Biomechanical mechanisms underlying exosuit-induced improvements in walking economy after stroke

Abstract

Stroke-induced hemiparetic gait is characteristically asymmetric and metabolically expensive. Weakness and impaired control of the paretic ankle contribute to reduced forward propulsion and ground clearance - walking subtasks critical for safe and efficient locomotion. Targeted gait interventions that improve paretic ankle function after stroke are therefore warranted. We have developed textile-based, soft wearable robots that transmit mechanical power generated by off-board or body-worn actuators to the paretic ankle using Bowden cables (soft exosuits) and have demonstrated the exosuits can overcome deficits in paretic limb forward propulsion and ground clearance, ultimately reducing the metabolic cost of hemiparetic walking. This study elucidates the biomechanical mechanisms underlying exosuit-induced reductions in metabolic power. We evaluated the relationships between exosuit-induced changes in the body center of mass (COM) power generated by each limb, individual joint power and metabolic power. Compared with walking with an exosuit unpowered, exosuit assistance produced more symmetrical COM power generation during the critical period of the step-to-step transition (22.4±6.4% more symmetric). Changes in individual limb COM power were related to changes in paretic (R2=0.83, P=0.004) and non-paretic (R2=0.73, P=0.014) ankle power. Interestingly, despite the exosuit providing direct assistance to only the paretic limb, changes in metabolic power were related to changes in non-paretic limb COM power (R2=0.80, P=0.007), not paretic limb COM power (P>0.05). These findings contribute to a fundamental understanding of how individuals post-stroke interact with an exosuit to reduce the metabolic cost of hemiparetic walking.

Keywords: Exoskeleton; Gait biomechanics; Gait energetics; Post-stroke gait; Robotics; Stroke rehabilitation.

Conflict of interest statement

Competing interestsPatents have been filed with the U.S. Patent Office describing the exosuit components documented in this paper. J.B., K.G.H., K.O. and C.J.W. were authors of those patents and patent applications (PCT/US2013/60225 – Soft Exosuit For Assistance With Human Motion; PCT/US2014/68462 – Assistive Flexible Suits, Flexible Suit Systems, and Methods for Making and Control Thereof to Assist Human Mobility; PCT/US2014/40340 – Soft Exosuit for Assistance with Human Motion; PCT/US2015/51107 – Soft Exosuit for Assistance with Human Motion). Harvard University has entered into a licensing and collaboration agreement with ReWalk Robotics. C.J.W. and K.O. are paid consultants to ReWalk Robotics.

© 2018. Published by The Company of Biologists Ltd.

Figures

Fig. 1.

Exosuit design and operation. (A) A soft exosuit for paretic ankle assistance after stroke. The exosuit consists of two separate textile modules that interface with the paretic limb, a low-profile insole inserted into the paretic shoe, and an off-board actuator that generates the mechanical power transmitted to the wearer's paretic ankle. The first textile module is a plantarflexion (PF) module that anchors at the waist, extends to the paretic leg, and serves as a proximal anchor for a Bowden cable attached posteriorly on the shank. The distal anchor of this Bowden cable is the heel of the low-profile shoe insole. The second textile module is a dorsiflexion (DF) module that anchors around the shank and serves as the proximal anchor for the second Bowden cable, which is attached anteriorly on the shank. The distal anchor for this second Bowden cable is a textile attached to the shoe insole on the dorsal surface of the foot. When retracted by an actuator, Bowden cables transmit mechanical power to the wearer, producing the ankle PF or DF torques. Load cells and gyroscopes are integrated into the exosuit to deliver well-timed assistive force with adequate magnitude through the textiles. (B) Illustration of exosuit actuation (top) and exosuit-generated force trajectory (bottom, where force is given as percentage body weight, % BW), presented with respect to the percentage of the paretic gait cycle. The exosuit delivers to the wearer's ankle PF force during late stance and pre-swing and ankle DF force during swing and initial contact.

Fig. 2.

Individual limb COM power across the gait cycle. (A) Group average center of mass (COM) power segmented into percentage gait cycle (top) and sub-phases (bottom) for two different conditions (exosuit unpowered and powered) on the paretic and non-paretic limbs. The gait cycle was divided into four different sub-phases representing paretic and non-paretic limb double support (PDS and NPDS) and single support (PSS and NPSS). Trailing limb double support for each limb is indicated with gray shading. (B) Symmetry indices of average positive COM power generation during the trailing limb double support. These indices represent interlimb symmetry of positive COM power generated during the trailing limb double support (gray shading in A). *Statistically significant change from exosuit unpowered to powered condition.

Fig. 3.

Correlation between COM power during trailing limb double support and net metabolic power. (A) Correlation of average positive COM power generated in trailing limb double support (TDS; x-axis) and average net metabolic power (y-axis) for the exosuit unpowered (top) and exosuit powered (bottom) conditions. Non-paretic COM power was linearly correlated to net metabolic power (P<0.05) in both conditions, while the correlation between paretic COM power and metabolic power was not statistically significant in either condition. (B) Correlation of the change in average positive COM power during trailing limb double support and the change in net metabolic power resulting from exosuit assistance. Exosuit-induced net metabolic power reduction was linearly correlated with the exosuit-induced change of non-paretic positive COM power during trailing limb double support, whereas a statistically significant correlation with paretic positive COM power was not observed. TDS, trailing limb double support.

Fig. 4.

Individual joint power across the gait cycle. (A) Group average of body COM power and sum of lower-limb joint power segmented into percentage gait cycle. (B) Group average of ankle, knee and hip joint power segmented into percentage gait cycle. The trailing limb double support phase of each limb is indicated with gray shading. total: total power generated by human and exosuit; exo: ankle and hip power generated by exosuit when powered. (C) Average of positive and negative power variables generated during the trailing limb double support (gray shading in A,B). *Statistically significant difference between the exosuit powered and unpowered conditions. Note that ankle and hip power generated by the exosuit is zero in the unpowered condition.

Fig. 5.

Correlation between ankle and COM power during trailing limb double support. (A) Correlation of average positive ankle power (x-axis) produced during trailing limb double support and average positive COM power over the same time period (y-axis) for the exosuit unpowered and powered condition. Positive ankle power was linearly correlated with positive COM power on both limbs in both conditions (P<0.05). (B) Correlation of the change of average positive ankle power and the change of COM power in exosuit-assisted walking. Exosuit-induced ankle power change was linearly correlated to the exosuit-induced change of positive COM power during trailing limb double support on both limbs (P<0.05). TDS, trailing limb double support.

Fig. 6.

Correlation between ankle power during trailing limb double support and net metabolic power. (A) Correlation of average positive ankle power (x-axis) generated during trailing limb double support and net metabolic power (y-axis) for the exosuit unpowered and powered conditions. Non-paretic ankle power was linearly correlated to net metabolic power (P<0.05) in both conditions. The correlation between paretic ankle power and metabolic power was not statistically significant for either condition. (B) Correlation of the changes in average positive ankle power and net metabolic power in exosuit-assisted walking. No statistically significant correlation was found between exosuit-induced ankle power changes and changes in metabolic power (P>0.1). TDS, trailing limb double support.