Heart rate variability (HRV) is the calculation of the time between heart beats. There can be dramatic time changes, or variation, between heart beats depending if the body is relaxed or stressed.
When the body is under stress (overtraining, mental stress, or lack of sleep) the sympathetic nervous system is stimulated producing less variability between heart beats. This will produce a low HRV score, meaning that day’s training should be of low intensity or shorter duration.
Whereas when the body is well rested, the parasympathetic nervous system is stimulated and this produces more variability between heart beats. This produces a higher HRV score meaning that day’s training can be higher intensity and/or duration.
By getting our clients to monitor HRV daily, trainers can better organize a daily/weekly training program and they can observe improvements in fitness by seeing HRV increase over weeks and months.
- Differentiate how the sympathetic and parasympathetic nervous systems affect heart rate variability.
- Describe how the variability of heart beats can determine the readiness for high or low intensity, or long or short duration exercise.
- List the uses for heart rate variability with all different types of clients.
Heart rate variability (HRV) is the science of recovery from heavy training, preventing overtraining, and how stress affects our clients’ ability to train. It accounts for all kinds of stress and monitors the day-to-day fluctuations of the length of time between heart beats to determine exercise intensity for that day’s training session.
Initial HRV research was done with rowers who were training for the World Rowing Championships (Lellamo, et al. 2002). The research found significant changes in the nervous systems (sympathetic and parasympathetic) of rowers involved in hard training. In general, research shows that HRV is lower in over-trained athletes than in rested ones (Mourot, 2004). Moreover, the research indicates that:
- A high HRV is a sign of good health, high level of fitness, good recovery, and a good time to train hard.
- A low HRV can suggest high stress, fatigue, poor sleep, poor hydration, too much alcohol, but most importantly to a trainer or coach, a day where the training should be of lower intensity or duration or a recovery day.
Rhythm of the Heart
Harald M. Stauss (2003) wrote this elegant synopsis of the rhythm of the heart in an article in The American Journal of Physiology:
The rhythm of the heart has not only fascinated cardiologists but also inspired poets and musicians. Indeed, the periodic beat of the heart was used to define the speed of music. In music notation, the traditional Italian term “moderato” originally referred to one beat of the measure per walking pace (76-80 paces/min) or heartbeat (∼72 beats/min). The use of the heartbeat to define the speed of music may imply that the periodicity of the beat of the heart is very constant. However, this is not necessarily the case.
Heart Rate Variability
Heart rate variability measures the time and the change in time between heart beats. It is not only the measure of a heart rate, rather, it is the measure of the time between heart beats and how the time between heart beats varies. There is natural variability between each heart beat due to physical or mental stressors, or lack of stress. An average heart rate of 60 beats per minute (bpm) does not mean the interval between successive heartbeats would be exactly one second; instead the heart beats may be different by 0.5 seconds to 2.0 seconds (Heart Rate Variability (HRV), n.d.). Specifically, HRV measures time between R spikes on an EKG trace. See image below.
Measuring time between heartbeats can be used to determine psychological and physiological stress and fatigue on the body during training, or it can measure a body and mind that are relaxed and well rested. The more relaxed and recovered the body (“stimulation” of the parasympathetic system), the more the time between heart beats varies, thus a higher HRV. Greater stimulation of the sympathetic nervous system (“fight or flight”) indicates a stressed body and will decrease HRV, which means that the time between heartbeats will not change as much from beat to beat.
The below heart rate readings demonstrate that the sympathetic HRV response time does not vary as much, indicating a low HRV and high stress response.
The below heart rate readings demonstrate that the parasympathetic HRV response time between heart beats varies more, indicating a high HRV and low stress response.
According to Lellamo’s team’s (2002) research done with rowers training for the World Rowing Championships, the Italian Junior National rowing team (n=7) increased their training loads from 75% to 100% of maximum 20 days before the World Championships. The findings indicate that when the rowers were training at 75% of maximum they were found to have lower heart rate and higher HRV. Whereas, when they trained at 100% of maximum, the researchers found increases in resting heart rate and diastolic blood pressure, and lower HRV.
Plews, et al. (2013) looked at parasympathetic nervous system measures of morning HRV. They found morning HRV provides not only the most accurate HRV, but the best practical application for monitoring fitness. The researchers also found that in moderately trained athletes, moderate training loads increase HRV in parallel with aerobic fitness. However, when training loads approach 100% of maximum, HRV started to reduce. This indicates the sympathetic nervous system is more prevalent because the body is under the stress of heavy training. But, HRV increased after periods of deliberately reduced training intensity which is also known as tapering. This indicates that when a person engages in heavy training, then decreases the intensity and/or duration, the parasympathetic nervous system is more prevalent and the body is more relaxed. They also found that HRV shows high natural day to day variation but when HRV is taken over a seven day average the numbers smooth out the variations. This makes trends more apparent and detection of overtraining easier. One of the recommendations the researchers make is to take HRV at a regular time in the morning, shortly after getting out of bed.
Buchheit et al. (2010) wanted to assess relationships between running performance and HRV. They had 14 subjects (age = 37 years) all of whom were moderately trained runners who performed at least one 10k race in the previous year. Maximal aerobic speed and 10 km running performance were assessed before and after the training program. Subjects participated in an eight-week running training program with a short tapering period at end of training to maximize performance. The results showed:
- The subjects with highest starting HRV were the fastest runners.
- The subjects with the lowest starting HRV showed the most improvement.
- Subjects with largest gains in running performance had largest increases in HRV.
- Improvement of 10% in 10k run time required an average of 15% improvement in morning HRV.
In personal training, trainers use rating of perceived exertion, and/or how a client “feels” to determine the training intensity for a workout. Or trainers will guess, or assume, the client is ready for a high intensity workout or a low intensity workout. Sports conditioning specialists use periodized training where there are weeks of training when an athlete is training very hard and is tired, and other times when the training is less intense and the athlete feels more refreshed. But these methods are subjective, whereas HRV is objective.
Tracking HRV can be used to prevent overtraining and improve sports and/or fitness performance. Buchheit et al. (2010) showed that HRV was a good tool to use to show objective improvements in fitness (running performance). An example of the utility of HRV is that a client can take his or her reading in the morning, text it to the trainer, and the trainer can then determine that day’s training load based on a high, medium, or low HRV reading.
Heart rate variability technology can be used for a wide variety of clients, from weight loss clients to athletic clients. Training is training and we want to make sure all clients are at their peak. In other words, a weight loss client can suffer from overtraining as much as an athletic client.
With HRV utilization, it can be determined if a client is truly tired, over-trained, suffering from stress or lack of sleep, or generally not ready for a high intensity or long duration training day. On the other hand, HRV monitoring can indicate to the trainer when a client it ready for a high intensity workout. In our technology-loving society, HRV can be a great tool for tech-savvy clients wanting to monitor their fitness. An HRV app coupled with a fitness tracking watch can be the two of the best tools for a client who wants to monitor progress and readiness for training.
Trainers are always looking for ways to improve the results their clients get for fitness, health, and sports performance. With technology emerging as useful tools in the fitness, HRV is a new and exciting tool trainers and clients can use to monitor health, recovery, and training.
By using a HRV app each morning to get a reading, a client and trainer can understand the physical readiness for high or low intensity training. Heart rate variability measures the time between heart beats so that when the body is recovered, there will be a more varied time between heart beats, giving a higher score indicating a day to train harder and/or longer. Whereas, when the body is not recovered, the time between heart beats will be more constant giving a lower score, meaning the body is in need of low intensity workout or a recovery workout. Heart rate variability can also be used to monitor improvements in fitness over weeks and months because the HRV score gets higher as fitness improves.
Buchheit, M. et al. (2010) Monitoring endurance running performance using cardiac parasympathetic function. Eur J Appl Physiol. 108(6):1153-67.
Heart Rate Variability (HRV). (n.d.). Retrieved May 13, 2016, from http://support.polar.com/us-en/support/Heart_Rate_Variability__HRV_
Iellamo, F. et al. (2002), Conversion From Vagal to Sympathetic Predominance With Strenuous Training in High-Performance World Class Athletes, Circulation. 105: 2719-2724.
Stauss, H. M. (2003). Heart rate variability. Am J Physiol Regul Integr Comp Physiol, 285:R927–R931. 10.1152/ajpregu.00452.
Plews, D. J. et al. (2013). Training adaptation and heart rate variability in elite endurance athletes: opening the door to effective monitoring, Sports Med. 43(9):773-81.