to bring the center of gravity over the foot.
5)There is increased knee flexion on the unaffected side during early stance. The most persuasive reason for this is that it provides a compensatory energetic advantage by increasing the power burst that occurs at the knee during stride when the knee goes into extension.

10. Gait Characteristics Joint moments Unaffected side Prosthetic side
Higher hip extensor moment during stance
Higher hip flexor moment during early swing
Higher knee extension moment during stance
Prosthetic side
Ankle D/F moment longer in duration and larger in amplitude during early stance
1) The hip extensor moment in early stance has two functions 1) to provide support to the linked multi-segment system and 2) to provide a small amount of power to contribute to the work of walking.
3) One study showed a higher than mean normal knee extension during stance on the unaffected side. Because a lower body position occurs during prosthetic foot roll over, the knee on the unaffected side is more flexed then mean normal. Thus requiring a larger then normal extension moment. This higher extensor moment can also be seen during late stance. Downward movement of the body attributed to heel compression and stump vertical movement on the prosthetic side would lead to increased knee flexion. Greater knee flexion during stance normally results in larger knee extension moments to prevent collapse. Another reason could be the desire to create more support and security. The knee extensors are major contributors to the support of the lower limb segments.
4) The ankle D/F moment in early stance is both longer in duration and larger in amplitude than mean normal on the prosthetic side. In normal subjects, foot flat occurs under the control of the D/F muscles, achieving an early foot flat position optimal for obtaining a balance position after heel strike. However, amputees using a SACH foot have no means of P/F the foot and therefore the beginning of the plantarflexor moment is delayed.

11. Gait Characteristics Joint Power Heel Contact (unaffected side)
Increased hip extensor activity (unaffected side)
Heel Contact (prosthetic side)
Increased hip abductor activity (unaffected side)
Increased knee extensor activity (unaffected side)
Increased hip extensor muscle bursts on both sides
Two articles gave very in depth information in regards to power generation in BKA. So I have tried to pick out the major components involved. And I have concentrated only on areas with increased muscle activity .
1) With heel contact on the unaffected side increased hip extensor activity was found. This could be explained by the fact that because of the loss of normal P/For and forefoot functions during the push off period, the hip extensors on the sound limb start to act earlier than expected to push the body from behind and also control the collapse of the trunk.
2) At the time of heel contact with the floor on the prosthetic side and when the unaffected side is at push off increased hip abductor activity was found and might compensate for the absence of normal D/For activity on the prosthetic limb by absorbing the effect of the impact produced during body weight transfer. Increased knee extensor activity also on the unaffected side accompanies this event to absorb any impact from weight acceptance
3) Increased hip extensor muscle power bursts was found on both sides at heel contact when compared to able bodied subjects. This could be explained in part by the fact that the hip extensors not only act to prevent the collapse of the knee but also could be considered as the first limb propeller parameter as well as the parameter that makes up for the loss of upright support as a result of the prosthetic ankle

12. Gait Characteristics Joint Power Midstance Push-off
Increased hip abductor activity (prosthetic side)
Push-off
Increased hip flexor power generation (prosthetic side)
1)It was shown that there is increased hip abductor activity on the prosthetic side during midstance and this can be explained by the greater need of the prosthetic side to maintain dynamic balance when propulsion is in progress. You have lost all that proprioceptive import from the foot and the ankle to help provide that balance.
2) Hip flexor activity at push off has been recognised as a major contributor to limb propulsion. Therefore increased muscle power generation on the prosthetic side is most probably used to compensate for the lack of normal ankle function. The hip flexors during push off on the prosthetic side should be considered a second main source of facilitating the propulsion of the prosthetic limb. The first is the increased hip extension activity on the unaffected side as I mentioned earlier.

13. Patients capability and general condition
Other causes of gait abnormalities
Prosthesis
Patients capability and general condition
Shape, length and size of the residual limb
Discomfort
Inadequate or incorrect re-education
Psychological, social or economic reasons
1)Different prothesis and components can greatly influence gait. As I was doing the lit search I found a lot of information regarding different types of prosthesis's. Different components involved and even the mass of the prosthetic limb and the influence it has on gait.
2) It is not possible to replace all movements with the prosthetic components. Prosthesis is mainly determined by the patients capability because the individual amputee is not always capable of managing sophisticated components. Therefore we must retrain the gait, bearing in mind the amputees physical ability and the design of the prosthesis supplied. Co-morbidities and there effects on gait
3) Impacts on the fitting of the prosthesis and how much force the patient can produce at the knee.
4) discomfort caused from the prosthesis, pressure areas, rubbing. Because we often treat amputees during the early stages in which the residual limb is still maturing, it is found that many gait deviations are caused by residual limb discomfort and volumetric changes

14. References
Bateni, H et al (2002) Kinematic and Kinetic Variations of Below-Knee Amputee Gait. Journal of Prosthetics and Orthotics, 14, 2-10.
Engstrom, B&, Van de Ven, C (1999) Therapy for Amputees. London: Churchill Livingstone.,
Robinson, J et al (1977) Accelerographic, Temporal, and Distance Gait Factors in Below-Knee Amputees. Physical Therapy, 57,

15. References
Sadeghi, H et al (2001)Muscle Power Compensatory Mechanisims in Below- Knee Amputee Gait. American Journal of Physical Medicine and Rehabilitation, 80,
Ruud, W et al (2004) Adaptions to Mass Perturbations in Transtibial Amputees: Kinetic or Kinematic Invariance. Archives of Physical Medicine and Rehabilitation, 85,