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Background  Past studies have utilized external interfaces like resistive bands and motor-generated pulling systems to increase limb propulsion during walking on a motorized treadmill. However, assessing changes in limb propulsion against increasing resistance demands during self-controlled walking has not been undertaken.    Purpose  We assessed limb propulsion against increasing fore-aft loading demands by applying graded fore-aft (FA) resistance at the center of mass during walking in a novel, intent-driven treadmill environment that allowed participants to control their walking speeds. We hypothesized that to maintain a target speed against progressively increasing resistance, participants would proportionately increase their limb propulsion without increasing vertical force production, with accompanying increases in trailing limb angle and positive joint work.    Methods  Seventeen healthy-nonimpaired participants (mean age 52 yrs., SD = 11) walked at a target, self-controlled speed of 1.0 m/s against 10, 15, 20, and 25% (% body weight) FA resistance levels. We primarily assessed linear slope values across FA resistance levels for mean propulsive force and impulse and vertical impulse of the dominant limb using one-sample  t -tests. We further assessed changes in trailing and leading limb angles and joint work using one-way ANOVAs.    Results  Participants maintained their target velocity within an a priori defined acceptable range of 1.0 m/s ± 0.2. They significantly increased propulsion proportional to FA resistance (propulsive force mean slope = 2.45, SD = 0.7,  t  (16) =14.44,  p  < 0.01; and propulsive impulse mean slope = 0.7, SD = 0.25,  t  (16) = 11.84,  p  < 0.01), but had no changes in vertical impulse (mean slope = − 0.04, SD =0.17,  p  > 0.05) across FA resistance levels. Mean trailing limb angle increased from 24.3° at 10% resistance to 27.4° at 25% ( p  < 0.05); leading limb angle decreased from − 18.4° to − 12.6° ( p  < 0.05). We also observed increases in total positive limb work (F (1.7, 26) = 16.88,  p  ≤ 0.001, η 2  = 0.5), primarily attributed to the hip and ankle joints.    Conclusions  FA resistance applied during self-driven walking resulted in increased propulsive-force output of healthy-nonimpaired individuals with accompanying biomechanical changes that facilitated greater limb propulsion. Future rehabilitation interventions for neurological populations may be able to utilize this principle to design task-specific interventions like progressive strength training and workload manipulation during aerobic training for improving walking function.    Electronic supplementary material  The online version of this article (10.1186/s12984-019-0577-x) contains supplementary material, which is available to authorized users.

journal of neuroengineering and rehabilitation

Fore-aft resistance applied at the center of mass using a novel robotic interface proportionately increases propulsive force generation in healthy nonimpaired individuals walking at a constant speed