I. Introduction
Gait speed (GS) is a primary indicator of walking function [1] and it is associated with quality of life [2]. Walking includes three primary subtasks: propulsion, limb advancement, and bodyweight support [3]. For propulsion, the trailing leg generates a forward oriented ground reaction force to accelerate the pelvis in the anterior direction [4], [5]. Early work examining GS and propulsion has determined that the GS increases with increased braking and propulsive impulses (integrated posterior and anterior ground reaction forces, respectively) [6]. Propulsion is determined by two components: 1) the plantarflexor moment generated about the ankle and 2) the trailing limb angle (TLA) [7]. The plantarflexor moment is generated primarily by the gastrocnemius and soleus muscles at late stance [8]. The trailing limb angle is the angle defined by the hip and foot landmark defined segment, relative to the vertical laboratory axis, commonly assessed at the moment of peak propulsive force [7], [9], [10], [11], [12], [13], [14]. As such, propulsion can increase by applying a greater plantarflexor moment while keeping TLA constant, or by increasing TLA while applying the same plantarflexor moment. Due to the association between GS and propulsion, training methods that modulate the components of propulsion during walking are attractive for rehabilitation of individuals with neuromotor impairment [3].