The heart responds immediately to an increase or decrease in stress levels. This phenomenon can be measured through beat-by-beat heart rate data (R-R intervals). An accurate measurement of the time interval between heart beats is called heart rate variability (HRV). Generally, larger HRV intervals mean less physiological and psychological stress. The results of HRV measurement can be used to limit the chance of over-training, under-recovery and injury; it also can be used to confirm recovery is of adequate duration, and training is at the right intensity, and vice versa. Moreover, measuring HRV reveals the effects of stressors induced by variable events such as jetlag and high altitude training.
With HRV, athletes can discover how their daily routines and particular psychological stressors are contributing to stress and recovery. This allows them to better plan for the future by adopting favorable training practices and finding the right balance of training and recovery. It is also easier to detect early signs of overtraining and illness, and to evaluate changes in training and how they are affecting the body.
The relation between stress and recovery can be numerically expressed by the Recovery Index. Readings of high frequency (HF), low frequency (LF), respiration rate and HRV are taken overnight during the first four hours of sleep. The Recovery Index is represented by a pair of numbers, where the first number shows stress reactions and the second indicates recovery reactions. By comparing their Recovery Index value at any time to a baseline value at rest, athletes can determine whether or not they have fully recovered after a training session.
A similar value to the Recovery Index is the HRV Index. This is a single number, again taken after an overnight recording of the same variables. A high number shows increased recovery, while a low number means poor recovery. For most people, during the day the value should be over 15 and more than 25. At night, it should be around 50% higher, meaning around 20 to 30. However, in athletes, the HRV Index value is usually at least 100, and often several hundred.
One of hardest things to know for athletes is whether they should rest or continue training when experiencing fatigue. To this end, the HRV Index and Recovery Index can be very useful tools. With both metrics, athletes can monitor HRV values to determine if they have recovered sufficiently to have reached their baseline, and as such are able to continue training without the risk of injury. For this to be effective, it is necessary to record Recovery Index and HRV Index values on a daily basis.