Are cervical multifidus muscles active during whiplash and startle? An initial experimental study.
The data from this study indicates that the multifidus muscles of some individuals are active early enough during whiplash to potentially increase the collision induced loading of the facet capsular ligaments.
The cervical multifidus muscles insert onto the lower cervical facet capsular ligaments and the cervical facet joints are the source of pain in some chronic whiplash patients. Reflex activation of the multifidus muscle during a whiplash exposure could potentially contribute to injuring the facet capsular ligament. Our goal was to determine the onset latency and activation amplitude of the cervical multifidus muscles to a simulated rear-end collision and a loud acoustic stimuli.
The cervical facet joints are a source of neck pain in about half of chronic whiplash patients. In addition to guiding better diagnostic and treatment techniques, this finding provides an anatomical focus to biomechanical studies aimed at understanding the aetiology of whiplash injuries. Pinching of the posterior synovial fold of the cervical facet capsular ligament is one possible injury mechanism, but more attention has been devoted toward excess strain of the capsular ligament itself. Injurious levels of strain have been observed in some capsular ligaments when loads simulating a rear-end collision were applied in-vitro. More recently, allodynia (measured as paw withdrawals in a rat model) has been correlated to levels of capsular ligament strain relevant to whiplash injury, and Group III and IV afferents from the facet joint capsule have demonstrated a graded response to mechanical loading in an in-vivo goat model.
Anatomically, the cervical facet capsule contains fine, unmyelinated nerves that likely have nociceptive function. Distending these ligaments by injection of contrast media has produced whiplash like pain patterns in normal individuals. Tendons of the cervical multifidus muscles insert directly onto the capsular ligaments and it has been postulated that multifidus activation during the neuromuscular response to a rear-end automobile impact could increase the strain in the capsular ligaments above that imposed passively by the impact induced head and neck dynamics. Prior work has shown early multifidus activation during a whiplash response in one of three subjects, however, it remains unclear whether this reflex response will be present in a larger group of subjects.
The neuromuscular response to a whiplash exposure contains both a postural and a startle response. This combined postural and startle response was observed in surface electromyograms of the sternocleidomastoid and cervical paraspinal muscles with and without the loud sound of a vehicle crash, although muscle activity was larger when the acceleration was accompanied by a loud sound. These prior findings suggest that a startle response (presumably evoked by the crash motion, the noise, or some combination of the two) amplifies the superficial neck muscle response during an unexpected rear-end collision. If either the crash induced motion or a loud noise also evokes a reflex response in the deep multifidus muscles, then this muscle contraction could add to the loads borne by the facet capsule during a rear end automobile collision and possibly affect the capsular ligament’s injury potential.
The broad goal is to establish whether reflex activation of the cervical multifidus muscle contributes to straining the facet capsular ligament and thereby contributes to the genesis of whiplash injury. One step in this broader goal and specifically address whether a reflex contraction of the multifidus muscle is evoked by either the postural response (a
seated horizontal acceleration without the noise of impact) or a startle response (a loud noise without a postural perturbation). If either of these stimuli generates a reflex response in multifidus, our broader goal warrants continued investigation; if neither
stimulus generates a reflex response, then we can conclude that multifidus likely does not play a role in facet capsule injury in whiplash.
Based on the data in this study, the cervical multifidus muscles of some individuals are active during either postural or startle responses. Of nine subjects, eight subjects exhibited multifidus activity following the stimuli and seven subjects responded within 160 ms to at least one of the two stimuli. As a result, this study establishes that multifidus could play a role in straining the capsular ligament of a large segment of the population during a rear end collision. Of course the single subject in the current study who responded to neither stimulus may represent a segment of the population whose multifidus muscles do not react to these stimuli, and this subject should not be dismissed outright as atypical given the low number of subjects we tested. Despite this caveat, these findings lend support to the proposition that multifidus activation may play a role in the genesis of some whiplash injuries, although more work is needed before we can conclude whether it actually contributes to injuring the facet capsule ligament.
In summary, cervical multifidus muscle activity is evoked by both horizontal sled accelerations and acoustically startling tones. This finding represents one step towards understanding whether reflex activation of the multifidus muscle can exacerbate a whiplash injury involving the cervical facet capsule ligament.