Abstract: Objective To investigate the effects of bilateral sternocleidomastoideus contraction on neck flexion and extension. Methods The electromyograms (EMG) signals of left sternocleidomastoideus of 24 healthy persons were recorded respectively when neck relaxation and flexion in the supine position, and neck relaxation and extension in the prone position. The biomechanical measurement and analysis were done using anatomical specimens and models. Results The sternocleidomastoideus registered electrical resting potential when neck relaxation in the supine position, and when neck relaxation and extension in the prone position; the motor unit action potential of sternocleidomastoideus raised the type of mixed phase when neck flexion in the supine position. Conclusion The contractions of bilateral sternocleidomastoideus can lead to the head and neck flexion while neck joints are not fixed, and the contractions of bilateral sternocleidomastoideus can lead to the head extension under the state of fixing neck joints below atlanto-occipital.

Key words: sternocleidomastoideus, flexion and extension, effect