Ankle Robotics for Error Augmentation After Stroke

This is a single-arm pilot study evaluating robotic error augmentation of paretic ankle propulsion during gait training in adults with chronic stroke. Ten participants with chronic (>6 months) stroke and residual ankle weakness will complete 30 training sessions using the AMBLE portable ankle exoskeleton. Assessments are conducted at baseline, after 15 sessions, and after 30 sessions. The hypothesis is that 30 sessions of robot-assisted treadmill gait training enhances paretic ankle plantarflexion propulsive torque and gait symmetry.

Approximately half of stroke survivors are left with chronic mobility disability, typically involving ankle weakness that impairs walking and increases fall risk. Current care relies on ankle-foot orthoses, functional electrical stimulation, canes, and walkers, none of which address the underlying deficit in ankle motor control. Weakness of the paretic plantarflexors limits push-off during late stance, reducing forward propulsion and contributing to reduced walking speed and gait asymmetry.

The investigators previously developed the AMBLE, a lightweight portable ankle exoskeleton that uses impedance-based control with real-time gait event detection to deliver torque synchronized to specific phases of the gait cycle. Prior work by this group using laboratory-based and portable ankle robotics targeted dorsiflexion, demonstrating reduced foot drop and improved overground walking in both early and chronic stroke, and the present study extends an ongoing investigation of the device (NCT04594837). Paretic plantarflexion propulsion has not previously been targeted with the AMBLE. Recent engineering modifications enable graded resistance during the push-off phase of gait, permitting investigation of whether error augmentation applied to paretic ankle propulsion can enhance forward propulsion during walking.

Training sessions consists of two 10-minute bouts of treadmill walking at progressively increasing speed and one 10-minute bout of overground walking as able, with the robot donned on the paretic leg. Plantarflexion resistance is delivered through the ankle robot and calibrated individually, increased as tolerated up to the level just below the point at which hip circumduction or instability emerges. Resistance is applied after attainment of steady-state walking speed and intermittently removed at predetermined intervals to sample voluntary paretic ankle power. Treadmill speed is then progressed toward a target of 70-85% heart rate reserve and RPE 15-18, recorded every 3-5 minutes. Seated rest of 2-3 minutes is provided between bouts.

Visual feedback of ankle propulsion power is delivered through a tablet-based application on a faded schedule: continuously during weeks 1-2, for 1 minute every 5 minutes during weeks 3-4, for 1 minute at the end of each 10-minute bout during weeks 5-6, and as terminal summary feedback during seated rest in weeks 7-9.

Assessments include the 10-meter walk test at self-selected and fastest comfortable speeds; overground gait biomechanics using an instrumented walkway for spatiotemporal parameters and gait symmetry and two force plates for propulsive torque during push-off; and paretic and non-paretic ankle power normalized to body mass with kinetic symmetry, collected during unassisted walking with the device in evaluation mode. The Fugl-Meyer Assessment for the Lower Extremity is administered at baseline. Baseline and post-training measures will be compared using paired t-tests or nonparametric equivalents. Findings will provide preliminary effect estimates to inform the design of a larger trial.

Inclusion criteria

• Chronic stroke, more than 6 months post-stroke
• Clinical evidence of hemiparetic gait
• Ankle dorsiflexion and plantarflexion strength on the affected side graded 1/5 (trace contraction) to 4/5 (reduced strength) on manual muscle testing
• Able to independently complete the 10-Meter Walk Test at a walking speed less than 1.0 m/s, with or without an assistive device

Exclusion criteria

• Self-reported unstable angina, myocardial infarction within the past 3 months, congestive heart failure (NYHA Class II), or hemodynamically significant valvular disease
• Self-reported hypertension that is a contraindication to routine physical therapy (greater than 160/100 mmHg on two assessments)
• Self-reported hospitalization within the past 3 months for severe medical illness
• Self-reported symptomatic peripheral arterial occlusive disease
• Self-reported orthopedic conditions or chronic pain that significantly alter gait function
• Self-reported pulmonary or renal failure
• Self-reported active cancer
• Self-reported history of a non-stroke neuromuscular disorder restricting gait
• Aphasia or cognitive functioning that confounds participation, defined as unable to follow two-step commands or by judgment of the evaluating therapist
• Self-reported history of cerebellar stroke or observable cerebellar signs
• Self-reported botulinum toxin prescribed for the lower extremities within the previous 3 months