Heart Rate Variability

As Juerg Feldmann from FaCT-Canada likes to say, “The purpose of training is to make you worse. It’s the recovery afterwards that makes you better.” Proper training imposes a stress on the body. After a hard workout an athlete usually feels worse. In many cases, it may take a day or two before the athlete has recovered enough to repeat the workout. If the athlete ignores the feeling of tiredness and fatigue and continues to train hard, he or she risks pushing the body too hard ultimately resulting in an overtraining state.

Thus, an important consideration in training is knowing when to train and when to rest. While this seems obvious, it’s surprising how few athletes integrate this concept into their training. Many athletes are wedded to a pre-programmed plan which details daily workouts in advance. Such plans work for some athletes but not others since they fail to account for the individual’s response to training stress.

For those that do try to integrate strategic rest days, many do so simply by how they feel. This isn’t a bad way to go but not everyone is capable of separating out physical response from other factors, such as motivation or mood.

So is there a way to accurately gauge training stress and recovery? One approach lies in examination of an athlete’s heart rate variability.

Imagine a person sitting perfectly with constant heart rate of 60 beats per minute. If the heart rate was perfectly regular, each beat would occur exactly one second (or 1,000 milliseconds (ms)) apart. In reality, heart rate is not exactly regular. One beat might occur 1,015 ms after the previous beat, while the next beat occurs in 992ms. The variation between beats is defined as heart rate variability (HRV). In this example, the HRV between the two beats is 23ms.

In medicine, HRV has long been used in monitoring cardiac patients. Abnormally low levels of HRV are considered to be a risk factor for heart failure. Besides its use for critical care patients, sports scientists found HRV can provide insights into healthy athletes.

In the 1990’s Polar conducted extensive research into the relationship between HRV and cardiovascular fitness. Their initial research indicated that athletes had a higher resting HRV than sedentary subjects. They expanded their research to investigate HRV levels at different levels of exercise intensity. This research showed a consistent and repeatable relationship between HRV and exercise intensity.

Based on this research, Polar developed a feature for their heart rate monitors which calculates an individual’s recommended training zone (heart rate range, e.g. 120-150) based on HRV during a simple step test. They call this feature “OwnZone.” This is a useful test since it can be conducted in any workout as part of a warm-up. If an athlete is tired, the OwnZone is usually lower. Consistent training can lead to higher OwnZone levels.

Polar also conducted research into the relationship between HRV and overall cardiovascular capacity as measured by Vo2 max. Their initial research indicated a correlation between these two variables. Based on their findings, Polar gathered a large sample of athletes of different ages and levels of fitness. This study produced a large database composed of age, level of activity, sex, height, weight and HRV. Based on this database and the associated research findings, Polar developed a feature called the “Fitness Test” or FitTest.

To perform the FitTest, the athlete lays down perfectly still for approximately five minutes. Based on the HRV during this time, the monitor calculates a number which roughly corresponds to the person’s Vo2 max.

Polar suggests that the FitTest be taken periodically to gauge changes in fitness. Juerg Feldmann from FaCT-Canada thinks it can be used much more frequently to measure training effects. Taken daily it can show short-term training impacts. Further, he suggests that the intensity of a specific workout can be assessed by taking the FitTest before and after. Following a particularly hard workout, the FitTest can decrease by five or more points.

Finally, Polar used their HRV research in the development of their Optimizer test, which is an extension of the traditional orthostatic heart rate test. In the orthostatic test, the athlete takes a resting heart rate while lying down, then they slowly stand-up and note the maximum heart rate. They remain standing and record the final resting rate.

In the Polar Optimizer test, these three steps remain the same and the heart rate monitor automatically records these numbers as well as certain measures of HRV. Every time the test is done, the monitor records the results and builds a database. The monitor calculates a single number based on the readings from the test as well as previous tests. A “1” corresponds to a recovered state while a “2” represents a normal state, a “3” is overtrained. There are a total of nine possible states under the test.

Research continues on HRV and exercise. A study published in December 2007 compared two groups of athletes. One group exercised over several weeks according to pre-determined mix of low and high intensity workouts. The workouts of the second group were guided by their HRV. If HRV indicated a recovered state, the athlete would do a high intensity workout. Otherwise, the athlete would do a low intensity workout. Following the training period, the HRV group had significantly greater improvements in terms of Vo2 max and workload compared with the control group.

Training can improve performance but it must be done strategically to achieve the best results. HRV is a readily-available tool which an athlete can use to leverage their training to optimum effect.