Perturbing gait stability during treadmill walking with destabilizing functional electrical stimulation

Functional electrical stimulation is commonly used to enhance human movement through low-level electrical activation of muscles. More recently, destabilizing functional electrical stimulation (DFES) has been proposed as a method to perturb gait by inducing mistimed muscle activation. Here, we investigated strategies to introduce internal perturbations during treadmill walking using DFES by systematically varying stimulation timing and target muscle. Eleven healthy participants walked at three different speeds while DFES was applied to the tibialis anterior (TA), soleus (SOL), rectus femoris (RF), and biceps femoris (BF) muscles at 25%, 50%, 75%, and 100% of the gait cycle for a duration of 0.2 s. The gait cycle was time-locked to heel contact (0%). During the stimulation stride (Post0), RF50 produced the largest immediate reduction in anterior-posterior margin of stability, whereas SOL75 and BF75 produced the largest reductions during the first subsequent stride (Post1). These conditions were accompanied by distinct changes in joint kinematics and stride timing. In conclusion, this study identifies muscle- and phase-specific DFES conditions associated with larger reductions in gait stability and provides a controlled experimental approach for investigating internally driven perturbations in locomotor control.