HRV and training (not archery): Plews 2013 study

This is an opinion piece, not an experiment: five sport physiologists summarise what was known in 2013 about heart rate variability (HRV) as a sign of how endurance athletes adapt to training, and set out a way of reading it in elite athletes, with examples from rowers they were working with. In athletes with little or moderate training the picture they gather is fairly orderly: moderate loads raise HRV along with fitness, very heavy loads lower it, and a lighter phase lets it recover. After three weeks of overload, for example, HRV had fallen by 22% in swimmers and 38% in middle-distance runners, and after the lighter period it rose by 7% in the former and 38% in the latter. In elite athletes, by contrast, the studies point every which way: overtraining has been linked to higher, lower and unchanged HRV, and fitness can improve while HRV falls.

The authors put much of this confusion down to two things. The first is day-to-day noise: in an earlier paper of theirs, changes in HRV correlated with changes in 10 km time at r = -0.76 when weekly averages were used, but only at r = -0.17 with single-day values. The second is saturation: when resting heart rate is very low, HRV can drop even though vagal control is high, so a drop does not necessarily mean fatigue.

The cases they show point the same way. Two rowers at the 2012 World Cup in Lucerne both had low HRV before racing: for the athlete who finished second, 0.12% behind the winner, the authors read the drop as saturation; for the athlete who finished fifth, 1.92% behind the leader, as fatigue. In three rowers who won at the 2011 World Championships and the 2012 Olympics, HRV rose during the weeks of heaviest training and eased slightly just before the race. Finally, in four champions the relationship between HRV and the length of the heartbeat ranged from r = 0.91 to r = -0.03: it was different for each of them.

There is no experimental design and no sample: it is a narrative review with no stated criteria for choosing studies, alongside data from individual athletes the authors worked with. The tables summarising the studies are in the supplementary material, which is not part of the PDF read for this record. All the data shown were recorded on waking, lying down: six minutes of rest, of which the last five are used. The chosen index is Ln rMSSD, the logarithm of a measure of how much the length of one heartbeat differs from the next. The authors prefer it because its typical error is lower than that of the spectral indices (a coefficient of variation of 12.3% against 52.0% for normalised high-frequency power), it is little affected by breathing rate, a short recording is enough and it can be worked out in a spreadsheet.

To read the data they propose three tools. The first is the weekly average, or a seven-day rolling average, in place of a single day's value. The second is the ratio of Ln rMSSD to heartbeat length, which shows whether a fall in HRV comes with a slower heart, in which case it may be saturation, or a faster one, in which case fatigue is more likely. The third is the smallest meaningful change worked out for each athlete, with 90% confidence intervals. The rowers were followed for the 62 days before their main race, training on average 17 hours 21 minutes a week in 2011 and 16 hours 44 minutes in 2012.

The examples are a handful of athletes chosen by the authors themselves, nearly all of them winners, and read after the event: they illustrate a way of looking at the data, they do not show that it works. None of the proposed criteria, such as a fall in Ln rMSSD with a rising ratio as a sign of fatigue and a fall in both as a sign of freshness, was tested in advance on a group, and the authors say further research is needed to confirm it. Why HRV drops before competition in elite athletes remains, by their own account, unknown. The fifth place of athlete B is put down to poor adaptation to training load, but a bad race can have many other causes.

Everything concerns endurance sport, with many hours of aerobic work a week, and the effects discussed, saturation above all, depend on very low resting heart rates. Archery is not mentioned and there are no data on precision sports. The review dates from 2013, so later work on HRV monitoring is not covered.

For archers the study says nothing directly: it concerns rowers, not archers, and an archer's training load, made up of hundreds of arrows rather than hours of aerobic effort, is a different thing. What can be carried over to archery is the way the number is handled. Anyone measuring HRV each morning with an app or a chest strap will find here the reasons not to read much into a single day's value, to keep the same index and the same measuring conditions, and to compare each athlete with themselves rather than with a threshold that applies to everyone.

The study is also a reminder that in highly trained people a low HRV does not necessarily mean tiredness and a high one does not necessarily mean good form. In an archer with a normal resting heart rate saturation is less likely, but the principle holds: the number is read alongside how the archer feels and how they are shooting. In the registry the study bears on the claim about heart rate variability as a guide to training load from two sides: against it, for the uncertainty it documents in elite athletes, and in favour, for the way of reading the data it proposes.