Investigation of the effect of tonus on the change in postural control strategy using musculoskeletal simulation

Humans use different postural control strategies depending on perturbations. The shift from an ankle strategy to a hip strategy occurs at different perturbation magnitudes for different individuals. Although such differences relate to the differences in body parameters such as muscle strength, the parameter changes that affect the strategy shift are unclear. The relationship between tonus and strategy is especially unclear, but humans control tonus, which contributes to body stability. The objective of this study was to investigate the influence of tonus on postural control strategy.

Is there a trend toward the use of a hip strategy with a decrease in the magnitude of tonus?

Predictive simulations were performed for changing parameters of muscle weakness and increased sensory noise, which are considered the causes of different strategies and decreased tonus. A musculoskeletal model with 70 muscles and 15 degrees of freedom of joints was controlled using a neural controller model, and the support surface was translated backward to introduce perturbations. The parameters of the musculoskeletal and neural controller models were changed for 48 conditions of different muscle strengths, sensory noise, and tonus. The control parameters were optimized for each condition. Simulations were performed with the optimized control parameters to calculate an evaluation index to show the difference in postural control strategies (peak hip angle), and the relationship between the index and parameters was analyzed using analysis of variance and multiple regression.

The main effects of muscle weakness and decreased tonus and their interaction were confirmed. The results recognized a positive response to the research question.

The study emphasizes the importance of considering tonus while investigating the postural control strategy. Furthermore, it was suggested that when the magnitude of tonus was larger than a threshold, only the ankle strategy was used, regardless of muscle strength.