The first part of “The Beginners Guide to HRV“, covered a general overview of Heart Rate Variability. The second part expanded on that information by exploring the available HRV apps, what they actually measure and how to take your own readings. In this third and final part, I will share with you my experience of using HRV data to track response to High Intensity Resistance Exercise (HIT). We will cover some of the personal observations I have made including, how my HRV typically responds to a HIT workout, unusual responses and warning signs, and whether HRV recovery tracks with muscular recovery.
High Intensity Training and personal observations on HRV
I partake in other varying physical activities outside of my 1-2 High Intensity Resistance Exercise workouts per week. However, I do not do so in any formal or structured way, I simply do other physical activities, when I am in the mood for them. Whilst testing HRV though, I wanted to minimize any moderately intense forms of physical activity for a period to focus on the HRV response to high intensity resistance exercise or HIT.
I will typically see a dip below HRV baseline, the morning following a HIT workout (as well as a slight increase in resting heart rate). Sometimes, I am back to baseline in 48 hours and sometimes it takes 72 hours. Note however that even on these post workout days with a dip in HRV score, the majority of the time, I will still get a green color-code indicator from Elite HRV.
Is it possible for the HRV score to be back to baseline the day after a HIT workout?
An interesting observation made is on one occasion my HRV score was back to baseline the morning after a HIT workout. I had to test what was happening here, surely I couldn’t be good to go 24 hours after my previous full body HIT workout?
My prior workout had included a leg press but no leg curl or leg extension, and the next scheduled workout was due to start with a leg curl.
24 hours after the prior workout and after getting the morning green reading, I set up the leg curl, locked myself in, started the stopwatch and commenced contracting. I was able to perform a set to momentary muscular failure that lasted for 1:25, a good length but nearly 20 seconds down on my previous leg curl performance, enough of a reduction to be significant.
What did I learn?
- Leg press significantly fatigues my hamstrings.
- I also learned the valuable lesson that it is possible to have an ideal baseline HRV score, normal resting heart rate and green color-code but that does not necessarily mean that you are muscularly recovered.
Does the autonomous nervous system recover simultaneously with muscle tissue?
I was now very intrigued by the muscle recovery aspect, so performed the experiment again, this time allowing for 48 hours of recovery before testing my hamstring strength. On this occassion I was able to achieve 1:40 on the leg curl, approximately the same as my previous best (1:43). This suggests that my hamstring strength had returned to baseline: the hamstrings had recovered (to baseline level) but not significantly improved in strength with 48 hours rest.
I wanted to extend this experiment a little further, this time testing quadriceps recovery. The same initial workout was used: full body routine including leg press but no leg curl or leg extension, and this time I waited a full 72 hours before testing quadriceps recovery with a leg extension. The result on this day was that I exceeded my prior leg extension best TUL at that load by nearly 30 seconds, a significant improvement. In this instance, it appears that 72 hours after my previous workout, I was fully muscularly recovered (beyond previous baseline) and ready to go, whereas at 24 and 48 hours my hamstrings had not been, despite good morning HRV readings. I am leaving aside any possible fiber-type difference factors between my hamstrings and my quadriceps and the possible effect of that on the recovery speed of the respective muscle groups. However I believe looking at the recovery curve of my hamstrings, I would have performed very well on leg curl at 72 hours and my quadriceps did perform very well at 72 hours.
The point I want to make is that when you engage in High Intensity Exercise to Momentary Muscular Failure, it may not be wise to simply go by an HRV app’s color-codes alone to judge your workout frequency. It is possible for the autonomic nervous system to fully recover, before muscle tissue has fully recovered.
How accurate are the HRV apps when it comes to scheduling HIT workouts?
That may leave some people disappointed, as if what promised to be the Holy Grail of HIT frequency scheduling has lost its magic. I don’t see it this way at all. The color-coding is still a useful feature, particularly for a summation of your ability to tolerate stress in general on a particular day. This is of benefit for example, if you engage in other types of physical activity or sport in between your HIT workouts, and is also useful for measuring the effects of work, psychological related stress and so on.
Imagine that you plan to do a HIT workout next week on Tuesday, but on Tuesday morning you get a yellow or a red indicator. Perhaps you’re in cumulative sleep deficit, you’ve been working hard or are coming down with an infection that is not displaying any physical symptoms yet. That yellow or red indication would tell you that perhaps today is not the best day to perform a HIT workout. Your resilience to stress is low and a full on high intensity workout may push you over the edge and/or require a disproportionally long recovery period afterward.
Instead on that day you could do no physical activity, or just have a walk, do some gentle mobility work, or indeed take some time to meditate, do some focused breathing or anything that you find relaxing and rejuvenating. Wednesday, Thursday or Friday may be your ideal day that particular week to exercise intensely.
Going beyond the color-codes to understand significant warning markers in baseline HRV scores
The color-coding system the apps use is cool, it is however beneficial to explore a little deeper so as to get a better grasp as to what is going on with the autonomic nervous system when you get differing or more extreme morning HRV scores.
Let us look at two scenarios, ones which you will want to do your best to avoid from happening, or to react quickly if they indeed do occur.
When you have deviated away from your current HRV baseline by >5%, upward or downward overnight, this represents a significant change and warrants serious consideration. Often, this shift will be accompanied by a similar degree of change in resting heart rate. Usually, when HRV jumps up, Resting Heart Rate (RHR) will decrease and when HRV falls significantly, RHR will increase. Let’s look at what this means.
This is a screenshot of my HRV data during an unusual week where I had two baseline readings of concern, one where HRV elevated >5% and one where HRV decreased by >5%. On this graph the green and yellow bars represent HRV at morning baseline reading, the green line represents rolling HRV average and the red line represents resting RHR taken at the same time as the HRV readings. The grey bars represent High Intensity Resistance Training workouts. Notice that on the two days of concern represented by the yellow bars, when HRV score dropped, RHR increased and vice versa. Also note how the app got it right and designated those two days as yellow using the color-coding system, I listened and didn’t exercise on either of those days.
Specific response of HRV to High Intensity Resistance Exercise
After completing a workout consisting of both compound and isolation exercises for a total of 8 exercises, all taken to momentary muscular failure and completed within about 20 minutes, my HRV score drops by approximately 30 points, from the immediate pre-workout reading to the immediate post-workout reading. This drop does not have the same ramifications as a morning baseline drop, as it is an immediate and acute response to the workout. The morning following a HIT workout although my HRV baseline score will be depressed it is unusual for it to drop to a level of concern, i.e. >5% below baseline, and as mentioned earlier, my HRV score will then typically return to baseline within 48-72 hours.
I have experimented with workouts separated by only 48 hours when I am back at HRV baseline by that time, however following the second workout, morning HRV readings can elevate excessively beyond baseline, accompanied by a decreased resting heart rate. This suggests the onset of a degree of sympathetic exhaustion.
In my experience, within the context of my lifestyle and my genetics, in general 72 hours between workouts allows for a recovery of both HRV and therefore the autonomic nervous system, and muscle tissue. Now that I have made some preliminary findings about my response at the shorter end of the recovery spectrum (24-72 hours), I will in the future look at the effect of slightly longer periods between high intensity exercise bouts (94+ hours). Of course this has been a classic n=1 study and your recovery may vary.
The take home message
- You will likely get the most out of high intensity resistance exercise workouts, if you perform them on days, when your morning HRV score is at least within a couple of points of your current baseline.
- Make sure to factor in sufficient recovery time for muscle tissue damage repair; just because you got a green reading on the app, the morning after your workout, doesn’t mean you are ready to exercise to momentary muscular failure again. Go and do a more moderate or calming physical activity instead.
- It is important to observe more than your HRV score alone. Be aware of your in-workout performance at each HIT session and your gradual strength increases over macro time periods, i.e. months. Note your performance in any other physical activities and sports you engage in, as well as your mood, energy levels and so on. Make HRV tracking a valuable part of the big picture of all your tracking metrics, rather than the be all and end all, and remember that exercise related stress is only one factor of many that has an impact on your HRV.