A recently published paper investigated the effects of chiropractic adjustments on motor unit behaviour1. This study is the first publication for Lucien Robinault’s PhD, who is working with us for the next few years. This study was conducted in collaboration with Lucien’s home university, the University of Valenciennes in France, as well as researchers from the University of Maribor in Slovenia and Auckland University of Technology (AUT).

This project investigated neurophysiological changes at a motor unit level in the tibialis anterior muscle following a single session of chiropractic care. We used a technique called high density surface electromyography (HDsEMG), which measures the electrical activity of motor units, coupled with advanced signal processing. Previously, to do this sort of research we had to insert a needle into the participant’s muscle, which of course is not all that comfortable. And the information we could collect was limited compared to the wealth of information we can get from HDsEMG. For example, in a previous study we published2 we were able to record 85 single motor units (using the needle-based technique) across 19 participants. Whereas in this study, using HDsEMG we were able to decompose 8485 single motor units across the 14 participants we tested. We conducted this research as we are constantly seeking better ways to understand what happens neurophysiologically following chiropractic care. We have done previous research that has shown changes in motor neuron excitability and force production in lower limb muscles, but we wanted to understand why we saw these changes and to do that we needed to dig deeper and look at the motor unit activity.

The results of this study showed that there was a decrease in the conduction velocity of the motor units during ‘ramp and maintain’ ankle dorsiflexion at 5% maximum voluntary contraction (MVC). ‘Ramp and maintain’ means that the participants had to dorsiflex their ankle and then hold the contraction briefly before releasing. Conduction velocity is the speed in which an action potential travels along a skeletal muscle fibre and is correlated to the twitch torque production capacity of the motor unit. This significant reduction in conduction velocity is thought to represent increased recruitment of the motor units that are better suited to the production of low force and precise action and was only seen in the group that got adjusted, not in the control group.

This research furthers our scientific understanding of chiropractic care and how it influences the central nervous system. We now have more evidence to suggest that chiropractic adjustments alter neuromuscular control. To read the full study go to https://www.mdpi.com/2076-3425/11/1/105/htm

We would like to thank the NCMIC Foundation (Iowa, USA), the French government for their grant to the University of Valenciennes, and the Slovenian Research Agency for funding this project. We would also like to thank our collaborators at the University of Valenciennes, AUT and the University of Maribor for their excellent contributions to this publication. It takes a whole team to conduct such high quality research, and we are grateful to have developed such good working and funding relationships that have enabled this work to be done.

References

Robinault L, Holobar A, Crémoux S, Rashid U, Niazi IK, Holt K, Lauber J, Haavik H. The Effects of Spinal Manipulation on Motor Unit Behavior. Brain Sciences. 2021; 11(1):105. https://doi.org/10.3390/brainsci11010105
Haavik, H., Niazi, I. K., Jochumsen, M., Uginčius, P., Sebik, O., Yılmaz, G., … & Türker, K. S. (2018). Chiropractic spinal manipulation alters TMS induced I-wave excitability and shortens the cortical silent period. Journal of Electromyography and Kinesiology, 42, 24-35.