Figure 1

Experimental setup. (A) Overview of the camera set-up (with permission of the participant). (B) Kinematic diagram of the infant’s head and torso. Inertial reference frame N is defined by the xyz triad. Its origin corresponds with the marking on the floor at the participant’s feet; the same place at which the dummy’s sensors and the motion tracking system were zeroed at. Moving reference frame B rotates with the head and is defined by the x’y’z’ triad according to the anatomical definition (x-axis dummy face forwards, y-axis to the dummy right, z-axis upwards). The B-frame is tilted by angle θ with respect to the z-axis of the inertial reference frame N, and its origin o’ corresponds with the ICOR—both tilt angle θ and origin o’ vary over each timeframe. Torso reference frame F rotates with the torso and is defined by the x”y”z” triad according to the anatomical definition, with its origin at the torso accelerometer. The F-frame is tilted with respect to the z-axis by angle δ, which varies over each timeframe. Other variables are further explained in Eqs. (1) to (5) in the “Methods” section. (C) Components of the dummy’s head. (D) Position of the torso accelerometer. (E) Sensing axes according to a right-hand coordinate system with respect to human body axes: x-direction, longitudinal axis, anterior positive; y-direction, transverse axis, left positive; z-direction, vertical axis, superior positive. Flexion of the neck represents a positive rotation around the y-axis. The participant shown in the figure gave informed consent for being depicted.