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. 2023:31:11-21.
doi: 10.1109/TNSRE.2022.3214806. Epub 2023 Jan 30.

Effect of Increasing Assistance From a Powered Prosthesis on Weight-Bearing Symmetry, Effort, and Speed During Stand-Up in Individuals With Above-Knee Amputation

Grace R Hunt, Sarah Hood, Lukas Gabert, Tommaso Lenzi

Effect of Increasing Assistance From a Powered Prosthesis on Weight-Bearing Symmetry, Effort, and Speed During Stand-Up in Individuals With Above-Knee Amputation

Grace R Hunt et al. IEEE Trans Neural Syst Rehabil Eng. 2023.

Abstract

After above-knee amputation, the missing biological knee and ankle are commonly replaced with a passive prosthesis, which cannot provide net-positive energy to assist the user. During activities such as sit-to-stand, above-knee amputees must compensate for this lack of power using their upper body, intact limb, and residual limb, resulting in slower, less symmetric, and higher effort movements. Previous studies have shown that powered prostheses can improve symmetry and speed by providing positive assistive power. However, we still lack a systematic investigation of the effect of powered prosthesis assistance. Without this knowledge, researchers and clinicians have no framework for tuning powered prostheses to optimally assist users. Here we show that varying the assistive knee torque significantly affected weight-bearing symmetry, effort, and speed during the stand-up movement in eight above-knee amputees. Specifically, we observed improvements in the index of asymmetry of the vertical ground reaction force at the point approximating maximum vertical center of mass acceleration, the integral of the intact vastus medialis activation measured using electromyography, and the stand-up duration compared to the passive prosthesis. We saw significant improvements in all three metrics when subjects used the powered prosthesis compared to the passive prosthesis. We saw improvements in all three metrics with increasing assistive torque levels commanded by the powered prosthesis. We also observed increased weight-bearing asymmetry at the end of movement, and increased kinematic asymmetry with increasing assistance from the powered prosthesis. These results show that powered prostheses can improve functional mobility, potentially increasing quality of life for millions of people living with above-knee amputations.

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Figures

Fig. 1.
Fig. 1.
Experimental setup. The subject stands up with feet equally spaced on two force plates, without using their hands. Reflective markers track the movement of the subject’s body segments. (a) The subject stands up with their passive prosthesis. (b) The subject stands up with the powered prosthesis.
Fig. 2.
Fig. 2.
Index of asymmetry of the vertical ground reaction force (GRF), calculated using (1). Negative index of asymmetry indicates more weight on the intact side, and positive values indicate more weight on the prosthesis side. (a) Index of asymmetry at the peak of the sum of right and left GRFs. Bar heights indicate means, error bars indicate standard errors (N = 8 subjects). There was a significant effect of torque level (Table II). Significant differences from the passive trial are indicated with black plus signs. Significant differences from the 1.6 Nm/kg trial are indicated with red asterisks (Table II). Black curve is an exponential regression on the means of the powered torque levels, with the equation IOA(TL) = −41.81 e−0.7714TL−9.135. R2adj = 0.9861. (b) Index of asymmetry as a function of prosthesis-side stand-up completion, including time before the prosthesis knee movement starts (<0%) and after the prosthesis knee movement ends (>100%). Lines indicate means, shaded regions indicate standard errors (N = 8 subjects).
Fig. 3.
Fig. 3.
Intact vastus medialis muscle EMG. (a) Normalized vastus medialis EMG as a function of intact-side stand-up completion, including time before the intact knee movement starts (<0%) and after the intact knee movement ends (>100%). Lines indicate means, shading indicates standard errors (N=8 subjects). (b) Vastus medialis EMG integral calculated between −35% and 135% of intact-side stand-up completion, calculated using non-normalized time. Each subject’s EMG integrals were normalized to their passive prosthesis trial. The y-axis is cut between 0 and 0.6 to provide a more detailed view of the region of interest. Bar heights indicate means, error bars indicate standard errors (N=8 subjects). There was a significant effect of torque level (Table II). Significant differences from the passive trial are indicated with black plus signs. Significant differences from the 1.6 Nm/kg trial are indicated with red asterisks (Table II). Black curve is an exponential regression on the means of the powered torque levels, with the equation VMintegral(TL) = 0.565e−2.53TL + 0.751. R2adj = 0.9806.
Fig. 4.
Fig. 4.
Duration of the prosthesis-side stand-up, measured from 0% to 100% of prosthesis-side stand-up completion. Bar heights indicate means, error bars indicate standard errors (N=8 subjects). There was a significant effect of torque level (Table II). Significant differences from the passive trial are indicated with black plus signs. Significant differences from the 1.6 Nm/kg trial are indicated with red asterisks (Table II). Black curve indicates an exponential regression on the means of the powered torque levels, with the equation Duration(TL) = −1.23e−2.167 TL + 0.644. R2adj = 0.9865.
Fig. 5.
Fig. 5.
Joint angles and ground reaction forces (GRFs) from intact leg (left column) and prosthesis-side leg (right column). Lines indicate means, and shading indicates standard errors (N=8 subjects). (a) Intact ankle position. (b) Prosthesis ankle position. (c) Intact knee position. (d) Prosthesis knee position. (e) Intact-side hip position. (f) Prosthesis-side hip position. (g) Intact-side vertical GRF normalized to body weight. (h) Prosthesis-side vertical GRF normalized to body weight.
Fig. 6.
Fig. 6.
Differences between the intact-side stand-up duration and the prosthesis-side stand-up duration. Positive values indicate that the prosthesis knee performed the stand-up movement in less time than the intact knee.
Fig. 7.
Fig. 7.
Knee and hip asymmetry measures. Intact and prosthesis-side knee and hip angles were normalized, and asymmetry was calculated as a straight difference between normalized intact and normalized prosthesis positions. Lines indicate means, shading indicates standard errors (N=7 subjects). Positive values indicate that the prosthesis-side joint is more extended (further into the stand-up movement) than the intact-side joint (a) Kinematic asymmetry of the knee joints. (b) Kinematic asymmetry of the hip joints.
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