The role of sternocleidomastoid muscle in simulated low velocity rear-end impacts.

The objectives of this study were to evaluate behavior of sternocleidomastoid muscle (SCM) electromyogram (EMG) related to impact velocity, gender, awareness and kinematics of head movement in simulated low velocity rear-end impacts. Twenty-nine healthy adults (17 male) were subjected in random order to three rear-end impacts: Two unexpected impacts causing chair accelerations of 4.5 m/s(2) (slow) and 10.1 m/s(2) (fast) and one 10.1 m/s(2) expected impact. Normalized left and right SCM EMG, linear head acceleration, angular head acceleration and maximum angular head displacement were recorded. The magnitude of normalized SCM EMG peak response ranged 2-3 times higher (P< 0.001) in female subjects than their counterpart male subjects. SCM EMG magnitude was 3-4 times higher (P< 0.001) for the fast unexpected than slow unexpected impacts, but there was no significant difference (P> 0.05) for the fast expected compared to the fast unexpected impacts. The onset time of SCM peak EMG ranged from 78 ms to 114 ms later than peak of linear head acceleration for all groups. Onset time of peak SCM EMG was not significantly different (P> 0.05) than onset of angular acceleration for the slow and fast-unexpected impacts, but onset peak SCM EMG was significantly earlier than peak angular head acceleration (30 ms) (P </= 0.05) for the fast expected impact. SCM EMG magnitude increased with increased impact velocity. Gender differences exist for SCM EMG magnitude. Temporal and amplitude awareness of a simulated impact do not produce different magnitude of SCM EMG response. The temporal relationship between the SCM and angular head acceleration is different from the temporal relationship between the SCM and linear head acceleration.