From template to anchors: transfer of virtual pendulum posture control balance template to adaptive neuromuscular gait model increases walking stability

Abstract

Biomechanical models with different levels of complexity are of advantage to understand the underlying principles of legged locomotion. Following a minimalistic approach of gradually increasing model complexity based on Template & Anchor concept, in this paper, a spring-loaded inverted pendulum-based walking model is extended by a rigid trunk, hip muscles and reflex control, called nmF (neuromuscular force modulated compliant hip) model. Our control strategy includes leg force feedback to activate hip muscles (originated from the FMCH approach), and a discrete linear quadratic regulator for adapting muscle reflexes. The nmF model demonstrates human-like walking kinematic and dynamic features such as the virtual pendulum (VP) concept, inherited from the FMCH model. Moreover, the robustness against postural perturbations is two times higher in the nmF model compared to the FMCH model and even further increased in the adaptive nmF model. This is due to the intrinsic muscle dynamics and the tuning of the reflex gains. With this, we demonstrate, for the first time, the evolution of mechanical template models (e.g. VP concept) to a more physiological level (nmF model). This shows that the template model can be successfully used to design and control robust locomotor systems with more realistic system behaviours.

Quantifying changes in individual-specific template-based representations of center-of-mass dynamics during walking with ankle exoskeletons using Hybrid-SINDy

“…The uniform selection of SLIP mechanisms from data is consistent with common template walking models and supports the common perspective that elastic legs are foundational mechanisms for describing COM accelerations during walking across individuals [13, 21, 22, 26, 49]. Our observation that rotary mechanisms were not critical to reconstructing COM accelerations for all individuals may explain their less frequent application in template walking models [14, 20]. Note that each template signature mechanism describes characteristic coordination patterns between leg kinematics and COM accelerations.…”

“…Leg springs ([14, 20-22, 26]) and dampers ([28]), which produce force along the leg. Leg springs are common energetically-conservative mechanisms used to describe walking and running dynamics, and enable a double-limb support phase.…”

“…Our results in one stroke survivor suggest that characterizing altered coordination between leg orientation and ground reaction forces (i.e., template signatures) may unveil sub-classes of exoskeleton responses, enabling device customization to assist individuals in each sub-class. Finally, using similar physics-informed data-driven approaches to anchoring template signatures in physiological detail may reveal biological mechanisms that characterize responses to exoskeletons or other assistive devices [14,16].…”

“…Reduced-order representations of gait dynamics provide a foundation to quantify and understand complex exoskeleton responses [13][14][15][16][17][18]. For example, Full & Kodistchek (1999) used reduced-order representations of COM dynamics, such as the spring-loaded inverted pendulum (SLIP), to quantify strategies to achieve task-level goals including COM stabilization and perturbation responses during locomotion [16].…”

“…For example, inverted pendulum templates have been used to study COM energetics and the transition from walking to running [17, 19], as well as strategies for energetically-efficient COM acceleration [9, 23, 24] and lateral stabilization [10, 25]. Alternative template structures, such as the bipedal spring-loaded inverted pendulum (SLIP) and its variants further improved predictions of sagittal-plane ground reaction forces (GRFs) and revealed COM stabilization strategies across different speeds [14, 21, 22, 26, 27]. Extensions of the bipedal inverted pendulum and SLIP templates include leg damping elements, rotary springs, or curved feet.…”