Who, why and what?
The whole principle of monitoring Heart Rate Variability (HRV) as an athlete is that you can see the impact of individual workouts and cumulative loading on your recovery. It stands to reason that the longer lasting and more intensive the workout, the longer it will take to recover. But what exactly is the relation between workout intensity and recovery? Is this a linear model where recovery time increases with power, speed or heart rate? Are there some threshold effects where the training load perceived by the body changes significantly?
Norwegian researchers, led by well-respected endurance sports scientist Stephen Seiler decided to look more closely at the effects of intensity and duration on HRV recovery.
Experienced runners and orienteers were separated into trained and highly trained groups. They used 1-2 hour sessions of training with intensity controlled to remain below the individual first ventilatory (first lactate) threshold as well as high intensity interval sessions.
What did they find?
Where the intensity level was controlled to below threshold, HRV values were already recovered to pre-exercise levels within 5 minutes of ceasing exercise (see chart below). The duration (either 1 or 2 hours) did not alter this, although there was a tendency towards rebound to levels above pre-exercise values for the 120 minute sessions. Conversely, both the threshold, and very high intensity exercise caused significant delays in recovery of parasympathetic HRV.
The researchers were also interested to see whether the parasympathetic HRV reactivation was different between the highly trained and trained subjects. The chart below shows that HRV recovery was delayed in the less trained subjects by 60-90 minutes compared to the highly trained athletes.
The authors concluded three key findings:
1. There is an increase in autonomic stress (as evidenced by delayed HRV recovery) once the first ventilatory (lactate) threshold was exceeded. Therefore, this threshold seems to be a clear boundary for ANS disturbance, i.e. workouts below the level have little impact on HRV recovery, whereas intensity levels above impact HRV recovery.
2. There appeared to be no additional delay for HRV recovery when working at an intensity level of 95% VO2max compared with training at lactate threshold intensity.
3. More moderately trained subjects required 2-3 times longer to reach the same level of parasympathetic recovery as highly trained subjects.
The conclusions I personally took from this study are:
1. You should be able to train for long periods of time below the first lactate threshold without causing HRV to be negatively disturbed. Therefore you can continue to do this kind of base training day after day without stressing the cardiovascular system. Perhaps this is how world class endurance athletes manage such high training volumes.
2. Do your intervals very hard rather than moderately hard, because it won’t make a difference to your recovery.
3. If you are not a naturally gifted or highly trained athlete with a VO2max of 70+ ml/kg/min it will take you (much) longer to recover from a hard session.
As usual, we would be interested to hear from ithlete users who have personal experience to either back up or dispute these findings!