Nonlinear response of human trunk musculature explains neuromuscular stabilization mechanisms in sitting posture

The interaction of various sensorimotor system components plays a fundamental role in maintaining upright sitting posture by stabilizing the unstable human trunk. The mechanisms associated with human trunk stabilization during sitting have been researched in several studies. However, it is still a big challenge to determine and distinguish the roles of these mechanisms in human motor control due to the complex interrelation amongst them. Therefore, an in-depth mechanistic understanding of the complex interrelations in terms of physiologically neuromechanical parameters is of great significance in human motor research.

Neuromusculoskeletal conditions are associated with degraded trunk control, and affected individuals are mostly wheelchair users as they cannot maintain an upright seated posture. Thus, during sitting, external perturbations are likely to cause serious falls, which are also the leading source of injury in this population. Remarkably, studies have shown that the knowledge of mechanistic neuromuscular control in healthy individuals contributes significantly to diagnosis and impaired balance improvement. It also plays a crucial role in developing robust technologies for trunk stability restoration during impaired sitting, which also requires accurate quantification of the neuromuscular stabilization mechanisms.

Generally, the stability of a seated human body is achieved through two control mechanisms: passive control without time delay and active control with time delay. The sensorimotor system generates an active joint moment, by activating relevant muscles based on the provided sensory information and anticipated perturbations. While these control mechanisms for sitting and standing stability have been extensively studied, most studies have assumed time-invariant linear behavior for the neuromuscular control and neglected nonlinear or time-variant dynamics in their control models. In addition, previous studies used closed-loop system identification and offline optimization techniques that require time-consuming post-processing, limiting the applicability of such identification approach for rapid identification of impaired balance.

Herein, Dr. Alireza Noamani, Professor Albert Vette and Professor Hossein Rouhani from the University of Alberta have employed a nonlinear and physiologically-meaningful neuromechanical model to characterize the underlying neuromuscular stabilization mechanisms (active and passive) involved in human sitting. To accomplish this, the nonlinear trunk dynamics in healthy individuals were identified experimentally to describe the roles of the stabilization mechanisms. Specifically, adaptive unscented Kalman filters (AUKF) were used to identify nonlinear model parameters considering the time-varying properties and process and measurement noise distributions. Their work has been published in the Journal of Neural Engineering.

The authors demonstrated the effectiveness of the presented model in predicting the roles of both passive and active mechanisms involved in trunk stability during perturbed sitting. The passive mechanism permitted instant resistance against gravitational disturbances. The active mechanism, which exhibited a non-isometric behavior, activated the relevant trunk muscles to provide a delayed phasic response against external disturbances. This characterization approach not only accounted for the physiological uncertainties and nonlinear behavior of the neuromuscular mechanisms but also could allow real-time tracking and correction of neuromechanical parameters’ variations due to muscle fatigue and and external disturbances. It is worth noting that under some conditions, the model did not require offline optimization because the AUKF enabled dynamic adaptation of its properties.

In summary, this study provides a better mechanistic understanding of the key roles of active and passive stabilization mechanisms involved in sitting for objective and targeted evaluation and rehabilitative interventions. The high accuracy of the model in predicting trunk sway behavior allowed a comprehensive mechanistic understanding of the roles of the neuromuscular control system and related stabilization mechanisms involved in sitting. In a statement to Advances in Engineering, Professor Hossein Rouhani explained their findings would advance the design of assistive technologies for restoring seated stability.