FUNCTIONAL-ANATOMY OF THE HEAD-NECK MOVEMENT SYSTEM OF QUADRUPEDAL AND BIPEDAL MAMMALS

This biomechanical investigation quantified the range of motion of the different articulations of the head-neck ensemble in man, monkeys, ca ts, rabbits and guinea pigs. Radiography and dissections were used to establish the degrees of freedom of the system. The erect posture and rigidity of the cervical spine in mammalian vertebrates are possible b ecause the degrees of freedom of the movements of the cervical and upp er thoracic vertebrae in passive ranges of motion are asymmetric, and thus significantly restricted, when judged from the resting position. The total range of motion at the atlanto-occipital articulation varies between species. It is similar to 90 degrees-105 degrees in the quadr upedal mammals tested, and only 11 degrees or 13 degrees, respectively , in humans and monkeys. When at rest, bipeds and quadrupeds hold the atlanto-occipital articulation and the upper cervical joints (C1/C2, C 2/C3) in a flexed attitude. The total range of motion at the cervicoth oracic junction (C6-T2) is similar to 6 degrees-80 degrees in all vert ebrates-investigated (quadrupeds and bipeds). At rest, the vertebral a rticulations that form the cervicothoracic junction are held in their extreme extended positions in quadrupeds and monkeys. In man, the vert ebrae of the lower cervical spine are kept at a midposition between ma ximal flexion and maximal extension. This latter observation may be re lated to the permanent bipedalism of humans. Collectively, our data in dicate that biomechanical constraints such as bone structures (e.g. sp ecifically shaped articular processes) and ligaments may maintain the intrinsic configuration and self-supporting structure of the cervical spine. Furthermore, the specialised structures in the cervical joints allow movements more or less in particular planes of space, and thus b iomechanical constraints limit the number of possible solutions as to how an animal can perform a given orientating head movement. Although we have not entirely clarified the functional implications for head mo vement control of the different sagittal-plane ranges of motion in ver tebrates, we hypothesise that different mechanical requirements relati ng to the influence of gravity have caused the observed differences be tween the investigated bipedal and quadrupedal mammals.