What Happens When You Train

By performing a training session you give the body a stressful stimulus to disturb its homeostasis and create positive metabolic changes. After the immediate fatigue, the body responds by adapting during recovery so that it can cope with a larger training stimulus during the next training session. During the recovery stage your body heals, grows, and rebuilds.

Training is stressful as it provides a powerful stimulus for adaptation. The stressed state of the body persists for many hours, or sometimes days after training. The rate of recovery depends on a number of factors, including quality of sleep, diet, and the overall fitness of the individual. During stress, the sympathetic ‘fight or flight’ branch of the nervous system is more active, and the parasympathetic branch is less active. Performance is gradually restored during recovery, with parasympathetic activity now dominating.

Stress can come from multiple sources. It can be from physical stress during training, from poor nutrition and too much alcohol. Stress can also come from mental/psychological sources such as work or relationships. If the total quantity of stress gets too much, combined with too little (or ineffective) recovery, then the downward spiral towards overtraining begins.

There are various ways to assess recovery, including subjective measurements of fatigue and muscle soreness, as well as exertion tests and scoring systems. Heart rate variability, or HRV, is another well validated test to do this.


Heart Rate Variability is an accurate, non-invasive measure of the Autonomic Nervous System (ANS) – which responds to everything: how you exercise, recover, eat, sleep and perceive stress.

Unlike basic heart rate (HR) that counts the number of heart beats per minute, HRV looks much closer at the exact changes in time between successive heartbeats (also called inter-beat intervals, RR intervals, NN intervals, etc).

By following trends of HRV readings on a daily basis, you can gain insights into your nervous system, stress and recovery activity, and work on ways to improve those patterns over time.

The Nervous System

The Autonomic Nervous System (ANS) controls unconscious metabolic processes and influences the functions of internal organs. Such influences include:
Heart rate
Blood pressure
Body temperature
Electrolyte balance
Respiratory rate
Pupillary response
Sexual arousal

The ANS consists of two branches:

The sympathetic nervous system (SNS) controls the body’s “fight or flight” reactions in response to internal or external stressors. It stimulates blood glucose increases, pupil dilation, slows digestion, and increases heart rate and blood pressure. The SNS is ideally activated to overcome short term stress situations such as escaping a natural disaster or fighting an intruder. However, this same response also occurs when you exercise, perform challenging mental tasks, get into an argument, or have a bad day at work!

The parasympathetic nervous system (PNS) controls your body’s “rest and digest” responses and is associated with recovery. Parasympathetic activation conserves energy, constricts pupils, improves digestion, and slows heart rate. The PNS facilitates cellular rebuilding and systematic recovery.

The SNS and PSNS control the same organ systems but with opposite effects. Both branches are always working to maintain homeostasis in the body. A healthy nervous system has a balance between the Sympathetic and Parasympathetic branches, responding when appropriate. Heart Rate Variability is an accurate, non-invasive measure of the ANS and the balance between the SNS and PSNS branches.

What Does All This Mean?

The Sympathetic Nervous System’s physiological response to stress focuses on short term survival in lieu of long term health. This acute response can become chronic in the presence of stress from modern daily life such as work, relationships, finances, environmental exposures, diet, physical activity, lifestyle choices, etc. Chronically accumulated stress from multiple sources can all contribute to drastically reduced health and performance over the long term. Chronic stress, as we have discussed, leads to chronic inflammation and consequently to many disease states. Chronic stress also wreaks havoc on our hormonal systems, with a myriad of adverse effects.

A significant amount of research published over the past 50 years correlates Heart Rate Variability to:

  • Disease risk and progression
  • Overall morbidity and mortality
  • Biological aging and health
  • Mental health, mood, depression, anxiety, PTSD
  • Physical performance (especially in elite endurance and team sports to guide training and recovery)
  • Injury prevention
  • Guided rehabilitation
  • Mental cognition

What Do HRV Scores Mean?

Higher resting-state HRV scores signify the ability of the body to activate the Parasympathetic “rest-and-digest” response. Higher heart rate variability is correlated with increased fitness/conditioning, better overall health, increased vitality, improved resilience and recovery, and improved emotional response to perceived stress.

Lower resting-state HRV scores signify an activated Sympathetic “fight-or-flight” response or suppressed Parasympathetic activity. This can indicate an inability to fully recover or signify overtraining. This can be a temporary response to the previous days’ work-out, or from a poor night of sleep. Or this can be a chronic response to stress with a resultant reduction in health and an increased risk of disease. Lower resting-state HRV is correlated with a reduced level of fitness/conditioning, poor health, decreased vitality, increased negative emotional states, and increased risk of inflammation and disease.

How To Use HRV

A daily HRV measure can be thought of as an indication of overall recovery and readiness to train or work out. Be sure to measure your HRV at the same time every day, typically first thing in the morning. Weekly and monthly trends provide the best information. 

One might expect their daily HRV to directly reflect the previous day’s training load but it often does not, for the following reasons:

  • Recovery from the previous day’s training may already be complete by the following morning.
  • Training at lower intensities produces very little stress even when performed at quite high volumes.
  • Lifestyle stressors such as sleep quality, nutrition, and mental /emotional stresses have an important influence on the total stress load the body is experiencing.
  • High life stress loads means less capacity for additional stress due to training.

Here’s a nice summary I found online to determine what to do based on your HRV measurements:

Real World Application of HRV

Today, people use HRV to:

  • Improve resilience and adaptability
  • Reduce stress
  • Optimize training and recovery
  • Personalize nutrition and sleep
  • Improve mental health – mood, depression, anxiety
  • Improve mental performance and cognition
  • Identify risk of disease, morbidity and mortality
  • Measure systemic inflammation
  • Track progress and guide treatment plans
  • Re-balance the nervous system with live biofeedback
  • Objectively understand motivation and willpower
  • Provide early warning signs for changes in health, overtraining or maladaptation

Now that you can accurately and conveniently track your nervous system, you can make better health, training, and recovery decisions to reach your goals.