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

Abstract

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...

Effects of backward-directed resistance on propulsive force generation during split-belt treadmill walking in non-impaired individuals

“…In agreement with our results, previous studies on resistance and uphill walking at constant speeds, non-impaired individuals scale their peak propulsion forces and duration of propulsion according to the degree of resistance or level of inclination against which they are walking ( Lay et al, 2007 ; Ellis et al, 2014 ; Naidu et al, 2019 ). For instance, Naidu et al (2019) assessed limb propulsion generation in healthy participants against increasing backward-directed resistance at the COM while participants were walking at their CWS in an intent-driven treadmill environment. By conducting this study, they observed a proportional increase in the propulsion generation by incrementing backward resistance without significant changes in vertical limb loading.…”

“…We also found that walking against greater backward resistance resulted in greater trailing limb angle, which increased to a greater extent for the slower limb than for the faster limb. According to studies on resistance and uphill walking at constant speeds, healthy, non-impaired individuals modulate their peak propulsion forces and duration of propulsion according to the amount of resistance or level of inclination against which they are walking ( McGowan et al, 2009 ; Franz and Kram, 2014 ; Naidu et al, 2019 ). However, none of these studies have evaluated the amount of each limb’s propulsion generation contribution while each limb moves at a distinctly different speed.…”

“…We showed that there is a strong correlation between the propulsive impulse generation and TLA angle for the slow limb on the split-belt treadmill. Several studies have demonstrated that an increase in TLA results in increasing propulsion generation ( Hsiao et al, 2015a , b ; Miyazaki et al, 2019 ; Naidu et al, 2019 ). We propose that by using this strategy, participants were able to improve the amount and rate of propulsive-force generation to meet the demands of increased resistance while maintaining their position on the treadmill.…”

“…Considering the importance of propulsion force in regulating the walking speed, and since it is generated in the anterior-posterior direction, some studies have explored the effect of different levels of backward-directed resistance, the force applied in the opposite direction of the individual’s walking motion, applied at the COM while walking at constant speed ( Gottschall and Kram, 2003 ; McGowan et al, 2009 ; Naidu et al, 2019 ) or in an uphill environment ( McGowan et al, 2008 ; Gottschall and Nichols, 2011 ; Franz and Kram, 2012 ; Janshen et al, 2017 ) on propulsion generation. They demonstrated by adding backward resistance or increasing the inclination level, the amount of propulsion generated by each limb increases.…”