Dissection of the musculature in the cervical spines of four cadavers indicated that the semispinalis cervicis and capitis muscles appeared to be the muscles primarily responsible for extension of the cervical spine and head. The minimum force needed to balance a biomechanical model of the cervical spine in the neutral prone position against gravity measured 14.38 kg. The values obtained with this model, with roentgenograms of the cervical spine, and vector analysis were close enough to establish that the model is apparently valid for studying the extensor musculature of the cervical spine. Because the extensors appear to generate considerable force and may act as significant dynamic stabilizers of the cervical spine, the authors have altered their posterior surgical approach to minimize disruption of the extensor musculature.