Physical interface dynamics alter how robotic exosuits augment human movement: implications for optimizing wearable assistive devices

Abstract

Background: Wearable assistive devices have demonstrated the potential to improve mobility outcomes for individuals with disabilities, and to augment healthy human performance; however, these benefits depend on how effectively power is transmitted from the device to the human user. Quantifying and understanding this power transmission is challenging due to complex human-device interface dynamics that occur as biological tissues and physical interface materials deform and displace under load, absorbing and returning power.

Methods: Here we introduce a new methodology for quickly estimating interface power dynamics during movement tasks using common motion capture and force measurements, and then apply this method to quantify how a soft robotic ankle exosuit interacts with and transfers power to the human body during walking. We partition exosuit end-effector power (i.e., power output from the device) into power that augments ankle plantarflexion (termed augmentation power) vs. power that goes into deformation and motion of interface materials and underlying soft tissues (termed interface power).

Results: We provide empirical evidence of how human-exosuit interfaces absorb and return energy, reshaping exosuit-to-human power flow and resulting in three key consequences: (i) During exosuit loading (as applied forces increased), about 55% of exosuit end-effector power was absorbed into the interfaces. (ii) However, during subsequent exosuit unloading (as applied forces decreased) most of the absorbed interface power was returned viscoelastically. Consequently, the majority (about 75%) of exosuit end-effector work over each stride contributed to augmenting ankle plantarflexion. (iii) Ankle augmentation power (and work) was delayed relative to exosuit end-effector power, due to these interface energy absorption and return dynamics.

Conclusions: Our findings elucidate the complexities of human-exosuit interface dynamics during transmission of power from assistive devices to the human body, and provide insight into improving the design and control of wearable robots. We conclude that in order to optimize the performance of wearable assistive devices it is important, throughout design and evaluation phases, to account for human-device interface dynamics that affect power transmission and thus human augmentation benefits.

Keywords: Exoskeleton; Human augmentation; Joint kinetics; Physical human-robot interaction; Power transfer; Rehabilitation; Soft tissue; Wearable robot.

Fig. 1

Experimental setup. a Human subject walked on a force-instrumented treadmill while wearing a robotic exosuit that assists ankle plantarflexion. b Simplified representation of motion capture markers used for power calculations. For graphical simplicity, a single marker is used to represent the shank, and a single marker is used to represent the foot; however, in practice segmental kinematics were estimated from several markers distributed along each segment (as detailed in Methods text and depicted in Additional file 1). These markers were selected to mitigate confounds due to soft tissue motion

Fig. 2

Conceptual summary of exosuit-to-human power transmission. Power is generated at the cable end-effector. A portion of this power contributes to ankle plantarflexion (termed ankle augmentation power), while a portion is absorbed into the human-exosuit interfaces (termed proximal and distal interface powers). Power absorbed into the proximal (shank) and distal (foot) interfaces is due to viscoelastic deformation of interface materials and underlying biological tissues, as well as relative motion of the interface with respect to the body. Reporting convention: power absorbed by the interfaces is negative, and power returned by the interfaces is positive. Black arrows represent motions associated with each power term

Fig. 3

Conceptual summary of ankle power. Net ankle power results from the combination of ankle augmentation power (from the exosuit) and biological ankle power (from muscles, tendons, ligaments). Black arrows represent motions associated with each power term

Fig. 4

Force, power and work during walking. a Exosuit cable force measured via load cell. b Cable end-effector power, P _{cable_end} is parsed into power that goes into motion/deformation of the proximal, P _{prox_int}, and distal, P _{dist_int}, interfaces vs. power that contributes to augmenting ankle plantarflexion, P _{aug_indirect}. Power and force results are shown for a representative stride cycle. The left-hand gray box in the background indicates exosuit loading, the primary period of increasing force application. The right-hand gray box indicates exosuit unloading. c Positive and negative work (mean ± s.d.) during exosuit loading. d Positive and negative work (mean ± s.d.) over the full stride cycle. Net work is indicated by thick white line on each bar

Fig. 5

Net ankle power during walking. a Net ankle power, P _ankle, due to both biological and exosuit contributions, was estimated using standard 3D inverse dynamics. Ankle augmentation power, P _{aug_indirect}, represents power provided by the exosuit that augments ankle plantarflexion. Biological ankle power, P _{ankle_bio_indirect}, reflects the net power generated by muscles, tendons and other biological tissues about the ankle joint. Power results are shown for a representative stride cycle. b Positive and negative work (mean ± s.d.) during the walking cycle. Net work is indicated by thick white line on each bar