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. 2023 May 2;20(1):58.
doi: 10.1186/s12984-023-01177-w.

Can a powered knee-ankle prosthesis improve weight-bearing symmetry during stand-to-sit transitions in individuals with above-knee amputations?

Grace R Hunt  1 , Sarah Hood  2 , Lukas Gabert  2   3 , Tommaso Lenzi  2   3
Affiliations

Can a powered knee-ankle prosthesis improve weight-bearing symmetry during stand-to-sit transitions in individuals with above-knee amputations?

Grace R Hunt et al. J Neuroeng Rehabil. .

Abstract

Background: After above-knee amputation, the missing biological knee and ankle are replaced with passive prosthetic devices. Passive prostheses are able to dissipate limited amounts of energy using resistive damper systems during "negative energy" tasks like sit-down. However, passive prosthetic knees are not able to provide high levels of resistance at the end of the sit-down movement when the knee is flexed, and users need the most support. Consequently, users are forced to over-compensate with their upper body, residual hip, and intact leg, and/or sit down with a ballistic and uncontrolled movement. Powered prostheses have the potential to solve this problem. Powered prosthetic joints are controlled by motors, which can produce higher levels of resistance at a larger range of joint positions than passive damper systems. Therefore, powered prostheses have the potential to make sitting down more controlled and less difficult for above-knee amputees, improving their functional mobility.

Methods: Ten individuals with above-knee amputations sat down using their prescribed passive prosthesis and a research powered knee-ankle prosthesis. Subjects performed three sit-downs with each prosthesis while we recorded joint angles, forces, and muscle activity from the intact quadricep muscle. Our main outcome measures were weight-bearing symmetry and muscle effort of the intact quadricep muscle. We performed paired t-tests on these outcome measures to test for significant differences between passive and powered prostheses.

Results: We found that the average weight-bearing symmetry improved by 42.1% when subjects sat down with the powered prosthesis compared to their passive prostheses. This difference was significant (p = 0.0012), and every subject's weight-bearing symmetry improved when using the powered prosthesis. Although the intact quadricep muscle contraction differed in shape, neither the integral nor the peak of the signal was significantly different between conditions (integral p > 0.01, peak p > 0.01).

Conclusions: In this study, we found that a powered knee-ankle prosthesis significantly improved weight-bearing symmetry during sit-down compared to passive prostheses. However, we did not observe a corresponding decrease in intact-limb muscle effort. These results indicate that powered prosthetic devices have the potential to improve balance during sit-down for individuals with above-knee amputation and provide insight for future development of powered prosthetics.

Keywords: Amputee; Knee-ankle prosthesis; Powered prosthetics; Sit-down; Transfemoral; Wearable robotics.

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Conflict of interest statement

Lukas Gabert and Tommaso Lenzi are coinventors on patents and disclosures pertaining to Utah Bionics Leg, which was used in the study presented in the paper and licensed to Ottobock.

Figures

Fig. 1
Fig. 1
A subject sitting down with passive (left) and powered (right) prostheses during the experiment. Retroreflective markers on the subject track the movement of their body segments, and separate six-axis force plates record ground reaction forces from each leg. A wire connected to the powered leg synchronized data recording
Fig. 2
Fig. 2
Sit-down controller. Left: Prosthesis knee torque was controlled as a function of prosthesis knee position. A triangle-shaped profile was prescribed (yellow), and damping torque was added (blue). Right: prosthesis ankle equilibrium position was controlled as a function of prosthesis knee position (yellow). The equilibrium position was used in an impedance controller that added damping and stiffness around the equilibrium position, resulting in the prosthesis ankle position (blue). The start of the movement (standing) is indicated with circles, and the end of the movement (sitting) is indicated with squares. Black arrows indicate the progression of the movement
Fig. 3
Fig. 3
Weight-bearing asymmetry and prosthetic knee torque. (A) Index of Asymmetry (IOA) of the vertical GRF during sit-down with passive (gray) and powered (black) prostheses for all subjects from 0 to 100% of sit-down completion. Negative IOA indicates more weight on the prosthesis; positive IOA indicates more weight on the intact leg. Lines indicate across-subject means and shading indicates standard error (N=10 subjects). (B) Average IOA calculated between 0 and 100% of sit-down completion. Bar heights indicate the across-subject mean IOA (the across-subject average of the single-subject IOA means), and error bars indicate standard error (N=10 subjects). A paired t-test compared the across-subject means for passive and powered and found a significant difference (p = 0.0012). Colored dots overlaid on the bar plots indicate the single-subject IOA average (3 trials per subject) for each subject. The legend on the right shows which colored dot corresponds with each subject. (C) Prosthesis knee torque calculated using inverse dynamics. Shown during sit-down with passive (gray) and powered (black) prostheses for subjects who did not use hands, from 0 to 120% of sit-down completion. Knee extension torque is negative. Lines indicate across-subject means and shading indicates standard error (N=7 subjects). (D) Relation between prosthesis knee position and prosthesis knee torque during sit-down with passive (gray) and powered (black) prostheses for subjects who did not use hands, from 0 to 100% of sit-down completion. Lines indicate across-subject means and shading indicates standard error (N=7 subjects). Circle indicates start of the movement (standing), square indicates the end of the movement (sitting)
Fig. 4
Fig. 4
Muscle activity and torque from the intact leg. TF07 was removed from all parts of this figure (A-D) because of movement artifact in the vastus medialis EMG signal. (A) Normalized EMG from the intact-side vastus medialis muscle, shown from 0 to 120% of sit-down completion with passive (gray) and powered (black) prostheses. Lines indicate across-subject means and shading indicates standard error (N=9 subjects). (B) EMG peak (left) and EMG integral (right) between 0 and 100% of sit-down completion. Bar heights indicate across-subject means and error bars indicate standard errors (N=9 subjects). Colored dots overlaid on the bar plots indicate single-subject means (3 trials per subject). The legend on the right shows which colored dot corresponds with each subject. (C) Intact knee torque calculated using inverse dynamics. Shown during sit-down with passive (gray) and powered (black) prostheses for subjects who did not use hands (excluding TF07), from 0 to 120% of sit-down completion. Knee extension torque is negative. Lines indicate across-subject means and shading indicates standard error (N=6 subjects). (D) Relation between intact knee position and intact knee torque for all subjects who did not use hands (excluding TF07). Shown during sit-down with passive (gray) and powered (black) prostheses for subjects who did not use hands, from 0 to 100% of sit-down completion. Lines indicate across-subject means and shading indicates standard error (N=6 subjects). Circle indicates start of the movement (standing), square indicates the end of the movement (sitting)
Fig. 5
Fig. 5
Energy injected (positive) or dissipated (negative) by each joint during the sit-down movement, calculated between 0 and 100% sit-down movement for all subjects that did not use hands during sit-down. The energy injected or dissipated by each joint is calculated as the area under a torque-position curve for that joint. Bar heights indicate across-subject means and error bars indicate standard error (N=7 subjects). Colored dots overlaid on the bar plots indicate the single-subject means (3 trials per subject) for each prosthesis condition. The legend at the top of the figure shows which color of dot corresponds with each subject
Fig. 6
Fig. 6
Lower limb joint positions plotted from 0 to 120% of sit-down completion with passive (gray) and powered (black) prostheses for all subjects. Lines indicate across-subject means and shading indicates standard error (N=10 subjects). Dotted vertical lines indicate 0% and 100% of sit-down completion. Dotted horizontal lines indicate 0 degrees. Plots in the left column show intact-side joint angles, and plots in the right column show prosthesis-side joint angles. The rows show ankle joint positions (top row), knee joint positions (middle row) and hip joint positions (bottom row)
Fig. 7
Fig. 7
Ankle and hip joint torques plotted from 0 to 120% of sit-down completion with passive (gray) and powered (black) prostheses for subjects that did not use hands during sit-down. Lines indicate across-subject means and shading indicates standard error (N=7 subjects). Dotted vertical lines indicate 0% and 100% of sit-down completion. Dotted horizontal lines indicate 0 degrees. Plots in the left column show intact-side joint torques, and plots in the right column show prosthesis-side joint torques. The rows show ankle joint torques (top row) and hip joint torques (bottom row) calculated using inverse dynamics
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