This paper reviews a nonlinear complexity within the Human Biodynamics Engine (HBE), a world-class human neuro-musculoskeletal simulator, developed at the Department of Defense, Australia. The HBE development is based on an anthropomorphic tree of Euclidean motion groups SE (3), with 270 active degrees of freedom, realistic muscular mechanics and hierarchical neural-like control. The HBE is formulated in the fashion of nonlinear dynamics/control of highly complex biophysical and robotic systems and developed for the purpose of neuromusculoskeletal injury prediction. The following aspects of the HBE development are described: geometrical, dynamical, control, physiological, biomedical, AI, behavioral and complexity. Several simulation examples are provided.