Using Heart Rate and Pace During the Summer Transition

I have been training in zone 1 and 2 for a couple of months now.  For me, that involves keeping my heart rate below 130 BPM.  I have no particular run goals in mind right now other than building some leg durability and "keeping in touch" with decent run fitness so that if I decide to do an event, I can train up quickly.  Today was the first hot run of the year.  Last week I was wearing long sleeved shirts for my morning runs and today I probably lost a gallon of sweat.  This is the "transition" season between spring and summer.  During this time of the year, many of us have not heat acclimatized yet.  What it meant for me this morning was that I simply could not keep my heart rate below 130 on the second half of the run.  I was closer to 145 at an easy pace.  Why?  Because heart rate is an output and not an input.  Heart rate responds to work rate (pace), which is why it is used as a gauge of exercise intensity.  However, heart rate responds to other factors as well.  Heat is one of them.  In hot weather, the body shunts some blood away from working muscles and to the surface of the skin for cooling.  Also, as you sweat, blood volume is reduced, so the heart has to pump more often to pump the same amount of blood.  Both of these factors result in an increased heart rate for the same work rate (pace).  If you have the ability to track pace and heart rate, you can manage this transition period effectively and ensure that you get quality sessions, despite the changes in heart rate response.

1.  Should I simply maintain a pace that keeps me in my heart rate zone during this time of the year?  No.  If I did that this morning, I would have had to run too slowly.  In that case, my leg muscles would not have received an adequate training stimulus.  Aerobic fitness is primarily in the muscles.

2.  If I train at an elevated heart rate in this case am I overworking the heart?  No.  You don't slow down because your heart is tired.  Except for cases of clinical heart disease, cardiac fatigue is not the reason you cannot go a faster pace.  Fatigue is due to biochemical fatigue in the muscles, not the heart.

3.  So, should I train at an elevated heart rate all summer then?  No, not exactly.  Over time (approximately 2 weeks, with most adaptation happening in the first week) your heart rate will settle back down.  This happens due to heat acclimatization.  The primary response is that your body increases blood volume. However, if you train in the heat, you will probably have to accept a slightly higher heart rate (approx. 5 BPM).

4.  So, how should I manage this until I heat acclimatize? 

• The first thing you should do is pay extra attention to hydrating before you start your runs.  Then, hydrate during the run.  Both of these will help to preserve blood volume and result in keeping your heart rate closer to where it should be. 
• Second, if you can monitor pace, as well as heart rate, try to keep your pace close to where it was before the hot weather hit.  If you slow down too much in order to maintain your target heart rate range, you will not adequately train the muscles, which are the primary target for training.  However, there is a cost to training at the same pace, but a higher heart rate.  The higher heart rate is driven primarily by a higher central nervous system (CNS) sympathetic drive.  Over time this can result in a bit more CNS fatigue.  As a result, you will probably need to dial back pace just a little bit.  I would accept a slightly higher heart rate, by about up to 10 BPM higher by the end of the run.  More than that would be excessive and might be a sign that you need to be more aggressive with hydration.
• Third, acclimatization is going to happen fast.  If you pay attention to hydration, you should be back to within 5-8 BPM of normal within 2 weeks.  Because of the heat though, you are probably going to have to adjust your HR zones up by just a bit.  5 BPM is probably about right.
• Fourth, if you are using a software analysis tool like Training Peaks or others to compute training load and training stress balance, the higher heart rate for a given pace is going to lead to inaccuracies.  It will overestimate training stress.  Training stress is determined primarily by work accumulation.  HR is not a great estimate of work rate if it is elevated due to heat stress and lack of heat acclimatization.  Remember, HR is an output, not an input.

In summary, keep your pace close to where it was in cooler weather.  Dial it back just a little and temporarily accept a higher heart rate.  You should see your heart rate come down to within 5 BPM or so of normal in about 2 weeks.   This is an indication that you have heat acclimatized.  During the summer, adjust your HR zones up by about 5 BPM if training in the heat.  Ensure that you have a good hydration strategy to include hydration during a run.  Monitor pace and heart rate to ensure that you are getting it right.