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To identify factors that contribute to head acceleration during purposeful soccer heading.Prospective.Biomechanics laboratory.One-hundred soccer players (42 male, 58 female, 17.1±3.5 years (range: 12-24years), 168.5±20.3 cm, 61.5±13.7 kg). Exclusionary criteria: history of neurologic disorder, cervical spine injury, or head injury in the past 6 months.We implemented a soccer heading paradigm that consists of controlled soccer heading (12 headers at 11.2m/s from a JUGS machine) while measuring 3D motion capture kinematics (torso and head-to-torso range-of-motion), sternocleidomastoid and upper trapezius EMG activity (peak and area), and linear and rotational accelerations using a triaxial accelerometer and gyroscope. Head mass, neck girth, and sternocleidomastoid and upper trapezius strength were also evaluated.Linear and rotational accelerations at the head center-of-gravity.Multiple regression analyses suggest that the ten variables predicted linear (R2=0.272, p=0.002) and rotational (R2=0.259, p=0.004) acceleration; however none of the predictors individually made a significant contribution to the model. Linear and rotational accelerations were correlated with head mass (r=-?0.447, p<0.001; r=-0.467,p<0.001), neck girth (r=-0.465, p<0.001; r=-0.438, p<0.001), sternocleidomastoid strength (r=-0.386, p<0.001; r=-0.410, p<0.001), and upper trapezius strength (r=-0.311, p=0.001; r=-0.351, p<0.001).Our findings suggest that head mass, neck girth, and neck strength are important in minimising head acceleration, whereby greater size and strength are associated with lower head accelerations. Soccer players with smaller head-neck segment masses and lower neck strength, may be at risk for greater head acceleration. These athletes may experience a theoretical bobble-head effect, when the neck strength is not great enough to control the mass of the head.None.