Heart Rate Part 1

Heart rate is widely used by athletes as a means of ensuring they are training at the right intensity. It does have some limitations though and it’s important to understand this if you plan to use heart rate as your intensity metric .

Determining Anchor points for Heart Rate

As with any metric we need an anchoring point from which we can relate intensity to. For heart rate there are two common ones ; maximum heart rate and threshold heart rate. I dont recommend maximum heart rate as it comes with too high an error margin and is not linked to ones fitness. 
The determination of maximum heart rate is often done using an equation , 220 – ones age being the most popular one . This has an error margin of +/- 12 bpm. There are some slightly more accurate ones but they only improve the error margin to +/-10 bpm. This is nowhere you are accurate enough to use to prescribe training intensity. 
There are field tests you can use to determine heart rate accurately which gets around the inaccurate formulas. But the other problem with maximum heart rate is it’s not reflective of your fitness.. Maximum heart rate changes very little, if at all, in response to changes in fitness.

This means using maximum heart rate requires a constant adjustment of training percentages to account for changing fitness levels. Running at a set percentage of maximum heart rate will feel significantly easier for a fitter runner compared to a less fit one. However, if you base intensity on a percentage of your threshold, then two runners with different fitness levels running at set % of their respective thresholds will be running at different speeds but at the same relative intensity.

In another video we’ll talk about what threshold actually is, what threshold heart rate is, and how to determine threshold heart rate and maximum heart rate.

Response Time
Now one of the problems with heart rate is the lag time. When you increase your effort there is a gap between when your pace increases versus when your heart rate reflects that increase in pace.

To illustrate that, have a look at this example here.

Below is a session of 6 x 3 minutes with 2 minutes recovery

The average HR per rep was 158,162,164,166,166,168
Speed ( minute per k ) 3:21, 3:23, 3:24, 3:20, 3:24, 3:19

Heart rate has increased 10 beats over the 6 reps whereas pace has remained relatively constant. The pace to heart rate relationship is not fixed, heart rate was lower for the same pace in the first few reps compared to the last few reps.
It took until the 3rd rep for the heart rate to be relatively consistent in relation to pace. The use of heart rate to guide intensity in the early reps would have resulted in the athlete pushing too hard and slowing significantly in the later reps. 

For shorter reps the lag between effort and pace is even more pronounced.
Hence using heart rate to guide intensity for short reps is not recommended, one needs to use pace, power or even RPE.

Here is another workout 8 x 4 minutes

Heart Rate 155, 164, 165, 171, 169, 173, 172, 174
Speed ( minutes per km ) 6:11, 6:13, 6:12, 6:15, 6:19, 6:14, 6:17, 6:17
So you can see that although heart rate is increasing throughout the session by almost 20 beats the athletes pace is decreasing.

So we have two problems
1. Heart rate is slow to respond, it took until the 4th rep before it started to be consistent each rep
2. The relationship between heart rate and pace is not 1:1. Heart rate can increase even if pace decreases.

As heart rate is both slow to respond and does not have a 1:1 relationship with speed it is not recommended to use in short intervals as a means of guiding intensity.

As the reps get longer it becomes less of an issue

Here is a 4 x 10 minutes session

Heart Rate 158,163,165,165

Pace 3:33, 3:31, 3:35, 3:35

Now if you look closely, you can see in the first two reps there was a period where the heart rate started lower and increased untill eventually plateaued out.
But in the second two reps, it reached that point pretty quickly and was relatively steady for the remainder of those last two reps.
So for shorter reps it may not be any good, but for longer reps if you ignore heart rate for the first few minutes then after that, it’s a fairly good guide on whether you’re maintaining a constant effort.
So it could be used for longer threshold tempo type reps but you would need to educate your athlete that it might take a few minutes in the first few reps for their heart rate to reach a steady state.

 

Now given we are trail runners how does heart rate cope with trails and the ups and downs that come with that?

Here is a 2 x 30-minute workout on undulating trail

You can see that HR varies more than on flat terrain – there is a range of 20 bpm from lowest to highest heart rate with the lower heart rates occurring on the downhills.

For every downhill heart rate will drop and then will be slow to rise on the next uphill due to the lag between change of effort and heart rate response.

Whilst heart rate is not useless using it on up and down terrain if your athlete is trying to adjust their pace based on heart rate on trails you will need to give them some understanding of how it will fluctuate on the ups and downs and the lag between a change of effort and the corresponding change in heart rate.

Heart Rate Part 2 

Environmental Factors

Heart rate not only responds to the work your body is doing it also changes according to environmental factors. On hot days your heart rate will be higher relative to the same pace on a cold day. Hot and humid days it will be even higher.  This is not a problem as if you stick to the same heart rate targets your pace will adjust to the conditions. However there is one other problem with heart rate in warm conditions.

Cardiac Drift

This refers to an increase in heart rate when running at below threshold intensity that occurs despite pace not increasing. There are several reasons for this ( a full understanding of the mechanisms is still being investigated in the literature see here for more detail

The main one is blood is diverted to the skin to aid cooling to help with thermoregulation, this decreases stroke volume, hence less blood going to working muscles and therefore to maintain the same pace heart rate must increase.

The second is dehydration. A 3-5% body weight loss means a 3-5% blood plasma reduction.  The accompanying reduction in stroke volume means an increase in heart rate is needed to get the same amount of oxygen to the muscles to maintain the same effort.

The third reason heart rate will increase for same effort is when the pace is close to threshold and you run for a longer duration than your TTE (Time to exhaustion at threshold). For example if your threshold pace is  4:00  min per k and TTE is 40 minutes  and  you try to run 60 minutes  at 4:05 min ks towards the end of the 60 minutes  heart rate will start to rise as fatigue increases.  The same will happen over longer durations. If the pace in the long run is too fast ( although well below threshold ) your  heart rate will still tend to rise.  Even  running at an easy pace , if you run for long enough heart rate will rise, for example if your longest run us 1 hour and you try and run 2 hours at the same pace as your 1 hour runs its very likely your heart rate will rise as fatigue increases.

So the question we have to ask ourselves is when is cardiac drift something we need to pay attention to?

If it’s rising as a result of dehydration then we have two choices – we can either ignore it and continue running the same pace or slow down to keep heart rate in the target zone.  Doing long runs that get slower towards the end is an ineffective way to train that no good coach would recommend.
Despite what many believe dehydration does not have negative impact on performance if you drink to thirst. If your  heart rate is rising then in the absence of other factors I would ignore it and maintain same pace rather than stick to target heart rate zones and slow your pace down.  ( We will talk in another chapter on the effect of dehydration on performance )

If the reason for heart rate rise is the pace is too fast  or the athlete doesnt have the endurance  to maintain that pace  then a modification of pace is needed.
So if for example an athlete’s threshold is 4 min k pace then long run should be around 5 min k pace ( approx 75-80% of threshold ) but if they try running at 4:30 pace for three hours its very likely heart rate will rise  compared to if they ran at 5 min k pace.
Or if ones longest run is 60 minutes  and the athlete does a 2 hour run even if they stick to 5 min ks its likely heart rate will rise as fatigue sets in.

The problem now is how can we tell if the cardiac drift is a result of the athlete running too fast relative to their  fitness , is  due to dehydration or due to environmental conditions ? Even if we knew which one how could we determine how much of a rise is ok ?

There is no easy answer to this

An understanding of your athlete’s fitness and the weather conditions the athlete runs in is necessary to aid prescription of exercise intensity based on heart rate.  Even in  cool  conditions dehydration greater than 2% is not uncommon so you may have to give  graded prescription. For example  start with heart rate 135-145 and allow it to rise to 140-150 by the end. There is no definitive amount heart rate will rise due to any of these factors which makes it very difficult to provide accurate targets.

For those athletes that like using heart rate one approach would be to use heart rate to set intensity early in the run  and connect it to rate of perceived exertion and then later in the run  forget heart rate and focus on RPE instead.

Combining a heart rate target with some descriptions on how the run should feel is a even better approach  as you can define what  the RPE should feel like rather than relying on heart rate alone.

Heart Rate Part 3

 

One of the things that people say is a pro for heart rate is the fact that it encompasses all the body stresses.  Our mind and body has to deal with stress from several different sources ; emotional stress, physical stress, financial stress, work stress, relationship stress, environmental stress, fighting  off sickness, all of those stresses affect heart rate.

We have all had that experience  of going for an easy run when fighting off a cold and your heart rate’s a little bit higher than usual. It’s just your body saying, well, I’m trying to fight this cold off and you’re trying to make me go for a run, so I have to do more work than usual.

In a sense, that’s a good thing, but the problem is that the heart rate response to stress is not easily definable, and it can vary. To give you an example, if you’ve had a week off or a very, very easy week of training, but good sleep, no stress, and you go for your easy run, and what you’ll find is that your heart rate is probably lower than what it normally is for the same pace.

Training fatigue also influences heart rate. You can usually hold higher heart rates when fully tapered than you can in training . For example, my  threshold heart rate in training used to be 165-168 bpm  and I could hold that for 60 minutes at a real push. But fully tapered I could average 174 in a half marathon that took just under 80 minutes .

On fresh legs  we can usually run faster for the same heart rate or run longer with a higher heart rate  than we can in training.

The opposite can also happen .  If you are overtrained your heart rate can also be lower than usual for the same pace in an easy run, and on a hard run you’ll find it difficult to reach the usual heart rate you do  even though you are running the same or similar pace .

Thus heart rate can give us conflicting responses. We can have easy runs where sometimes heart rate is lower than usual for the same pace because you’re fresh, and sometimes because you’re tired.  Other times our heart rate can be elevated due to fighting off something or elevated because we are fresh.

It can be difficult to determine  what you should do based on the slightly  you elevated or slightly lower heart rate because depends on why that heart rate is higher or lower.

Now, if you combine it with a bit of common sense and a bit of perceived effort, you can probably work most of that out yourself. If you think you’re fighting off something and you go for an easy run and your heart rate is elevated it’s just confirming to you that, yes, you are fighting off something, maybe you should slow down.

Or if you’re going for a interval session or a tempo session and you’re really struggling to get your heart rate up to the same levels as you usually do, and your pace is not quite as high as you usually run, it’s probably telling you something that you are either stress level is reach too high, you are more fatigued, or you’re fighting off an illness.

But without the connection between heart rate and RPE,  pace or  power it is next to impossible to determine how to act on your heart rate data.

Caffeine

Caffeine is a stimulant and causes a rise in heart rate. If you go for your morning run after a shot of coffee, your heart rate is likely to be higher than if you don’t. Caffeine gels taken in a race may cause a slight rise in heart rate.

It is likely that if you set your heart rate targets well, that the slight rise in your heart due to caffeine will probably still have you in the zone where you need to be anyway. It’s only really  if you are sitting at the very top of your zone and you take some caffeine, it might push you two or three beats higher and have you wondering if you need to slow down or not.

Pre race nerves can have a big influence on heart rate. Many of us will have noticed a very elevated heart rate whilst standing on the start line of an important race.

When you start running, it’s can  take a while before that heart rate settles down to be reflective of our effort rather than oour heightened states of anxiety due to the race.

If you use heart rate, you have to train your athletes or yourself to ignore heart rate for that initial period at the start of a race until you’re confident that the pre -race nerves have now settled and  heart rate is a true reflection of the work you’re doing.

How long it takes for the heart rate to settle down is impossible to say with any certainty.

In a race dialling into our optimal pacing right from the start helps minimise our chances of blowing up later in the race and maximises our chances of reaching our potential.

Using a metric to guide intensity that is likely not accurate for an unknown period  of time from the start makes heart rate a poor metric to base race intensity on.

So in summary,

Heart rate needs to be anchored to something to allow us to use it for training prescription. Threshold heart rate is far better to use than maximum heart rate as threshold heart rate  is reflective of one’s fitness.

Heart rate is slow to respond to changes in effort which makes its use on runs where there are frequent changes of intensity problematic.

Cardiac drift affects heart rate in longer runs, but this drift can be ignored in most cases except when its caused by a lack of fitness

Heart rate is affected by external factors such as stress and environmental conditions. How much these factors influence heart rate is unknown.

Heart rate can still be a useful metric if combined with other metrics such as RPE , pace or power.

When analyzing heart rate data post-workout, we have to keep in mind all those other factors that can influence the heart rates response to exercise before giving post-workout feedback.