When discussing the significance of case reports and larger studies, we often return to the impact of the intervention on the nervous system. Usually, we are referring to things like sympathetic activity, parasympathetic activity, stress, and adaptability. So, we thought it was time to take a step back from individual studies and refresh our knowledge of what all of these terms mean and how they are connected.
It’s the tip of the iceberg when it comes to the many ways the nervous system can tell us what’s going on below the surface. We will be pulling some of these apart over the next few weeks, looking at objective measures one by one. So, for those who are new to the nervous system and its mysterious ways – let’s get into it.
The nervous system can be broken down into different arms that are responsible for different functions; autonomic and somatic, parasympathetic and sympathetic.
The somatic nervous system is involved in our conscious movements and activity. These nerves go to the skin and muscles and send messages to them when we want to make a movement, such as picking up a glass of water or smiling. The autonomic nervous system, in contrast, is responsible for all of our ‘unconscious’ activity, like breathing, digestion, and our heart beating.
We can further break down the autonomic branch of the nervous system into sympathetic and parasympathetic. This is where we spend a lot of our time discussing the impacts of chiropractic care. In times of stress, the sympathetic nervous system works to mobilise our energy stores and generally prepares the body to face whatever is coming. This reaction is known as the ‘fight or flight’ response and involves a rise in heart rate, more rapid breathing, and a diversion of energy from the digestion to other muscles. To facilitate this response, adrenaline and other stress hormones are released. The parasympathetic nervous system works to decrease heart rate and breathing, and generally recover the body. It is known as our ‘rest and digest’ response.
While the ‘fight or flight’ response is a highly effective and helpful response when appropriate, getting stuck in this state of heightened alert can have detrimental effects on an individual’s health. One aspect of the negative impact heightened sympathetic activity can have is the increased production of stress hormones.
Cortisol is a hormone released by the adrenal glands. It is released to help you wake up each morning, in response to exercise, and also in response to stress. Prolonged periods of increased cortisol levels can interrupt regular sleep patterns and digestion, increases blood sugar levels, and dampens normal immune function. There are a host of symptoms and complaints individual’s with chronically elevated cortisol levels may have, including weight gain, fatigue, muscle weakness, frequent illness and slow recovery from sickness, digestive issues, disrupted menstrual cycles, and poor sleep.
Many of you are probably very familiar with the above information, it is nothing we haven’t spoken about before – but it does form the important foundation of why we care so much about the nervous system and sympathetic activity.
As a doctor of the nervous system, your chiropractor is interested in caring for this complex system that enables you to adapt and respond to the world around you. That’s why we check and adjust for subluxations – which we often say arise from ‘trauma toxins or stress.’
Now let’s take a look at the measures commonly used in chiropractic care, and what research says about them. First cab off the rank – Heart Rate Variability.
Heart rate variability is an indirect measure of sympathetic activity. Simply put, it is a measure of the variation in time between each heartbeat.  Generally, a lower and more variable heart rate indicates your heart and body can readily respond to changes internally and externally. It is more common for individuals with higher resting heart rates to have lower variability, as when the heart is beating faster, there is less time between beats and thus less opportunity to vary the rate. As we have discussed above; sympathetic activity works to increase heart rate, and thus heart rate and its variability can be helpful indicators of ongoing increased sympathetic activity.
The other thing HRV shows us is adaptability. How well your heart rate adapts to your physical state can be a measure of how well you adapt to your environment. Lower HRV, lower adaptive ability. Higher HRV, higher adaptability and responsiveness – ie. Measures of physical resilience. So you can see why it matters.
HRV in research and practice
Since the early 2000s HRV has been utilised in chiropractic research, with studies continuing to suggest a relationship between chiropractic care and HRV. During this time, there has been some discussion over the significance of HRV in clinical and research settings.  The discussion is centred around the value of taking one measure of HRV during a consultation and whether it is a reliable or meaningful way of quantifying changes in sympathetic activity.
Researchers compared standalone measurements to multiple measurements taken over a period of time and have come to the general consensus that a series of measures provide more useful values. Particularly with the widespread use of consumer wearables (think smart watches and such), recording HRV over a longer period and using this information to determine a true baseline for an individual is more feasible than before. This is an exciting development – albeit one that hasn’t been put into widespread clinical use yet.
It is important to note that although more accessible and in theory more meaningful, data from consumer wearables have many limitations and should always be coupled with a reliable method of interpretation.
HRV is still a staple in many studies, both case reports and larger studies, as an indirect measure of sympathetic activity and nervous system function. It also continues to be researched in its own right as a measure used in clinical practice and in research .
We are sure there will be more ways for HRV to create meaningful data in research and practice. Tune in next week when we talk about surface EMG as an objective measure of sympathetic nervous system activity.
- Billman GE. Heart Rate Variability – A Historical Perspective. Frontiers in Physiology. 2011;2:86
- Ebrall, P. (2022). HEART RATE VARIABILITY IN CHIROPRACTIC: FUZZY MEASURES FROM A CONSUMER WEARABLE. Journal of Contemporary Chiropractic, 5(1), 85–96
- H. Stephen Injeyan, Budgell B. Mitigating Bias in the Measurement of Heart Rate Variability in Physiological Studies of Spinal Manipulation: A Comparison Between Authentic and Sham Manipulation. Journal of Manipulative and Physiological Therapeutics. 2022;45(2):104–13.