Age-related differences in sit-to-stand strategies: a multi-objective optimization simulation approach

Sit-to-stand (STS) is a fundamental activity involved in daily human activities, which requires coordination among multiple objectives including stability, velocity, and muscular effort. Aging is associated with a shift in STS strategies, such that older adults (OA) typically adopt a stabilization strategy, in which the center of mass (CoM) is positioned within the base of support (BoS) before seat-off. On the other hand, younger adults (YA) often employ a momentum-transfer strategy that initiates seat-off with the CoM located posterior to the BoS and relies on horizontal momentum. However, whether these age-related differences in STS strategy arise primarily from a decline in muscle function or from changes in the prioritization of CoM control remains unclear. In this study, the influence of age-related alterations in muscle function on STS strategies was investigated using predictive musculoskeletal simulations. Multi-objective optimization was applied to YA and OA models to identify muscle activation patterns that achieved an upright posture and maximized the horizontal CoM distance from the BoS at seat-off, while simultaneously minimizing CoM velocity and total muscle activation. The resulting Pareto front indicated that the OA model consistently maintained the CoM within the BoS at seat-off, whereas the YA model exhibited both stabilization and momentum-transfer strategies. Furthermore, the OA model showed narrower ranges of variation in CoM position and velocity than the YA model. These findings suggested that age-related changes in muscular function constrained CoM control, restricting the feasible solution space in a manner consistent with the greater reliance on stabilization strategies during STS.