Aerobic exercise has a tremendous impact on our morbidity and mortality due to its' ability to positively impact our emotional, mental, and physical health. In particular, the cardiovascular system undergoes dramatic improvements in response to aerobic exercise. These benefits are noted in many of the variables we track clinically including blood pressure, VO2 Max (aerobic capacity), respiration rate, and heart rate. Heart rate changes are well documented in the literature and include a decreased heart rate at a given level of exercise in a trained versus untrained individual. This is due to the cardiovascular system becoming more efficient in its' ability to deliver a higher volume of blood carrying nutrients and oxygen to the body. This efficiency is also noted as a lower resting heart rate in trained individuals. Recently in the literature articles have discussed heart rate variability as a clinical measure of training in endurance athletes (Meeusen et al. Med Sci Sports Exer. 2013).
Heart rate variability is an indirect, clinical evaluation of the heart's rhythm due to the influence of autonomic nervous system. This physiological event is described as the time interval between heart beats. A low variability heart rate would have consistent durations between beats (increased sympathetic aka flight or fight influence) whereas a high variability would reflect inconsistent, changing durations between beats (increased parasympathetic aka rest and digest). Tracking heart rate variability on training and rest days may allow clinicians and endurance athletes to determine if the aerobic training is having a positive or negative impact on the athlete.
An athlete who is overtraining (weeks to months) or overreaching (days to weeks) is caused by an accumulation of training with inadequate rest which results in short term or long term decrements in performance or physiological markers. These athletes would demonstrate maladapting responses to cardiovascular exercise including decreased performance, overuse injuries, and emotion or mental stress or fatigue. These athletes have been shown to have a higher resting heart rate (>4-5 beats per min), a lower maximum or training heart rate (<7-8 bpm), and low heart rate variability (Bosquet et al. Br J Sp Med. 2008). Limitations into this research show large variability between athletes, testing positions, and variability within normal heart rate ranges.
In short, heart rate variability is a promising tool, but further research is needed to improve the consistency of the research. Tracking heart rate may help complement our other clinical tests and measures, as well as, the athlete's subjective report on determining if an athlete is overtraining. Other key measures to track include training performance, sleep quality and quantity, diet, mental alertness/fatigue, and mood.