In-vivo measurement of loads and displacements in the head and neck is very difficult. Musculoskeletal biomechanical models are useful tools for investigating biomechanical phenomena in this system, but they require several assumptions and simplifications regarding tissue mechanical properties and intervertebral kinematics (IVK). In particular, IVK show considerable variation among subjects [1], and quantifying the influence of this variation on model estimates is important for the application of models toward understanding neck biomechanical function. Variation in IVK parameters may affect model estimates of neck strength (neck muscle moment, the product of muscle force and muscle moment arm), as well as the location of the head center of mass, which influences the gravitational load on the neck due to the weight of the head. The magnitude of gravitational load relative to neck extension strength, referred to here as fatiguability, is an estimate of demand on neck muscles and may be related to chronic neck pain induced by forward head postures [2]. The goal of this study was to quantify variation in model estimates of flexion strength, extension strength and fatigability over sagittal plane postures, due to variation in IVK.

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