Muscle co-contraction is important in stabilizing the spine. The effects of movement speed and direction on the cervical co-contraction were, however, not yet investigated. Surface electromyographies of three paired cervical muscles were measured in 17 young healthy subjects. The subjects performed voluntary neck movements in sagittal and coronal plane at fast, medium, and slow speeds. The co-contraction ratio was defined as the normalized integration of antagonistic electromyographic activities divided by that of total muscle activities. The results showed that the co-contraction ratio at fast speed (0.42 +/- 0.21) was smaller than that at medium (0.45 +/- 0.20) and slow (0.46 +/- 0.19) speeds (P </= 0.001). The co-contraction ratio was different between flexion and extension, but was similar between left and right lateral bending. The implications of co-contraction patterns include: (1) agonistic activities increase with speed but antagonistic activities do not increase accordingly; (2) neck extensors are highly activated during sagittal motions; and (3) muscular activations are symmetric during coronal motions. In conclusion, the co-contraction patterns from young healthy subjects demonstrate normal neuromuscular control in regulating the stiffness of cervical spine, and may further be references to assess the neck disorders in elderly or patients.