Sex differences in human neck musculoskeletal biomechanics and modeling

Abstract

Females have a higher incidence of neck injuries and neck pain compared to males. This may be related to sex differences in neck musculoskeletal biomechanics, which have not been examined in detail previously. In this study, the intrinsic sex differences in neck skeleton and muscles were quantified and incorporated into subject-specific models. This thesis had three specific aims: (1) to specify the sex differences in the neck skeleton and muscles; (2) to build generic female and male neck models, and develop a generalized subject-specific (and sex-specific) neck modeling procedure; (3) to optimize and validate models based on experimental data.

Specific Aim 1 investigated the sex differences related to the neck skeleton and neck muscle size using X-ray and MRI data, respectively. No significant sex difference was found in vertebral shape (wedging or concavity) or in kinematic parameters such as intervertebral motion distribution or instantaneous axis of rotation when normalized by vertebral size. The total neck muscle volume and the percentage of the total neck volume which was composed of muscle were larger in males than in females, but no sex difference was found in the individual neck muscle volume distribution. A method was developed to predict individual neck muscle volumes from external measurements.

Specific Aim 2 created a unique female model based on the anatomical data of the Visible Human Female, and improved an existing male neck model. Subject-specific neck models were scaled from the generic female or male model, incorporating sex-specific muscle data.

Specific Aim 3 developed a hybrid EMG-assisted optimization method to adjust the model so that model-predicted moments more closely approach experimentally recorded moments. A common gain and the individual gains for muscle force and moment arm were applied to the models to correct the systematic errors, and error in estimates of forces and moment arms, respectively. The optimized results were validated by separate isometric data.

The models and procedures developed in this work will enable studies of sex differences in head and neck biomechanics through static and dynamic analyses. These future studies can help elucidate the causes of increased incidences of neck injuries and pain in women.