Predictive simulation of sit-to-stand based on reflexive-controllers

Abstract

Sit-to-stand can be defined as a set of movements that allow humans to rise from a sitting position to a bipedal standing pose. These movements, often categorized as four distinct kinematic phases, must be coordinated for assuring personal autonomy and can be compromised by ageing or physical impairments. To solve this, rehabilitation techniques and assistive devices demand proper description of the principles that lead to the correct completion of this motor task. While the muscular dynamics of the sit-to-stand task have been analysed, the underlying neural activity remains unknown and largely inaccessible for conventional measurement systems. Predictive simulations can propose motor controllers whose plausibility is evaluated through the comparison between simulated and experimental kinematics. In the present work, we modelled an array of reflexes that originate muscle activations as a function of proprioceptive and vestibular feedback. This feedback encodes torso position, displacement velocity and acceleration of a modelled human body with 7 segments, 9 degrees of freedom, and 50 actuators. We implemented two controllers: a four-phases controller where the reflex gains and composition vary depending on the kinematic phase, and a simpler two-phases controller, where three of the kinematic phases share the same reflex gains. Gains were optimized using Covariance Matrix Adaptation. The results of the simulations reveal, for both controllers, human-like sit-to-stand movement, with joint angles and muscular activity comparable to experimental data. The results obtained with the simplified two-phases controller indicate that a simple set of reflexes could be sufficient to drive this motor task.

A planar neuromuscular controller to simulate age-related adaptation strategies in the sit-to-walk movement

“…We found that a 2-phase reflex based neural controller can realistically simulate the task of standing up (38), which is in line with Munoz et al (2022) (14) planar sit-to-stand simulation, and is different from the commonly reported 4-phase kinematic phases (37). In our model, the transition time is part of the optimization.…”

“…In the development of the sit-to-walk controller, we have tested several combinations of controllers, using muscle-reflex controllers and existing gait controllers (12). The most realistic and stable controller was a combination of two reflex-controllers, which is in line with what other research groups found for the sit-to-stand movement (14), followed by gait control.…”

“…Our planar sit-to-walk framework allows for asymmetric strategies, like asymmetric foot positioning, and asymmetries in muscular capacities. Previous studies on sit-to-stand simulation did not allow for asymmetry (9,14) which limits the available compensation strategies, and may not properly reflect daily life movements as a result (16). Asymmetric strategies in standing up may have potential implications for long-term joint degeneration due to altered joint loading and provide valuable insights for clinicians in advising patients on the best strategy to avoid over- or unloading after surgery or injury.…”

“…Geyer and Herr (2010) (11) were pioneers to develop a robust gait controller with a limited set of proprioceptive feedback loops, a method subsequently applied in various gait simulation studies (12)(13)(14). These reflex controllers have been sporadically studied beyond gait, with Muñoz et al (2022) using the controller to simulate a sit-to-stand motion (15). This existing sit-to-stand feedback controller has several limitations when it comes to realistic simulation of adaptation strategies in standing up (15).…”

“…These reflex controllers have been sporadically studied beyond gait, with Muñoz et al (2022) using the controller to simulate a sit-to-stand motion (15). This existing sit-to-stand feedback controller has several limitations when it comes to realistic simulation of adaptation strategies in standing up (15). Asymmetry in muscular activation and kinematics is common in STS, especially in an elderly or patient population (7,16,17), but was neglected.…”