Modulating reflexes enables speed control in simulated human walking and running

Human locomotion is characterized by remarkable versatility, enabling smooth speed modulation, acceleration, deceleration, and transitions between gaits. This versatility is thought to emerge from interactions between the musculoskeletal system, spinal circuits—including reflexes and central pattern generators—and supraspinal control. The relative contributions of these components remain a subject of ongoing debate. In this study, we explore the potential role of reflex modulation in achieving the observed flexibility of human locomotion. Using a neuromusculoskeletal model controlled by reflexes between agonist/antagonist muscle groups, we show that a wide range of walking and running speeds can be generated using reflex modulation. We devise a modulation strategy of key reflexes, which allows for setting a desired target speed before simulation (offline modulation) as well as dynamically adjusting speed during run-time (online modulation). Furthermore, we can demonstrate that switching between parameter sets allows for smooth transitions between walking and running. This study aims to understand the capabilities of reflexes as a core component of locomotor control and therefore isolates reflexes from other components of the spinal cord. While the study, thus, does not aim to fully replicate human motor control, it demonstrates that reflex modulation can account for speed control in walking and running as well as gait transitions, highlighting the possible contributions of reflexes to versatile locomotion.