Whiplash and whiplash-associated disorders are a major cause of neck pain. Although muscle damage sustained during whiplash exposure is expected to heal quickly, the cervical muscles may play a role in the development of chronic whiplash pain secondary to facet joint capsule (FJC) damage. The purpose of this study was to investigate cervical muscle response and changes in muscle recruitment patterns after physiologic and non-physiologic FJC loading, under quasi-static and dynamic loading conditions. Fifteen in-vivo caprine FJC preparations were subjected to an incremental tensile loading paradigm. EMG activity was recorded from the right trapezius (TR) and multifidus (MF) muscle groups at the C5 and C6 levels; and from the sternomastoid (SM) and longus colli (LC) muscle groups bilaterally at the C5-C6 level; during FJC stretch and for 10 minutes after the capsule was unloaded. Capsule load during the displacement applications was recorded via miniature load cell, and 3D capsule strains (based on stereoimaging of an array of markers placed on the capsule surface) were reconstructed using finite element methods. EMG traces from each muscle were examined for onset of muscular activity, mean and peak activity during the application of FJC stretch, and for the presence of sporadic afterdischarge (i.e. spasms) during the 10 minutes between each displacement application.
All muscles were responsive to both high-rate and low-rate non-physiologic FJC stretch. Capsule load was a better indicator of the tensile stretch threshold for muscle recruitment than capsule strain in both test conditions. MF was the first muscle group to be recruited regardless of the activation criterion under consideration (i.e. strain, load, or latency) or the rate of FJC stretch. LC recruitment occurred significantly sooner under high-rate vs. low-rate FJC stretch. Peak capsule loads (pooled by test increment) increased with increasing FJC displacement, and were significantly correlated with pooled global and local maximum principal strains in both test conditions. In addition to the mechanical properties of the facet joint, muscle activity caused by distraction of the facet joint contributed to the magnitude of the observed peak FJC strains under low-rate FJC stretch, and to the magnitude of the observed peak loads under both highrate and low-rate FJC stretch. Afterdischarge was observed in all muscles in both test conditions.
The results of this study provide the first evidence of extensive ligamento-muscular reflex pathways between the FJC and the cervical musculature, which are responsive to both low-rate and high rate FJC stretch. The observation of sporadic muscle activity after low-rate FJC stretch demonstrates the potential role the cervical muscles may play in the generation and/or maintenance of neck pain. The response of the cervical muscles to the high-rate loading, especially the observation of sporadic muscle afterdischarge, indicates that these muscles may play a role in the experience of whiplash pain even after any initial muscular lesions have healed. Since the source of whiplash pain remains undiagnosed in up to 50 % of whiplash pain patients, defining an alternative source of whiplash pain will hopefully enable expansion of treatment options available for whiplash-associated disorders and alleviate the suffering of a larger proportion of whiplash patients.
