HRV as an "early warning system" for injury risk.
(More lessons from our horsey friends)

Alan Couzens, MS (Sports Science)

March 18th, 2015

"You've got to know when to hold 'em, know when to fold 'em", know when to walk away, know when to run...." - The Gambler

My last few posts have been devoted to (obsessed with? :-) the wonderful world of heart rate variability. Specifically, with how to…

Begin to practically implement HRV to guide your training

Actively improve your HRV to enhance your training response

But in this piece, I want to talk about how you can use your HRV data to significantly decrease your risk of injury.

"OK, Couzens, I was with you up until now, but you’ve gone a bit far here! How can my heart tell me what’s about to happen to my achilles? I know the old 'hip bone’s connected to the knee bone' song, but there are limits here!"

I hear you and, if it wasn’t for some recent chats with Christine (an expert in HRV, who I introduced you to in my last post), I wouldn’t have made the connection either, but in her world (elite race horses) she has spent several years looking specifically at this relationship between HRV and injury occurence and the results of her studies are interesting to say the least…

Her research has discovered significant relationships between a number of equine maladies and preceding HRV data. For race horses, the most serious of these maladies is, of course, a catastrophic injury during an event. This can literally be a life or death proposition for the horse, which highlights the huge benefit of anything that can predict this before it occurs to the horse &, hopefully, to the astute horse owner.

Christine’s studies were able to show a significant relationship between the (equine) athletes’ heart rate variability numbers (in this case LF/HF) and their likelihood to suffer a catastrophic injury during or leading up to a race. In the presence of sufficient data and sufficient time, this relationship is easy enough to observe. Biomechanical ‘failings’ in horses are horribly obvious. In human endurance athletes, often less so….

Chances are, when you go out for your training runs, you don’t have a little guy on your back 'whipping you on' irrespective of how you feel. If you feel a little ‘niggle’, consciously or not, you’ll probably back off some OR compensate in some way to minimize the pain. If you’re smart, you’ll recognize this compensation and abandon or modify the session, but chances are you have some form of the jockey with the whip buried deep in your psyche who’ll push you on to some extent. So, you’ll complete your run with a compensated, inefficient gait. This is where our (very human) problems start….

When endurance athletes present with an injury it is generally chronic, a result of days of the above scenario stacking on top of each other. Fundamentally, brought on by a blindness to mild pain and compensations that are not individually ‘catastrophic’ as in the case of our equine friends, but, summated are absolutely so - when we look at an athlete who is forced to throw an entire season of training out the window because of one of these ‘niggles’ that just gets progressively worse and worse.

The above scenario is all too common in a large chunk of the population that I work with – serious, age group athletes in their late 30’s to early 50’s. Guys whose ‘inner jockey’ might be feeling a little time crunched and who might be a bit more ‘whip happy’ than in previous years. Whip happy ‘inner jockey’ + tendons that don’t have the same tissue quality as they did ‘back in the day’ = serious problems.

Fortunately, for us humans, we write the inner jockey’s checks and we can take control back & make sure that he works for us & not against us. This all starts with opening our eyes to signs that things might be a ‘little off’ and tethering the whip on those days. With new technology, these signs of something ‘being off’ are so much more visible (& indeed quantifiable) than in years past.

Studies have shown a relationship between certain biomechanical measures/tendencies and the risk of injury. One of these key biomechanical signs that ‘something is off’ is a longer ground contact time (measured in milliseconds per stride). The longer the ground contact time, the greater the risk of injury (Willems et al., 2005, 2006). This makes intuitive sense: Athletes who are ‘heavier on their feet’ must cover more ground by extending their ROM ‘striding out’ rather than by ‘flying through the air’.

These differences in stride pattern are obvious to the trained eyes of an experienced coach, though perhaps a little less so to the runner themselves. Fortunately, technology now gives us the option to track ground contact in real time! The Garmin 920XT offers GCT feedback as you’re running. When this is combined with cadence, it gives the athlete some great biomechanical feedback as to when ‘something is off’.

Coming full circle, though, an athlete (& coach) can get a great ‘heads up’ on when something is likely to ‘be off’ (& the athlete is at a higher risk of injury) before the athlete even gets out of bed! As Christine’s work has shown, tired (equine) athletes, athletes with low HRV, consistently present with biomechanical deficits that lead to injury. It shouldn’t surprise us that this same link between tiredness (indicated via diminished heart rate variability) and biomechanical inefficiencies (increased ground contact time and decreased cadence) also show up in humans.

The following 2 scatter plots show the relationship between mean ground contact time vs HRV (rMSSD) and mean run cadence vs HRV (rMSSD) for an athlete over a block of training. This data is taken directly from Garmin Connect and Kubios.

HRV vs Ground Contact Time

HRV vs Run Cadence

The trends are quite evident and unsurprising. The more tired the athlete (lower HRV), the more ‘heavy footed’ they are. Ground contact time goes up & cadence comes down. The research would suggest that risk of injury significantly increases as this occurs. Drilling a little deeper into the chart, a threshold is evident at an rMSSD of 30ms. Things really head south for this athlete at this point. His gait is likely to change substantially. He will be ‘running tired’ & at a significantly greater risk of injury. Coincidentally enough, this cut off of ~30ms also coincides with his prescription for an 'easy day' via the spreadsheet method that I outlined here.

The above focus on metrics is not to diminish the importance of human decision making in the process. We don’t want to become mindless autobots so focused on the numbers that we forget to look at the human they supposedly represent. Rather, I look at the above steps as a ‘safety checklist’ of what you’re likely to see if you choose to proceed. Get up in the morning – HRV is low and feeling a little worse for wear? Mental note. Head out the door to train and notice cadence is a bit low, GCT is a bit long and you feel a bit ‘heavy or flat-footed’? Maybe you rethink doing a hard main set today. Can’t even hold a good cadence/stride in your warm up? Maybe you rethink the merits of running today. Choice is always a factor of human sport. Now with technology, those choices can be a little more informed.

Train smart,




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