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Assistance magnitude versus metabolic cost reductions for a tethered multiarticular soft exosuit

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Science Robotics  18 Jan 2017:
Vol. 2, Issue 2, eaah4416
DOI: 10.1126/scirobotics.aah4416
  • Fig. 1 Soft exosuit.

    The soft exosuit used in the study consists of a waist belt, two calf wraps, and four vertical straps. An actuator is used to retract the inner cable of a Bowden cable assembly and to apply a force local to the ankle to assist with plantar flexion. Some of this force is also transmitted through the vertical straps to the front of the waist belt to assist with hip flexion.

  • Fig. 2 Changes in net metabolic rate.

    Changes in net metabolic rate of the four active conditions continually decreased with increasing exosuit assistance relative to the powered-off condition. Change in net metabolic rate was −1.017 ± 0.137 W kg−1 (mean ± SEM) under the MAX condition, which corresponds to a 22.83 ± 3.17% decrease compared with the powered-off condition. Asterisks indicate significant differences relative to the powered-off condition (paired t test; PHIGH = 9 × 10−5; PMAX = 2 × 10−4).

  • Fig. 3 Changes in ankle moment and power.

    (A) Total (solid line) and exosuit (dashed line) ankle moment normalized to body mass over the gait cycle for each condition, averaged across participants. Gray shading outlines approximate region during which assistance was applied. Bar graph on the right indicates the total (solid bar) and exosuit (striped bar) peak ankle moment. n = 7; error bar indicates SEM. ANOVA tests were run for an effect on exosuit assistance (defined as peak exosuit ankle moment). Peak total ankle moment decreased (P = 4 × 10−9) with increasing exosuit assistance. Peak biological (total minus exosuit) ankle moment also decreased (P = 3 × 10−24) with increasing exosuit assistance. (B) Total and exosuit ankle power. Bar graph indicates total (white bar), exosuit (striped bar), and biological (solid bar) average positive ankle power and average negative ankle power. Average positive total power increased in magnitude (P = 4 × 10−8) and average negative total power decreased in magnitude (P = 4 × 10−6) with increasing exosuit assistance. Both average positive (P = 2 × 10−19) and negative (P = 7 × 10−3) exosuit ankle power increased in magnitude with increasing exosuit assistance. Average negative biological (total minus exosuit) ankle power decreased in magnitude (P = 7 × 10−6) with increasing exosuit assistance. Average positive biological ankle power decreased but not significantly (P = 0.099). PF, plantar flexion; DF, dorsiflexion; Tot., total; Exo., exosuit.

  • Fig. 4 Changes in hip moment and power.

    (A) Total (solid line) and exosuit (dashed line) hip moment normalized to body mass over the gait cycle for each condition, averaged across participants. Gray shading outlines approximate region during which assistance was applied. Bar graph on the right indicates the total (solid bar) and exosuit (striped bar) peak hip moment. n = 7; error bar indicates SEM. ANOVA tests were run for an effect on exosuit assistance (defined as peak exosuit ankle moment). Peak total hip moment decreased (P = 3 × 10−4) and peak exosuit hip moment increased (P = 9 × 10−19) with increasing exosuit assistance. Peak biological hip moment during push-off decreased (P = 1 × 10−17) with increasing exosuit assistance. (B) Total and exosuit hip power. Bar graph indicates total (white bar), exosuit (striped bar), and biological (solid bar) average positive hip power and average negative hip power. Both average positive (P = 7 × 10−11) and negative (P = 2 × 10−13) exosuit hip power increased with increasing exosuit assistance. Average positive total hip power decreased (P = 7 × 10−6), whereas negative total hip power remained unchanged (P = 0.985). Average positive (P = 2 × 10−7) and negative (P = 3 × 10−9) biological hip power decreased in magnitude with increasing exosuit assistance. Ext., extension; Flex., flexion.

  • Fig. 5 Changes in joint kinematics.

    Left: The sagittal plane joint angle for the hip, knee, and ankle over the gait cycle averaged across all participants for each experimental condition. Gray shading outlines approximate region during which assistance was applied. Right: The peak flexion and extension angles of each joint. n = 7; error bar indicates SEM. ANOVA tests were run for an effect on exosuit assistance (defined as peak exosuit ankle moment). (A) Maximum hip extension decreased (P = 5 × 10−9) and maximum hip flexion increased (P = 0.026) with increasing exosuit assistance. (B) Knee flexion (P = 0.380) and extension (P = 0.779) remained unchanged. (C) Dorsiflexion decreased (P = 7 × 10−9) and plantar flexion increased (P = 3 × 10−10) with increasing exosuit assistance.

  • Fig. 7 Experimental setup.

    While a participant is walking with the soft exosuit, an off-board actuation system generates assistive forces and Bowden cables transfer the forces to the exosuit. Sensors (load cells and gyroscopes) are attached to measure real-time data. Body segment motions (motion capture), ground reaction forces (instrumented treadmill), and metabolic rate (indirect calorimetry) are measured.

  • Fig. 6 Summary of biologically inspired controller.

    (A) Model of the system stiffness, where Δxmotor is the summation of (i) the travel required to track the ankle motion (Δxankle) and (ii) the travel due to the soft functional textile stretching and the human tissue compressing for a desired force profile (Δxsuit). (B) Static suit stiffness curve based on the Bowden cable position and force applied to the soft functional textile, as previously described by Asbeck et al. (37). (C) Values for Δxankle and Δxsuit across the gait cycle and the resulting calculation of Δxmotor, which was approximated as Δxmotor_approx for the controller developed in this study.

Supplementary Materials

  • robotics.sciencemag.org/cgi/content/full/2/2/eaah4416/DC1

    Supplementary Text

    Fig. S1. Structure of the waist belt component.

    Fig. S2. Structure of the calf wrap component.

    Fig. S3. Structure of the Y-strap part of the calf wrap component.

    Fig. S4. Structure of the vertical strap component.

    Fig. S5. Changes in ground reaction forces and center of mass velocity.

    Fig. S6. Changes in center of mass power.

    Fig. S7. Hip and ankle force profiles.

    Fig. S8. Changes in biological joint moment.

    Fig. S9. Changes in biological joint power.

    Fig. S10. Net biological joint power.

    Table S1. Changes in net metabolic rate for each participant.

    Movie S1. Varying assistance level with a soft exosuit: Methods and metabolic results.

    References (4244)

  • Supplementary Materials

    Supplementary Material for:

    Assistance magnitude versus metabolic cost reductions for a tethered multiarticular soft exosuit

    B. T. Quinlivan, S. Lee, P. Malcolm, D. M. Rossi, M. Grimmer, C. Siviy, N. Karavas, D. Wagner, A. Asbeck, I. Galiana, C. J. Walsh*

    *Corresponding author. Email: walsh{at}seas.harvard.edu

    Published 18 January 2017, Sci. Robot. 2, eaah4416 (2017)
    DOI: 10.1126/scirobotics.aah4416

    This PDF file includes:

    • Supplementary Text
    • Fig. S1. Structure of the waist belt component.
    • Fig. S2. Structure of the calf wrap component.
    • Fig. S3. Structure of the Y-strap part of the calf wrap component.
    • Fig. S4. Structure of the vertical strap component.
    • Fig. S5. Changes in ground reaction forces and center of mass velocity.
    • Fig. S6. Changes in center of mass power.
    • Fig. S7. Hip and ankle force profiles.
    • Fig. S8. Changes in biological joint moment.
    • Fig. S9. Changes in biological joint power.
    • Fig. S10. Net biological joint power.
    • Table S1. Changes in net metabolic rate for each participant.
    • References (4244)

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    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). Varying assistance level with a soft exosuit: Methods and metabolic results.

    Files in this Data Supplement: