Model and measurement studies on stages of prosthetic gait.   

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Prosthetic limb gait phenomena in transfemoral amputees observed in the real world

Maintaining sufficient balance is essential during gait, which can be seen as a process of continuous falling according to the German philosopher Hans Vaihinger (1852 - 1933) 17.

’Gehen ist ein reguliertes Fallen: mit jedem Schritt fällt der Mensch auf eine Seite durch Veränderung seines Gleichgewichtes und sucht den Fall durch Vorsetzung des anderen Fusses zu hemmen; auf dem antagonistischen Spiel solcher Funktionen beruht nicht bloß das Gehen, sondern beruhen auch sonstige organische Bewegungen.’ 1

This continuous falling, which is an antagonistic interaction between the two lower limbs, can be divided in four specific stages of prosthetic gait: gait initiation, weight bearing, prosthetic limb swing and gait termination. During these stages the prosthetic limb user has to control the inequality between the two limbs that take part in the teamwork, which is necessary during gait, to control the motions of the center of mass (CoM) by changing the orientation of the ground reaction force (GRF) and its origin, the center of pressure (CoP) under the prosthetic foot.

Gait initiation

Gait initiation is a task that challenges the balance control system by forcing an individual from a state of stable balance to an unstable motion during walking 18; 19; 20; 21. Gait initiation demands a complex integration of neural mechanisms, muscle activity and biomechanical forces 22; 23; 24. In persons with an amputation of the lower extremity, gait initiation may cause difficulties, because it is not possible to use an active ankle strategy 25 and the reduced sensory input system in the prosthetic limb. The lack of ankle strategy, which normally contributes to the posterior displacement of the CoP at gait initiation and thereby creating a forward momentum 21; 26 off the CoM, has to be compensated for with other strategies 27; 28. The lack of propulsive force during the end of the stance phase of the trailing prosthetic limb due to the absence of the calf muscles also influences the amputees’ performance 25; 29; 30; 31.

Weight bearing

The preference of experienced TF amputees to initiate gate with their prosthetic limb leading, indicates that they have implicit knowledge of the active control possibilities in their sound ankle, which they use to gain forward velocity 27. Because of these active control possibilities it seems advisable to initiate gait with the prosthetic limb leading. When considering the first step after gait initiation, in which the leading prosthetic limb becomes the stance limb and is used for weight bearing, the leading limb has to be placed in such a manner that sufficient knee stability is reached when loading the limb. The magnitude and orientation of the GRF under the prosthetic foot is determined by the angle at which the limb is placed, the internal moment of force around the hip joint, the angular velocities and gravitational forces on the body segments. When this GRF generates an external moment of force around the knee joint that remains within the limits of the knees stability, the knee will not buckle and stable stance will be achieved. In contrast to experienced prosthetic limb users, inexperienced patients are taught to initiate gait with their sound limb leading in the initial stage of therapy in our rehabilitation facility. This strategy ensures a stabilizing external extension moment on the prosthetic stance limb during gait initiation and minimizes the risk of falling during the first step, as the sound limb, with more control possibilities, becomes the stance limb. Consequently, in the second step the prosthetic limb becomes the stance limb again, with the same need to stabilize the knee.

Prosthetic limb swing

To make these steps, the prosthetic limb has to be swung forward, while the sound stance limb establishes a base of support that appropriately maintains stability to avoid slipping or falling. To move the limb forward in a safe and precise manner, the TF amputees have to take into account not only the properties of the prosthetic limb, but also the environment in which they are walking, especially during obstacle avoidance, which is a common problem during daily living activity 32. During a very complex combination of movement strategies, the swing limb must clear an obstacle successfully to avoid tripping 33; 34. The applied joint moments of the swing limb and the obstacle-foot distance during stance determine the clearance achieved during obstacle avoidance 35. Active flexion of the knee, as seen in able-bodied (AB) subjects and transtibial (TT) amputees 36; 37; 38; 39; 40, is not possible with a TF prosthetic limb. TT amputees increase swing hip elevation and hip and knee flexion as a function of obstacle height during obstacle avoidance. An increase of the knee flexion on the prosthesis side is achieved by modulating the moment of force at the hip, not at the knee, as seen on the amputee’s sound side 41; 42. In addition, the stance limb hip flexion, knee flexion and (on the sound side) ankle plantarflexion increase slightly with increased obstacle height, but the stance limb hip elevation does not. The lack of a knee strategy in TF amputees is compensated for by circumduction at the hip on the prosthesis side and by plantar flexion on the sound side 43. However, the extension strategy also has disadvantages. Not only does it make the prosthetic limb more visible, but also changes in the gait cycles are necessary when accelerating and decelerating the prosthetic limb in a lateral direction. Therefore, more degrees of freedom must be controlled. Additional free space is necessary for the clearance as the foot moves farther outward.

Gait termination

Successful gait termination with a TF prosthetic limb requires indirect control over this device with limited degrees of freedom. Gait termination studies in AB subjects show that several strategies are used to reduce the forward motion of the CoM 28; 44. By placing the leading limb on the ground in front of the body, a CoP under the foot is formed. The GRF originating from this CoP is used to decelerate the CoM. Also, by decreasing the push-off force with the trailing limb the forward motion is reduced 45; 46; 47; 48. During gait termination, the leading limb is for the most part responsible for the production of the necessary braking force 47. As a result of the absence of active control in the ankle joint, a prosthetic limb produces less braking ground reaction force under the leading prosthetic limb, compared to the force under the sound limb in a sound limb leading situation 44. Studies in prosthetic limb users show that the motion of the CoP is directly related to the stiffness of the prosthetic ankle, the orientation of the shaft, the position of the foot and the type of foot that is used 49; 50.

 

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