These legs were made for propulsion: advancing the diagnosis and treatment of post-stroke propulsion deficits

Abstract

Advances in medical diagnosis and treatment have facilitated the emergence of precision medicine. In contrast, locomotor rehabilitation for individuals with acquired neuromotor injuries remains limited by the dearth of (i) diagnostic approaches that can identify the specific neuromuscular, biomechanical, and clinical deficits underlying impaired locomotion and (ii) evidence-based, targeted treatments. In particular, impaired propulsion by the paretic limb is a major contributor to walking-related disability after stroke; however, few interventions have been able to target deficits in propulsion effectively and in a manner that reduces walking disability. Indeed, the weakness and impaired control that is characteristic of post-stroke hemiparesis leads to heterogeneous deficits that impair paretic propulsion and contribute to a slow, metabolically-expensive, and unstable gait. Current rehabilitation paradigms emphasize the rapid attainment of walking independence, not the restoration of normal propulsion function. Although walking independence is an important goal for stroke survivors, independence achieved via compensatory strategies may prevent the recovery of propulsion needed for the fast, economical, and stable gait that is characteristic of healthy bipedal locomotion. We posit that post-stroke rehabilitation should aim to promote independent walking, in part, through the acquisition of enhanced propulsion. In this expert review, we present the biomechanical and functional consequences of post-stroke propulsion deficits, review advances in our understanding of the nature of post-stroke propulsion impairment, and discuss emerging diagnostic and treatment approaches that have the potential to facilitate new rehabilitation paradigms targeting propulsion restoration.

Keywords: Diagnosis; Intervention; Locomotion; Propulsion; Rehabilitation; Robotics; Sensors; Walking.

Fig. 1

Relationship between peak paretic propulsion and walking a speed and b distance. Speeds and distances indicative of unlimited community ambulation are in red. Those indicative of home ambulation are in blue. See [17, 28, 29] for primary data

Fig. 2

Forward propulsion results when a plantarflexor moment (M _PF) is generated with the limb oriented behind the body [102, 103]

Fig. 3

a Combining isometric strength testing with supramaximal muscle electrostimulation allows assessment of the extent and nature of post-stroke muscle weakness (i.e., maximum voluntary strength, strength capacity, and the ratio of these force measurements is the Central Drive). b Central drive is a key explanatory factor of paretic propulsion and propulsion asymmetry across individuals with a wide range of walking speeds. See primary source [105]

Fig. 4

a The body’s forward acceleration during walking results from the interaction between the propelling trailing limb and braking leading limb. By summing the antero-posterior ground reaction forces (AP-GRF) generated by each limb, b distinct body acceleration profiles can be identified across individuals with different motor control deficits. The low acceleration subtype generates little forward acceleration during the paretic double support phase, whereas the high acceleration subtype demonstrates positive acceleration during paretic double support and then remains largely negative across the gait cycle

Fig. 5

Left- The FastFES intervention targets deficits in paretic propulsion by combining fast treadmill walking (to increase the trailing limb angle) and FES to the paretic plantarflexor muscles (to increase the plantarflexor moment). Right- When compared to control training without FES at both fast and comfortable speeds, only FastFES training was shown to improve the paretic plantarflexor moment [31]. Interestingly, the two control groups also improved paretic propulsion, but did so by improving trailing limb angle. Consequently, only the FastFES training group reduced the energy cost of walking at both comfortable and fast walking speeds [119]

Fig. 6

Left- A gait-restorative soft robotic exosuit commercially-adapted by ReWalk Robotics that recently gained FDA approval for use during stroke rehabilitation. Right- The exosuit technology was developed to assist both ankle dorsiflexion and plantarflexion function during post-stroke walking (see prior work [47, 143, 144, 147, 150])