BRAMBLE, D.M.*; LIEBERMAN, D.E.; DALEY, M.A.: Mechanics of Head-Neck Stabilization in Running Humans: Implications for Hominid Evolution

Adequate head-neck stabilization ought to be vital to any running mammal but the phenomenon has been little studied. We investigated the mechanical basis of cranio-cervical stability in humans during treadmill walking and running. Quantitative measures of head-neck and arm motion were obtained from high-speed videography as well as surface mounted accelerometers. Corresponding activity patterns in the trapezius and biceps muscles were also monitored. Our findings show that the mechanical interactions between the head-neck complex and arms are strikingly different in the walk and run regardless of speed. The timing of trapezius muscle activity relative to the step cycle differs in the two gaits (i.e., initiated at midstance in walking vs. heelstrike when running). During running the mass of the arms appears to be coupled to the cranio-cervical complex through the superior trapezii and the associated nuchal ligament. Indeed, the combined kinematic, kinetic, and electromyographic profiles suggest a biomechanical model in which the momentum of the runner’s arms is used to counteract the tendency of the head and neck to pitch forward at each footstrike. The same, gait-specific, mechanical linkage may also help to reduce impulsive loading of the head when running (as compared to fast walking). Analyses of occipital morphology in Recent and fossil crania suggest that this novel mechanism, in which the nuchal ligament is a key player, may be unique among hominoids to the genus Homo, thus providing evidence of specialization for running late in human evolution.