Controlling Knee Swing Initiation and Ankle Plantarflexion With an Active Prosthesis on Level and Inclined Surfaces at Variable Walking Speeds

Abstract

Improving lower-limb prostheses is important to enhance the mobility of amputees. The purpose of this paper is to introduce an impedance-based control strategy (consisting of four novel algorithms) for an active knee and ankle prosthesis and test its generalizability across multiple walking speeds, walking surfaces, and users. The four algorithms increased ankle stiffness throughout stance, decreased knee stiffness during terminal stance, as well as provided powered ankle plantarflexion and knee swing initiation through modifications of equilibrium positions of the ankle and knee, respectively. Seven amputees (knee disarticulation and transfemoral levels) walked at slow, comfortable, and hurried speeds on level and inclined (10°) surfaces. The prosthesis was tuned at their comfortable level ground walking speed. We further quantified trends in prosthetic knee and ankle kinematics, and kinetics across conditions. Subjects modulated their walking speed by ±25% (average) from their comfortable speeds. As speed increased, increasing ankle angles and velocities as well as stance phase ankle power and plantarflexion torque were observed. At slow and comfortable speeds, plantarflexion torque was increased on the incline. At slow and comfortable speeds, stance phase positive knee power was increased and knee torque more flexor on the incline. As speed increased, knee torque became less flexor on the incline. These algorithms were shown to generalize well across speed, produce gait mechanics that compare favorably with non-amputee data, and display evidence of scalable device function. They have the potential to reduce the challenge of clinically configuring such devices and increase their viability during daily use.

Keywords: Biomechanics; gait; powered knee and ankle prosthesis; sloped surface; transfemoral amputee; walking speed.

FIGURE 1.

State machine of walking consisting of 4 finite states (early-mid stance, late stance, swing flexion and swing extension). Onboard mechanical sensor thresholds were used to switch between states and were constant across conditions. Specifically, to detect heel strike and toe off, thresholds of the axial shank force were used. To switch between early-mid stance and late stance, ankle dorsiflexion thresholds were used. To switch between swing flexion and swing extension states, knee flexion velocity thresholds were used.

FIGURE 2.

Conceptual example depicting the use of the decreasing axial shank force during late stance to control powered ankle plantarflexion and knee swing initiation by modifying ankle and knee equilibrium angles, respectively. Left-hand column shows how at a fixed walking speed, tuning of a proportionality constant, C, can increase or decrease the rate of the function output. In this example, final equilibrium angle of the ankle is set to −12° (plantarflexion) and final equilibrium angle of the knee is set to −45° (flexion). For proportionality constants >1, the function is constrained to stop at each final value. Similarly, the right-hand column shows how the function output changes for an altered walking speed, with fixed tuning parameters (final equilibrium angles and proportionality constants). Note, changes of the equilibrium angles occur sooner and at a faster rate for increasing walking speed. Conversely, these changes occur later and at a slower rate for decreasing walking speed. Axial shank force data across a range of walking speeds are averaged from a group of non-amputees .

FIGURE 3.

Three of the various above-knee amputee users of the active knee and ankle prostheses, using the device to walk inside the laboratory up an incline as part of this experiment (left and middle), as well as outside the laboratory over level ground (right).

TABLE 1.

Above-knee amputee subject characteristics, including if subjects wore a microprocessor (MP) or non-microprocessor (NMP) controlled knee as their prescribed “home” prosthesis.

TABLE 2.

Individual subject (TF) and group averaged walking speed over level ground and an incline. Speed and surface main effects were significant as well as all pairwise comparisons.

FIGURE 4.

Group-averaged prosthetic knee and ankle angles and velocities during level ground walking. One standard deviation of the comfortable speed condition is shaded. Peak knee flexion, ankle dorsiflexion and ankle plantarflexion angles were compared as well as peak knee flexion and ankle plantarflexion velocities. Significant () speed (), surface () and interaction () effects are shown. Significant slow to hurried (□), slow to comfortable () and comfortable to hurried (■) pairwise comparisons using a Bonferroni adjustment for significance are indicated. Significant differences between level and incline walking at slow (○), comfortable () and hurried (■) conditions are also indicated.

FIGURE 5.

Group-averaged prosthetic knee and ankle angles and velocities during incline walking. One standard deviation of the comfortable speed condition is shaded. Peak knee flexion, ankle dorsiflexion and ankle plantarflexion angles were compared as well as peak knee flexion and ankle plantarflexion velocities. Significant () speed (▲), surface () and interaction () effects are shown. Significant slow to hurried (□), slow to comfortable () and comfortable to hurried (■) pairwise comparisons using a Bonferroni adjustment for significance are indicated. Significant differences between level and incline walking at slow (○), comfortable () and hurried (●) conditions are also indicated.

FIGURE 6.

Group-averaged prosthetic knee and ankle torques and powers during level ground walking. One standard deviation of the comfortable speed condition is shaded. Knee torque at terminal stance and peak ankle plantarflexion torque were compared as well as average stance phase positive and negative knee and ankle power. Significant () speed (▲), surface () and interaction () effects are shown. Significant slow to hurried (□), slow to comfortable () and comfortable to hurried (■) pairwise comparisons using a Bonferroni adjustment for significance are indicated. Significant differences between level and incline walking at slow (○), comfortable () and hurried (●) conditions are also indicated.

FIGURE 7.

Group-averaged prosthetic knee and ankle torques and powers during incline walking. One standard deviation of the comfortable speed condition is shaded. Knee torque at terminal stance and peak ankle plantarflexion torque were compared as well as average stance phase positive and negative knee and ankle power. Significant () speed (▲), surface () and interaction () effects are shown. Significant slow to hurried (□), slow to comfortable () and comfortable to hurried (◀) pairwise comparisons using a Bonferroni adjustment for significance are indicated. Significant differences between level and incline walking at slow (○), comfortable ( ) and hurried ( ●) conditions are also indicated.