Research ArticleNEUROPROSTHETICS

Proprioception from a neurally controlled lower-extremity prosthesis

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Science Translational Medicine  30 May 2018:
Vol. 10, Issue 443, eaap8373
DOI: 10.1126/scitranslmed.aap8373
  • Fig. 1 Agonist-antagonist myoneural interface.

    (A) Two AMIs were surgically constructed within the left leg residuum of a patient to enable control of prosthetic subtalar and ankle joint movements. Prosthetic subtalar and ankle movements are shown in (A1) and (A2), and (A3) and (A4), respectively. In (A1), the prosthetic subtalar joint everts (arrow) when the peroneus longus contracts, stretching the tibialis posterior; in (A2), the subtalar joint inverts (arrow) when the tibialis posterior contracts, stretching the peroneus longus. In (A3), the prosthetic ankle joint dorsiflexes (arrow) when the tibialis anterior contracts, stretching the lateral gastrocnemius; in (A4), the ankle joint plantar-flexes (arrow) when the lateral gastrocnemius contracts, stretching the tibialis anterior. Dashed arrows indicate muscle contraction and stretch. (B) Ultrasound strain and EMG data for the subtalar AMI, showing coupled motion when the peroneus longus is stretched during volitional contraction of the tibialis posterior [inversion movement (A2)]. The correlation coefficient of these two signals is 0.94. (C) Ultrasound strain and EMG data for the ankle AMI, showing coupled motion when the tibialis anterior is stretched during volitional contraction of the lateral gastrocnemius [plantar flexion movement (A4)]. The correlation coefficient of these two signals is 0.91. (B) and (C) are representative traces from subject A (n = 5 trials per motion). EMG values are normalized to calibrated maxima for each muscle.

  • Fig. 2 Volitional control of joint position and impedance.

    (A) Schematic showing how subject A activates the AMI muscle associated with his intended motion. This activation is recorded as EMG and generates a movement command for the motors within the prosthesis. The subject can stiffen a prosthetic joint by simultaneously coactivating both the agonist and the antagonist muscles within the AMI associated with that joint. Afferent signals describing prosthetic joint movement are communicated to the patient’s nervous system via muscle spindle response to differential stretch relationships within each AMI muscle. (B) Average performance maps for volitional control tasks (n = 100 samples from subject A, n = 350 samples from group T). The scores for each metric are presented by target area; the location of each rectangle within the axis represents the target area in joint space, ranging from full plantar flexion (PF) to full dorsiflexion (DF) and from full eversion (EV) to full inversion (IN). The shade of the rectangle indicates the subject’s score in that target area, where lighter shades are indicative of better performance. (C) Representative sample traces of joint position (angle), EMG, and ankle stiffness during free-space volitional control experiments for subject A (n = 100 total samples) and one subject from group T (subject T2, n = 50 total samples). Dashed vertical lines divide the trial into segments by target motion, indicated by the text at the top of each segment. The shaded region of each plot represents the portion of that trial in which the subject was instructed to stiffen the joint. The range of ankle angles shown is the full range of the prosthetic ankle: from 15 degrees of PF to 10 degrees of DF. The range of subtalar angles shown is the full range of the prosthetic subtalar: from 15 degrees of EV to 15 degrees of IN. Ankle and subtalar angle plots show target position (black) and actual position (purple). The ankle EMG plot shows signal recorded from the lateral gastrocnemius (light blue) and the tibialis anterior (dark blue). The subtalar EMG plot shows signal recorded from the tibialis posterior (light green) and the peroneus longus (dark green). EMG values are normalized to calibrated maxima for each muscle. Stiffness values are normalized such that a value of 1 represents coactivation of the tibialis anterior and the lateral gastrocnemius at each muscle’s calibrated maximum.

  • Fig. 3 Simultaneous subtalar and ankle control during a gait task requiring volitional eversion.

    Joint position and EMG during the swing phase of gait, as subject A (n = 10 trials) and each subject from group T (n = 32 trials) step onto the side of a block positioned on the floor to require eversion (arrow) of the prosthetic subtalar joint, as shown in the schematic. Shaded traces indicate mean ± 1 SD. Positive and negative subtalar angles correspond to eversion (EV) and inversion (IN), respectively. Positive and negative ankle angles correspond to dorsiflexion (DF) and plantar flexion (PF), respectively. The subtalar EMG plot shows signal recorded from the peroneus longus (light green) and the tibialis posterior (dark green). The ankle EMG plot shows signal recorded from the lateral gastrocnemius (light blue) and the tibialis anterior (dark blue). EMG values are normalized to calibrated maxima for each muscle.

  • Fig. 4 Reflexive control during stair tasks.

    Ankle position and EMG while each subject (A) ascends and (B) descends stairs. Shaded traces indicate mean ± 1 SD for subject A (n = 10 trials for each of ascent and descent) and each subject from group T (n = 32 trials for each of ascent and descent). The ankle EMG plots show signal recorded from the lateral gastrocnemius (light blue) and the tibialis anterior (dark blue). Arrow indicates direction of movement. EMG values are normalized to calibrated maxima for each muscle.

  • Fig. 5 Closed-loop torque control.

    (A) Schematic of the prosthesis-in-the-loop control architecture, in which afferent feedback of prosthetic joint torque is provided via FES of the antagonist muscle. The patient perceives this stimulation as a natural sensation of ankle torque. (B) Magnitude estimation of perceived dorsiflexion torque as a function of stimulation current delivered to the tibialis anterior. Perceived torques are normalized to the maximum reported value. For clarity in plotting, each point represents the mean value of five independent trials. Error bars represent the SE, and the R2 coefficient reported on the plot is that of the mean values. (C) Discrimination performance as a function of differences in stimulation current. The reference current for all forced choice trials was 2 mA. Points indicate percentage of test stimuli correctly identified as stronger or weaker than the reference over 20 pairwise trials, and the green line represents a cumulative normal distribution fit to the raw data. (D) Representative sample traces of lateral gastrocnemius EMG (blue), torque (purple), and stimulation current (green) during closed-loop torque control trials for the “stimulation on” (n = 79 total trials) and “stimulation off” (n = 79 total trials) cases. Numbers at the top of the plot correspond to percent effort commands. Stimulation currents are normalized to 9 mA. EMG values are normalized to calibrated maxima for each muscle. (E) Summary data for closed-loop torque control trials in each of the stimulation on (n = 79 trials), stimulation off (n = 79 trials), and “unaffected limb” (n = 80 trials) cases. An asterisk above a bar indicates that the bar is significantly different from all other bars in the plot (P < 0.025). Where no significance was seen, a P value for the comparison is shown. Error bars represent a 99.9% confidence interval on the mean.

  • Table 1 Summary data for terrain traversal trials.

    Summary data for terrain traversal trials.. The metric in the second column was calculated for each trial of the task named in the first column and averaged within each subject to give an overall subject score. Subject A’s overall subject score for each task (from n = 10 trials per task) is reported in the third column. The fourth column reports mean ± 1 intersubject SD for group T (n = 4 subjects, n = 32 total trials per task). Late swing eversion, late swing dorsiflexion, and late swing plantar flexion were calculated as the maximum eversion, dorsiflexion, and plantar flexion angles, respectively, achieved between 80 and 100% of the swing phase of the relevant task.

    TaskMetricSubject A
    (n = 1)
    Group T (n = 4)
    Eversion blockLate swing
    eversion
    8.8 degrees of
    eversion
    4.8 (±5.9) degrees
    of inversion
    Stair ascentLate swing
    dorsiflexion
    7.3 degrees of
    dorsiflexion
    7.0 (±3.8) degrees
    of plantar
    flexion
    Stair descentLate swing
    plantar
    flexion
    11.9 degrees
    of plantar
    flexion
    2.3 (±3.2) degrees
    of plantar
    flexion

Supplementary Materials

  • www.sciencetranslationalmedicine.org/cgi/content/full/10/443/eaap8373/DC1

    Materials and Methods

    Fig. S1. Control diagrams showing how EMG from the AMI muscles drives movement of the prosthetic joint.

    Fig. S2. Raw EMG recorded from fine-wire electrodes during volitional movement of the phantom limb.

    Movie S1. Ultrasound video of coupled AMI motion.

    Movie S2. Volitional control.

    Movie S3. Reflexive control.

    Movie S4. Candid videos showing prosthesis embodiment.

    Movie S5. Visual confirmation of stimulated muscle contraction.

    Table S1. Individual subject data for volitional control tasks.

  • Supplementary Material for:

    Proprioception from a neurally controlled lower-extremity prosthesis

    Tyler R. Clites, Matthew J. Carty, Jessica B. Ullauri, Matthew E. Carney, Luke M. Mooney, Jean-François Duval, Shriya S. Srinivasan, Hugh. M. Herr*

    *Corresponding author. Email: hherr{at}media.mit.edu

    Published 30 May 2018, Sci. Transl. Med. 10, eaap8373 (2018)
    DOI: 10.1126/scitranslmed.aap8373

    This PDF file includes:

    • Materials and Methods
    • Fig. S1. Control diagrams showing how EMG from the AMI muscles drives movement of the prosthetic joint.
    • Fig. S2. Raw EMG recorded from fine-wire electrodes during volitional movement of the phantom limb.
    • Legends for movies S1 to S5

    [Download PDF]

    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). Ultrasound video of coupled AMI motion.
    • Movie S2 (.mp4 format). Volitional control.
    • Movie S3 (.mp4 format). Reflexive control.
    • Movie S4 (.mp4 format). Candid videos showing prosthesis embodiment.
    • Movie S5 (.mp4 format). Visual confirmation of stimulated muscle contraction.
    • Table S1 (Microsoft Excel format). Individual subject data for volitional control tasks.

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