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To test the hypothesis that the head kinematic and the cervical muscle EMG responses to unexpected impacts of increasing low-velocity in the posterolateral direction will be greater than to expected impacts.Ten healthy volunteers were subjected to right posterolateral of 5.0, 8.6, 12.2, and 15.4 m/s2 acceleration in expected and unexpected conditions.The effect of a posterolateral impact on the cervical muscle responses is unknown.Bilateral EMG of the sternocleidomastoids, trapezii, and splenii capitis were recorded and compared. The acceleration of the chair, torso, and head was recorded and compared.At an acceleration of 15.4 m/s2, the sternocleidomastoids generated the highest EMG in the unexpected impact conditions with lower percentages for the expected impact (50% of their maximal voluntary contraction). The splenii exhibited between 30% and 40% EMG, the trapezii generated approximately 25%. The EMG was significantly affected by the levels of acceleration (P<0.01) for the sternocleidomastoids and splenii. The time to onset and peak EMG for all muscles progressively decreased with increasing levels of acceleration and regressed significantly on it (P<0.01). Head accelerations were greater in the unexpected impacts (P<0.05).The cervical muscles play a primary role in posture control. They may be a at risk of injury in whiplash especially in unexpected impacts. In the case of the right posterolateral impacts the sternocleidomastoids and splenii show a differentially higher response compared to the trapezii (P<0.05). This may suggest an increased risk of injury to these muscles.The results presented contribute to an understanding of head and neck responses in posterolateral impacts.