Robotic Ankle-Foot Orthosis: Gait-Triggered Foot-Drop Correction

Patient Physiology

Tibialis anterior strength 30%

Gait cadence 100 steps/min

Assistive Device

Device type

Assist intensity 70%

Live Readouts

Stance

Toe clearance

—

Ankle angle

0.0°

Cadence

100 spm

Drop-foot events

How it works

After stroke or a peroneal-nerve injury, the tibialis anterior muscle can no longer lift the foot (dorsiflex the ankle) during the swing phase of walking. The toe drags or catches the ground — "foot drop". A powered ankle-foot orthosis (AFO) — driven by functional electrical stimulation (FES) of the peroneal nerve, or a small robotic actuator — detects heel-off and lifts the foot through swing so it clears the ground, then releases before heel-strike.

gait cycle:  φ(t) = (t · cadence/60) mod 1
stance:      0 ≤ φ < 0.6      swing: 0.6 ≤ φ < 1.0

unassisted swing target:
  θ_natural = lerp(−20°, +10°, strength)   (weak → drop, strong → clears)

device-assisted target (swing only, gated by an attack/release ramp g(φ)):
  θ_target = lerp(θ_natural, +12°, assist · g(φ))

actuator response (first-order lag):
  dθ/dt = (θ_target − θ) / τ        τ_FES ≈ 0.18 s,  τ_robotic ≈ 0.05 s

toe clearance = world-Y of the toe marker after forward-kinematic
chaining of hip → knee → ankle rotations, read directly off the 3D rig.

Tibialis anterior strength — how much dorsiflexion the patient can generate unaided; low values reproduce classic drop-foot.
Device type — FES stimulates the muscle itself (slower, biological activation delay); a robotic AFO drives a motor directly (faster, more precise).
Assist intensity — how strongly the device pulls the ankle toward its +12° dorsiflexion target during swing.
A drop-foot event is logged whenever the toe's computed ground clearance drops to zero in mid-swing — the geometric signature of a toe-drag or stumble.

Sagittal-plane single-leg rig — treadmill view

Assisted ankle path Swing phase

⚠ Couldn't load the 3D engineThree.js failed to load from the CDN. Check your connection and reload.