The effect of spinal manipulation on biceps brachii muscle activity
Authors:   , Victor Aldo 
Published: January 2022

Background The electromyographic response post-spinal manipulation may support the elucidation of the underlying neurophysiological mechanism of spinal manipulation on motor activity as well as on arthrogenic muscle inhibition. The literature shows conflicting evidence regarding the excitatory or inhibitory nature of the reflexive electromyographic response and the excitability of the homonymous motor neuron pool post-spinal manipulation. The current study investigated whether the electromyographic response post-spinal manipulation is affected by a facilitated golgi tendon organ Ib inhibitory di-synaptic spinal reflex as part of the convergent input on the homonymous motor neuron pool excitability. Objectives The objectives of this placebo-controlled, single-blinded, repeated measures design were: 1) to determine electrical activity and muscle force of the biceps brachii muscle immediately before and after an Activator Adjusting II Instrument placebo spinal manipulation, 2) to determine electrical activity and muscle force of the biceps brachii muscle immediately before and after a C5/C6 spinal manipulation, 3) to compare the electrical activity and muscle force of the biceps brachii muscle between the control and intervention groups pre- and post-test. Method Each participant performed three sets of modified stretching of the biceps brachii muscle with two minute rest intervals between each set in a single appointment, of which at a standardized fourth second during each set an intervention was applied to the ipsilateral C5/C6 segment. The first intervention (AAI 1) entailed the application of an Activator II Adjustment Instrument placebo spinal manipulation; the second intervention (AAI 2) entailed the application of an Activator II Adjustment Instrument placebo spinal manipulation; and the third intervention (SMT) entailed the application of spinal manipulation. One-second electromyography (EMG) segments were taken during the force plateau of each set; the EMG signal was processed through Root Mean Square (RMS) analysis and the muscle force data were obtained by using the Biopac - MP 150 Data Acquisition system and AcqKnowledge analysis software. Results The objective data analysis revealed a noteworthy scientific finding of a medical anomalous inverse relationship between the muscle force and EMG RMS immediately post-spinal manipulation. The immediate post-SMT intervention revealed an increase in the biceps brachii muscle force by 4.76 % and a decrease in the biceps EMG RMS by 9.03 % with a summation of percentage difference between the muscle force and EMG RMS of 13.79 %. The immediate post-placebo AAI 1 intervention showed a decrease in the biceps EMG RMS by 1.86 % and a decrease in the biceps brachii muscle force by 0.85 % with a summation of percentage difference between the muscle force and EMG RMS of 1.01 %. The immediate post-placebo AAI 2 intervention showed a decrease in the biceps EMG RMS by 0.05 % and a decrease in the biceps brachii muscle force by 1.97 % with a summation of percentage difference between the muscle force and EMG RMS of 1.92 %. Conclusion Further research is warranted to add statistical significance to the inverse relationship between muscle force and EMG RMS observed immediately post-spinal manipulation. This knowledge obtained, may have clinical relevance for rehabilitation practitioners and physical therapists by providing evidence based support for the suggestion that optimal management of patients with muscle weakness suspected to be of arthrogenic nature could include the application of spinal manipulation to the segmentally innervated facet joints before traditional strength training is initiated.