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Musculotendon and Fascicle Strains in Anterior and Posterior Neck Muscles During Whiplash Injury

Study Design. A biomechanical neck model combined with subject-specific kinematic and electromyographic data were used to calculate neck muscle strains during whiplash. Objectives. To calculate the musculotendon and fascicle strains during whiplash and to compare these strains to published muscle injury thresholds. Summary of Background Data. Previous work has shown potentially injurious musculotendon strains in sternocleidomastoid (SCM) during whiplash, but neither the musculotendon strains in posterior cervical muscles nor the fascicle strains in either muscle group have been examined. Methods. Experimental human subject data from rear-end automobile impacts were integrated with a biomechanical model of the neck musculoskeletal system. Subject-specific head kinematic data were imposed on the model, and neck musculotendon and fascicle strains and strain rates were computed. Electromyographic data from the sternocleidomastoid and the posterior cervical muscles were compared with strain data to determine which muscles were being eccentrically contracted. Results. SCM experienced lengthening during the retraction phase of head/neck kinematics, whereas the posterior muscles (splenius capitis [SPL], semispinalis capitis [SEMI], and trapezius [TRAP]) lengthened during the rebound phase. Peak SCM fascicle lengthening strains averaged (±SD) 4% (±3%) for the subvolumes attached to the mastoid process and 7% (±5%) for the subvolume attached to the occiput. Posteriorly, peak fascicle strains were 21% (±14%) for SPL, 18% (±16%) for SEMI, and 5% (±4%) for TRAP, with SPL strains significantly greater than calculated in SCM or TRAP. Fascicle strains were, on average, 1.2 to 2.3 times greater than musculotendon strains. SCM and posterior muscle activity occurred during intervals of muscle fascicle lengthening. Conclusions. The cervical muscle strains induced during a rear-end impact exceed the previously-reported injury threshold for a single stretch of active muscle. Further, the larger strains experienced by extensor muscles are consistent with clinical reports of pain primarily in the posterior cervical region following rear-end impacts.