Spinal manipulation (SM) is used by clinicians for the treatment of several chronic pain conditions. The effectiveness of different spinal manipulations targeted at the lumbar spine in patients with low back pain is supported by an increasing number of high-quality randomized clinical trials1and systematic reviews. Although these techniques have shown some effectiveness in clinical practice, most of clinical studies solely investigated the effects of spinal manipulation on overall reports of pain and function and the underlying mechanisms by which manipulation produce clinical effects remain largely unknown.

The neurophysiologic mechanisms by which manipulation inhibits pain, however, are matters of speculation and still under investigation. Proposed hypotheses have suggested that manipulation has the potential to remove the source of mechanical pain or induce stimulus-produced analgesia. Spinal manipulation induces sufficient force to simultaneously activate both superficial and deep somatic mechanoreceptors, proprioceptors, and nociceptors. The effect of this stimulation is a strong afferent segmental barrage of spinal cord sensory neurons, capable of altering the pattern of afferent input to the central nervous system and inhibiting the central transmission of pain. Other suggested mechanisms have been the activation of the endogenous opiate system, the alteration of the chemical mediators or the effects of joint cavitation. An understanding of the mechanism by which manipulations cause a hypoalgesic response is subject to further research and is currently far from complete. A review of the literature found several studies exploring immediate changes in mechanical pain sensitivity provoked by spinal manipulative procedures. Mobilisation/manipulation to the cervical spine has been shown to provide a hypoalgesic effect as measured by pressure pain thresholds (PPTs) in patients suffering from mechanical neck pain and lateral epicondylalgia. A hypoalgesic effect has also been demonstrated following mobilization to peripheral joints in the upper and lower limbs. Mobilizations to the lumbar spine have been shown to produce an immediate and significant widespread hypoalgesic effect in asymptomatic subjects However, Perry et al. that found unilateral mobilizations on the lumbar spine respectively had side specific response.

Besides analgesic effect, it has been presented spinal manipulation can reduce the increased resting muscle tone or spasm, which can be monitored by surface electromyography (sEMG). If the presence of a hypertonic muscle is functionally associated with a spinal dysfunction that is correctable by SM, it would consequently follow that the associated higher EMG level would diminish after appropriate SM. In a descriptive study DeVocht JW et al. found that manipulation induces an immediate change, usually a reduction, in resting EMG level in patients with low back pain. Herzog J reported the observation of a single but very dramatic decrease in resting EMG activity in thoracic musculature within 1 second of SM. One possible segmental mechanism could be that the manipulation may induce a reflex muscle relaxation by modifying proprioceptive group 1 and 2 afferents. However, few randomly controlled trials have directly investigated the effect of spinal mechanical manipulation on basal electromyographic activity (BEA) in asymptomatic subjects.

Spinal mechanical manipulation has been widely used in clinical manual therapy. However, because mechanical thrusts usually produce no cavitations, whether mechanical techniques produce the same hypoalgesic effects and muscle relaxation as manual techniques remains untested. To further elucidate the physiologic mechanisms associated with spinal mechanical manipulation, it is essential to investigate its effects in asymptomatic individuals who do not have any active central sensitization. In fact, recent studies have supported the use of asymptomatic subjects in studies related to neurophysiological mechanisms of spinal manipulations. Further research is therefore required to clarify if there is a hypoalgesic effect or muscle relaxation in response to spinal mechanical manipulation in the lumbar region in asymptomatic subjects.