Heart Rate Variability Analysis- how to improve your training performance

Heart rate variability analysis – a training tool you can’t ignore

Heartrate variability analysis (HRVA) is fast becoming a versatileinstrument for athletes and coaches. In this article Alan Ruddockprovides further evidence for HRVA’s efficacy and gives a step-by-stepguide for sportsmen and women who wish to use HRVA as a monitoring toolto enhance training response

In endurance sports, several ‘endurance markers’ are used to monitorathletes, assess the effects of training and determine trainingintensity. The most frequently applied models are the ventilatorythreshold (VT) and respiratory compensation point (RCP). To identifythese endurance markers, athletes normally undertake an incrementalexercise test to volitional exhaustion, usually within, but notrestricted to, an exercise physiology laboratory.

VT and RCP are identified using cardiopulmonary gas analysisequipment, which continuously measures oxygen and carbon dioxideconcentrations and the flow of air inhaled and exhaled by athletesduring exercise. As exercise intensity increases during the test, sodoes lactateand hydrogen ion production, and as the body attempts to ‘buffer’ thehydrogen, an increase in carbon dioxide occurs. The response of thephysiological system is to increase ventilation (the total volume ofgas being inspired and expired) to expel the carbon dioxide.

The point at which lactate begins to accumulate rapidly in theblood, causing an increase in ventilation is important to sportscientists because several studies have reported a strong correlationbetween these markers and endurance performance. For instance, anathlete’s running speed at a lactate concentration of 2.5mmol/L-1appears to be highly predictive of distance running performance atdistance events such as the 10,000m and marathon(1).

The problem, however, is that blood lactateand ventilatory assessments are primarily restricted by laboratoryprotocols and expensive equipment that makes testing large numbers ofathletes out in the field extremely difficult.

Consequently, an inexpensive and non-invasive method to assess ventilatory thresholds has been proposed using HRVA.
HRVA is a non-invasive assessment of the autonomic nervous system’s (ANS) controlof heart rate. Heart rate variability reflects the time intervalsbetween ‘R spikes’ of the ‘QRS complex’ displayed by anelectrocardiogram (see figure 1 for explanation).

The ANS functions without consciousness or volition and regulatesnumerous bodily functions via the sympathetic and parasympatheticnerves. Activity of the sympathetic nerves constricts blood vessels,decreases gastric movement, constricts sphincters and increases heartrate while parasympathetic nerves have the opposite effects. In hearttissue, sympathetic nerve endings are situated on the myocardium, whileparasympathetic nerves on the sino-atrial node, atrial myocardium andatrio-ventricular node. Together these nerves act to control both heartrate and force of contraction.

HRV and ventilation

Several theories have been proposed to explain why HRV andventilation are closely related. These theories propose that the ANSdetects changes in blood pressure, cell chemistry, force sensors, localtissue metabolismand circulating hormones, which causes an appropriate response via thesympathetic or parasympathetic nerves in an attempt to maintainhomeostasis.

The most studied theory in relation to HRV and exercise is that ofrespiratory sinus arrhythmia (RSA). RSA is an interaction betweenrespiration and circulation whereby normal heartbeat and blood pressurevary secondary to respiration. The synchronicity can be seen in heartrate variability where R-R intervals are shortened during inspirationand prolonged during expiration.

During inspiration, the activity of the cardiac parasympatheticnerve is almost abolished, which results in a shortening of the R-Rinterval. In contrast, during expiration, the cardiac parasympatheticnerve reaches its maximum thus extending the R-R interval(2). Think ofit like this: when you inhale, you feel your muscles that controlinhalation contract, which is when the sympathetic nervous system ispredominant. When you expire you feel more relaxed – this is when theparasympathetic nerves are dominant. This link between respiration andHRV has led sports scientists to research into the effectiveness of HRVto detect ventilatory thresholds.

There are numerous ways to process/analyse HRV data and it is farbeyond the scope of this article to explain how. Instead, interestedreaders are directed to the HRV standards and guidelines for clinicaluse for a detailed explanation(3).

However, while the study of HRV and ventilatory thresholds is afairly new domain, back in the late eighties a piece of research wascarried out that identified VT using breathing frequency (breaths perminute)(4). Since then ventilatory thresholds have been identifiedusing different methods with reasonable reliability.

Recently, researchers from France looked at the relationship betweenHRV and ventilatory thresholds(5). Using both sedentary and athleticparticipants they found that the HRV method used to detect ventilatorythresholds was highly correlated with a gas analysis method fordetecting ventilatory thresholds. In other words, the point at which VTwas detected using gas analysis was close to the point at which the HRVmethod estimated VT. The researchers concluded that HRV was a usefultool for identifying ventilatory thresholds in both sedentary andathletic populations.
Other researchers have used different methods of HRVA analysis todetect ventilatory thresholds and despite differences in methods ofanalysis compared to previous studies, these groups have found HRVanalysis to be a potentially reliable tool for detecting ventilatorythresholds.

For example, a group from France used a field-based incrementalexercise test to determine whether ventilatory thresholds could bedetected using HRV in professional French football players(6). Theseresearchers used a commercially available Polar S810 heart rate monitorto record R-R intervals. After data analysis they found that HRVanalysis underestimated VT by 0.25kmh-1 and RCP by 0.5kmh-1.Statistically, there were no significant differences between the HRVmethod and gas analysis in detecting either threshold. This led theresearchers to conclude that the study provided an advance in exercisephysiology since coaches now have the ability to assess ventilatorythresholds of athletes using inexpensive HR monitors.

This method is based upon the principle that when athletes areoverreaching, overtrained or even recovering from a training session,parasympathetic nervous system activity will be reduced. In order fortraining to be effective, the body needs to have recovered from theprevious training session (see PP246 for a more detailed explanation)but if too much time between training bouts has elapsed then gainsbegin to slowly reduce. The idea then, is to monitor theparasympathetic nervous system using HRV; this method allows coachesand athletes to determine the state of autonomic function and thereforetailor future training sessions appropriately. 

• Parasympathetic nervous system activity reduced – decrease training load;
• Parasympathetic nervous system activity increased – increase training load.

Pioneering Finnish researchers have shown that by using this method VT, maximal rate of oxygen uptake (VO2max)and running speed at VO2max can be improved by four weeks of trainingconsisting of running sessions at either a low intensity (65% maxHR[maximum heart rate], duration 40-mins) or high intensity (85% maxHR,duration 30-mins)(7). The scientists took 30 recreational runners agedbetween 22 and 40 and split them into three groups, 10 in the HRV group(HRV), 10 in the training group (TRA) and 10 in the control group (CON).

• In the HRV group the researchers recorded HRV on a daily basis. Before any training began for the HRV group, a 10-day rest period was implemented to collect baseline HRV measures. Then, during the intervention, if HRV dropped below the calculated reference value, the training load was reduced;
• The participants in the TRA group undertook a standard pre-planned training programme with no measure of HRV;
• Participants in the control group undertook no training.

The results showed that the HRV training group significantlyincreased their VO2max (from 56 to 60mls/kg/min-1), their maximalrunning velocity at VO2max (from 15.5 to 16.4kmh-1) and their runningspeed at VT (from 12.0 to 12.7kmh-1). By contrast, only maximal runningvelocity at VO2max significantly increased for the participants in thetraining group. Moreover, the participants in the HRV group undertookmore training at a low intensity than the TRA group. The researchersconcluded that whenever HRV is lowered, a lower training stimulus mightbe beneficial to gain a favourable response to endurance training.

Step-by-step guide to using HRV in your own training

The method that the Finnish researchers used to adjust trainingintensity using HRV is a relatively simple process that can be appliedto guide your own endurance training. To do this you will need a Polarheart rate (HR) monitor that records R-R intervals (RS800 series, S810series), Polar Precision Performance SW 4.0 software, which you candownload for free from www.polar.fi. You will also need a spreadsheetpackage such as Microsoft Excel.

Step1 – The first thing you’ll need to do is to setup your HR monitor to record R-R intervals, referring to your usermanual to do this. Before you start using HRV to guide your training,you need to undergo a period of rest (no training) to collect restingbaseline values; this period should last between seven and 10 days.

When you’ve decided on when this period will be, you’re ready tostart recording. The best time to record HRV is in the morningimmediately after you have woken up and emptied your bladder. This willensure you get the most reliable measure of parasympathetic activity asit is less likely that it will be affected by external influences suchas physical activity, dietary intake or psychological stressors. If youregularly measure your morning HR, you need to record HRV when standingup for 5 minutes. This is so your HRV measure isn’t ‘swamped’ byparasympathetic nervous system activity, which occurs when you lie down.

Step 2 – After you’ve recorded this HRV data,import it into the Polar Precision Performance SW 4.0 software. Presentthe data as a ‘curve of the HR values’ and then right click on thecurve to display ‘curve properties’. Under the HR tab, change the HRformat to ‘RR Intervals (ms)’. This will display your RR intervals.

Step 3 – In the ‘edit menu’, select the entireexercise then click the right mouse button on the graph and select‘Error Correction’ in the dialog box. Now select ‘OK’ to remove anyerrors in the R-R plot.

Step 4 – Right click on the graph and choose the‘Selection Info’ option. This will bring up a dialog box withinformation regarding the HRV measure. Look for the ‘HF (0.15 – 0.40Hz)’ value and make a note of it. Let’s assume (for the next step) youget an HF value of 321.94.

Step 5 – The next stage is to calculate the naturallogarithm of the HF value. To do this open Microsoft Excel and in acell type the command ‘=Ln(321.94)’. This will give you a result forthe log of HF (HF Ln) of 5.77ms2. Of course, you’ll have your own HFvalues – simply substitute the example given above for the calculatedHF (0.15-0.40 Hz) value displayed by the Polar software which you’llobtain from your own data.

Step 6 – Following your baseline period, you needto calculate the standard deviation of these values by typing ‘=STDEV’in Excel and then selecting all the reported HF Ln values for yourbaseline/rest period. Calculate the average of the HF Ln values andthen subtract the standard deviation you’ve just calculated to obtainthe daily reference value for HRV guided training. For each subsequentday calculate the average and standard deviation of the total HF Lnvalues; this will produce a moving day-by-day reference value.

Step 7 – When you record your HRV after yourbaseline period (ie when you begin training), you must then calculateHF Ln values using the method described above. If your HF Ln value isabove the reference (baseline) value then it is likely that your bodyis rested and ready for a high-intensity training stimulus. If the HFLn is below the reference value then you must reduce the trainingstimulus. Figure 3 shows you how this data will appear in Excel.

Conclusion

HRV analysis provides a non-invasive assessment of theparasympathetic and sympathetic nervous systems. Recent articles in PPand the scientific community have shown that HRVA has the potential tobe a versatile tool for athletes, coaches and sports scientists.Researchers are close to providing a simple and valid method forestimating ventilatory thresholds during field-testing by using HRVAbut at the moment there exists no definitive method. However, it ispossible for athletes and coaches to use HRV analysis to guideendurance training by estimating the level of recovery throughquantifying parasympathetic nervous system activity. This method hasbeen proved successful by Finish researchers and with some application,may well aid your training too!

Alan Ruddock MSc, CSCS, YCS is a researcher in exercise physiology at Sheffield Hallam University, UK