The Effect of Curvature on Neck Muscle Moment Arms

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Abstract

Moment arm transforms muscle force to joint moment. Thus, it is dependent on the lineof-action in which a muscle force acts. Musculoskeletal models of the cervical spine commonly model muscles with straight paths. However, straight lines do not best represent the natural curvature of muscles in the neck, because muscles are constrained by bone and soft tissue. Implementing curvature in muscle paths has been shown to affect moment arm. Confirming the accuracy of neck muscle moment arms is a difficult task as there are multiple methods to estimate moment arms, and few studies that report moment arms. One method to validate moment arms is by comparing model moment estimates to experimentally measured moment. The purpose of this study was to create a musculoskeletal model of the cervical spine with accurate curved muscle paths to estimate moment arms of curved and straight paths using different moment arm methods, and confirm the accuracy of the estimates.

Two subject-specific cervical spine musculoskeletal models derived from in-vivo MRI (magnetic resonance image) of approximate 50th percentile males were created that were kinematically functional from 30° flexion to 30° extension. Curved paths were modeled using moving muscle points and straight paths were modeled as straight lines. Moment arms were estimated using three methods: tendon excursion, partial velocity and geometric. Experimentally measured moment and muscle activations were determined in a human subject study in sagittal plane postures. Model moment estimates were determined using human subject muscle activations and compared to experimentally measured moment.

Curved paths provided significantly different moment arm estimates than straight paths for 10 of 15 muscles and estimates were also found to be significantly different between moment arm methods for 11 of 15 muscles (p<0.05, repeated measures two-way ANOVA). Model moment estimates of curved paths predicted more accurate extension moments than straight paths when compared to experimentally measured moment in anthropometric matched subjects to the subject-specific models. Using straight lines to model muscle paths can lead to overestimating total neck strength. However, moment arm methods for curved paths should be investigated further as different methods of calculating moment arm can provide different estimates.

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