From Muscle–Tendon Units to Whole–Body Locomotion: Modelling Active and Passive Joint Dynamics

Animal movement emerges from the combined action of neural activation, muscle dynamics, and the passive mechanical properties of the body. Muscles and joints do not simply relay neural commands but transform them through intrinsic viscoelastic dynamics, shaping how forces and motion emerge. Indeed, how active and passive forces interact across muscles and joints in the body remains a fundamental open question in biomechanics. This thesis addresses this question by focusing on the joint as a fundamental unit of neuromechanical organisation. Here we develop a hierarchical modelling framework that progress from a single muscle-tendon unit representation, to single- and antagonistic- muscle joint models, and ultimately to a multi-joint, whole-body insect model. At each level, the balance between active muscle-like actuation and passive joint viscoelasticity is systematically analysed to identify the mechanisms that govern joint torque generation and movement. Using experimental data from the locust femur-tibia as benchmark, the results showed that a single joint model actuated by a single muscle-tendon unit successfully reproduced realistic extension-flexion cycles and revealed that history-dependent mechanisms in muscle dynamics play a key role in shaping motion. Extending the model to include antagonistic muscle actuation revealed distinct dynamics regimes governed by the balance between active and passive contributions. These regimes determine joint amplitude, stability and frequency response under symmetric and asymmetric configurations. We show that joint passive can effectively compensate for imbalances between asymmetric actuators, preserving the dynamic range of the joint within specific viscoelastic regimes. Finally, the joint model was embedded in a physics-based simulator and used as a modular building block to construct multi-joint limbs and a whole-body insect model inspired by dung beetle morphology. The dynamical regimes identified at the joint level re-emerge in this mechanically more realistic setting, indicating that these regimes are intrinsic to muscle-joint mechanics. Under a simple closed-loop control framework, the insect model reproduced stable tripod-like walking and revealed an operational regime in which passive joint mechanics and low-level control are sufficient for robust locomotion, even in uneven terrain. Taken together, these results advance our understanding of how active and passive properties interact to shape joint dynamics and offer guidance for the design of bioinspired robotic system, in which viscoelastic properties can be exploited rather than neglected.