Closed-loop control of split belt treadmill speed induces targeted bilateral adaptations in ankle kinetics

On a conventional split-belt treadmill, one leg walks exclusively on a fast belt while the other is on a slow belt. While this setup effectively induces asymmetrical adaptations beneficial for unilateral deficits like post-stroke hemiparesis, its design is less ideal for symmetrical impairments such as bilateral plantarflexor (PF) weakness common in aging. We introduce a novel split-belt treadmill paradigm with two distinct conditions that vary each belt's speed based on its corresponding limb's phase of gait. In the 'TrailFast' condition, each belt accelerated across the stance phase and decelerated during swing. In the 'LeadFast' condition, each belt accelerated during swing and decelerated across the stance phase. The natural phase offset between limbs transformed these inputs into consistent inter-limb speed differences during double support. These speed differences were intended to alter the pre-swing position of the feet to evoke adaptations in PF force production bilaterally. Twenty healthy adults completed a continuous 6-minute adaptation period in each condition. Upon returning to constant-speed walking, the conditions produced opposing, bilateral after-effects in PF impulse (p < 0.001). Specifically, TrailFast significantly increased PF impulse by 0.054 N·m·s/BW (p < 0.001), while LeadFast significantly decreased it by 0.061 N·m·s/BW (p < 0.001). Our results demonstrate that this phase specific perturbation can elicit robust, bilateral, and direction-specific adaptations in ankle kinetics. This instruction-free paradigm, particularly the TrailFast condition, presents a promising approach for developing interventions that target bilateral PF weakness by implicitly training greater PF output.