Morphology and control roles in perturbed standing recovery: a robotic study

Postural balance is essential for both humans and robots, as failures increase fall risk and limit robotic performance in real-world settings. Although humans and robots share fundamental balancing mechanics, biological complexity limits the isolation of individual muscle functions and direct principle transfer to robots. In this study, we use EPA-Walker, a bio-inspired robot actuated by electric motors and pneumatic artificial muscles (PAMs), as a physical platform to investigate perturbed standing. Here, we focus on the PAM-driven actuation to systematically examine the roles of muscle morphology and control, and to validate biomechanical findings in a robotic setting. To enable a clear upper body perturbation, a Control Moment Gyroscope (CMG) was integrated. We evaluated two stabilization paradigms: passive standing, in which joint compliance was tuned through static PAM pressurization, and active balancing, in which a bio-inspired ground reaction force (GRF) feedback controller generated muscle reflexes. The results showed that biarticular thigh muscles, particularly the hamstrings and rectus femoris, played the most prominent role in enhancing robustness through both morphology in the passive experiments and reflex control in the active experiments, consistent with findings from human perturbation studies. While ankle muscles, such as the soleus, were essential for stable standing mainly through their passive morphological contribution, their reflex-based action could also improve robustness in a more specific manner through center-of-pressure regulation. Activating a single muscle could significantly improve robustness beyond morphology, enabling recovery from $3Nm$ perturbations. Furthermore, synergistic reflex of biarticular muscles, especially hamstrings and gastrocnemius, extends the robustness to larger perturbations $\left(5Nm\right)$ . Our contribution highlights the synchronization of control strategies with the underlying morphological design through a universal sensory feedback signal, namely, GRF. These findings demonstrate the value of bio-inspired robots as testbeds to understand the potential principles underlying human motor control and support the transfer of such principles to legged robots and assistive systems.