Model and measurement studies on stages of prosthetic gait.   

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Predictions on four stages of gait

Measurement data and mathematical models were used to investigate principles of TF amputee prosthetic gait in gait initiation, weight bearing, prosthetic limb swing and gait termination. These four stages were described in separate chapters in this thesis. Based on the predictions, outcomes and findings, we found strong indications that confirm the idea that during gait TF amputee mainly depend on the sound limb to accelerate and decelerate their body, by influencing CoP and GRF and changing timing of actions 2; 3; 4; 5; 6. Also, we can confirm the idea that TF amputee can use the GRF to extend their prosthetic knee during stance when bearing body weight on the prosthetic limb 7, and, on the other hand, TF amputee can also use GRF forces and fast hip motions to overcome the knee extension aiding spring 8; 9. Important to note is that TF amputee appear to have to learn to take into account the environment in which they are functioning and learn to combine body segment properties and dynamics, which enables them to enlarge their (limited) leeway during difficult tasks.

Gait initiation

In chapter 1 - Gait initiation we investigated how TF amputee control the spatial and temporal parameters which modulate the propulsive forces, the positions of the CoP and the CoM during prosthetic gait initiation. Force measurement data were used to calculate the CoM velocity curves in horizontal and vertical direction. Our forward dynamics model predicted that whether the TF subjects initiate gait with their prosthetic limb or with their sound limb, their horizontal end velocity is equal. The subjects compensated the loss of propulsive force under the prosthesis with the sound limb, both when the prosthetic limb is leading and when the sound limb is leading. In the vertical CoM velocity a tendency for differences between the two conditions was found. When initiating gait with the sound limb, the downward vertical CoM velocity at the end of the gait initiation was higher compared to when leading with the prosthetic limb. We concluded that our subjects used a gait initiation strategy which depended mainly on the active ankle function of the sound limb and therefore they changed the relative durations of the gait initiation anticipatory postural adjustment phase and the step execution phase.

Weight bearing

In chapter 2 - Weight bearing, we focused on the occurrences of stabilizing and destabilizing external moments of force on a prosthetic knee during stance, in the first steps after gait initiation, in inexperienced users. Primary aim was to identify the differences in the external moments during gait initiation with the sound limb leading and the prosthetic limb leading. A prosthetic limb simulator device with a flexible knee was used to test AB subjects, with no walking aid experience. Inverse dynamics calculations were preformed to calculate the external moments. The subjects learned to control the prosthetic limb within 100 steps, without walking aids, evoking similar patterns of external moments of force during the steps after the gait initiation, either with their sound limb loading or prosthetic limb leading. Our inverse dynamics model predicted that critical phases in which a sudden flexion of the knee can occur were found just after heelstrike and just before toe off, in which the external moment of force is close to the maximum internal moment produced by a knee extension aiding spring in the opposite direction. We conclude that from a safety perspective it does not matter which limb is the leading limb during gait initiation.

Prosthetic limb swing

In chapter 3 - Prosthetic limb swing, conditions that enable a prosthetic knee flexion strategy in TF amputee during obstacle avoidance were investigated. This study explored the hip torque principle and the static ground principle as object avoidance strategies. A prosthetic limb simulator device for AB subjects was used to study the influence of applied hip torques and static ground friction on the prosthetic foot trajectory. Inverse dynamics was used to calculate the energy produced by the hip joint. A two-dimensional forward dynamics model was used to investigate the relation between obstacle-foot distance and the necessary hip torques utilized during obstacle avoidance. Our inverse and forwards dynamics model predicted that a prosthetic knee flexion strategy is facilitated by the use of ground friction and by larger active hip torques. This strategy requires more energy produced by the hip compared to a knee extension strategy. We concluded that when an TF amputee maintains sufficient distance between the distal tip of the foot and the obstacle during stance, he or she produces sufficiently high, yet feasible, hip torques and uses static ground friction, the TF amputee satisfies the conditions to enable stepping over an obstacle using a knee flexion strategy.

Gait termination

In chapter 4 - Gait termination we investigated how leading limb angles combined with active ankle moments of a sound ankle or passive stiffness of a prosthetic ankle, influence the CoM velocity during the single limb support phase in gait termination. Also, we studied how the trailing limb acceleration influences the CoM acceleration during this phase. We analyzed force plate data from a group of experienced TF amputee using a prosthetic limb, and the outcome from a two dimensional mathematical forward dynamics model. We found that when leading with the sound limb, the subjects came almost to a full stop in the single limb support phase, without the use of the prosthetic limb. When leading with the prosthetic limb, the CoM deceleration was less in a relatively short single limb support phase, with a fast forward swing of the trailing sound limb. Slowing down the heavier trailing sound limb, compared to the prosthetic limb, results in a relatively larger braking force at the end of the swing phase. Our inverse and forwards model predicted that only narrow ranges of leading limb angle and ankle moments could be used to achieve the same CoM velocities with the mathematical model as the average start and end velocities of the prosthetic limb user. We conclude that users of prosthetic limbs have a narrow range of options for the dynamics variables to achieve a target CoM velocity. The lack of active control in the passive prosthetic ankle prevents the TF amputee from producing sufficient braking force when terminating gait with the prosthetic limb leading, forcing the subjects to use both limbs as a functional unit, in which the sound limb is mostly responsible for the gait termination.

 

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