Recent epidemiological evidence shows that the potential for whiplash injury varies with both the average acceleration and speed change of a rear-end collision. The goal of this study was to examine the gradation of neck muscle responses and the head and neck kinematics to rear-end collision pulses in which the acceleration and speed change were independently varied. Thirty subjects (15F, 15M) underwent 36 consecutive rear-end collisions consisting of three different average accelerations (ā = 0.5, 0.9 and 1.3 g) and three different speed changes (Δv = 0.25, 0.50 and 0.75 m/s). Onset and amplitude of the sternocleidomastoid (SCM) and cervical paraspinal (PARA) muscle responses were measured using surface electromyography. Kinematic measures included linear and angular accelerations and displacements of the head and torso. The results showed that the amplitude of the muscle and kinematics responses was graded to both collision acceleration and speed change. The magnitude of early peaks in the head/neck kinematics correlated more strongly with collision acceleration (r 2 = 0.63 to 0.69), whereas the magnitude of later kinematic peaks correlated more strongly with collision speed change (r 2 = 0.59 to 0.95). Onset of the SCM muscle response correlated only weakly with collision acceleration and speed change (r 2 ≤ 0.07). A collision pulse descriptor obtained by multiplying average acceleration and speed change (āΔv) yielded the strongest and most consistent correlations with neck muscle (r 2 = 0.48 to 0.58) and head/neck kinematic responses (r 2 = 0.78 to 0.94). This measure of collision severity is also consistent with the recent epidemiological evidence that whiplash symptom intensity and duration increases with both average acceleration and speed change.