Reflex responses associated with manipulative treatments on the thoracic spine: A pilot study

Objective: To test systematically if spinal manipulative treatments (SMT) and the audible release associated with SMT cause activation of spinal muscles. 
Design: Experimental pilot study. 
Setting: Human Performance Laboratory, The University of Calgary. 
Participants: One male and one female asymptomatic volunteer. 
Intervention: Slow and fast SMTs to the left transverse process of thoracic vertebrae using a reinforced hypothenar contact. The treatment forces were directed in a posterior-to-anterior direction with the subjects in a prone position. 
Main Outcome Measures: Forces applied by the chiropractor during SMT. Measurements of the audible release using skin-mounted accelerometers. Electromyographical activity of selected spinal muscles. 
Results: Electromyographical (EMG) activity was observed consistently 50-100 msec after the onset of each of the fast SMTs, whether the treatment resulted in an audible release or not; for slow SMTs, there was never any visible electromyographical activity of the target muscles, whether the treatment resulted in an audible release or not. 
Conclusion: The results of this study suggest that fast treatment thrusts elicit muscle activation, whereas slow force application does not. The timing of the onset of the EMG response suggests that activation may be produced by a reflex response originating in the muscle spindles. It also appears that the audible release does not (by itself) evoke muscle activation or a joint proprioceptive reflex response as has been speculated in the literature.