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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 versi...

Cardiorespiratory fitness is assessed through graded exercise tests that determine the maximum amount of sustained mechanical work that an individual can perform while also providing health and fitness related information. This manuscript describes a novel method to perform graded exercise tests that uses posteriorly directed resistive forces. The purpose of this investigation is to validate a novel resistance based test in comparison to a traditional speed and incline based test in a cohort of non-impaired individuals. Twenty non-impaired individuals, 8 males, 12 females, mean age 28.4± 9.6, range 20-54 years old. Participants performed two maximal exercise tests. The speed and incline based test used the Bruce protocol and increased treadmill incline and speed every three minutes. The resistance based test used a robotic device interfaced with the treadmill that provided specified horizontal resistive forces at the center of mass calculated to match each Bruce Protocol stage while individuals walked at 1.1 m/s. Participants obtained ∼3% higher maximum V˙O2 measure using the speed and incline based method (dependent t-test p=0.08). V˙O2 peaks between tests were strongly correlated (r=0.93, p<0.001). Peak values of secondary physiologic measures (i.e., max heart rate, respiratory exchange ratio) were within 3% between tests. We found a significant linear relationship between mass-specific work rate and measured V˙O2 stage-by-stage for both tests, but no significant difference between each linear fit (p=0.84). These data suggest horizontal resistive forces while walking on a treadmill, can be used to increase aerobic effort in a way that closely simulates work rates of the Bruce Protocol.