The coordination of axial thorax and pelvis rotations during gait has been shown to be affected by several pathologies. This has been interpreted as an indication of increased apparent axial trunk stiffness, but arm swing may also affect these rotations. The objectives of this study were to assess the effect of trunk stiffness and arm swing on the relative timing (‘coordination’) between thorax and pelvis rotations, and to assess if apparent trunk stiffness can be inferred from thorax-pelvis kinematics. A forward dynamic model was constructed to estimate apparent trunk stiffness from observed thorax and pelvis rotations and arm swing moment around the longitudinal axis of the trunk of 30 subjects. The effect of independent manipulations of trunk stiffness and arm swing moment on thorax-pelvis coordination and gain of axial thorax-pelvis rotations were assessed using the same forward dynamic model. A linear regression model was constructed to evaluate whether forward dynamic model-based estimates of axial trunk stiffness could be inferred directly from thorax-pelvis rotations. The forward dynamic model revealed that axial trunk stiffness and arm swing moment have opposite effects on axial thorax-pelvis coordination. Apparent axial trunk stiffness could not be predicted from observed thorax-pelvis rotations.