Broken Heart Rate Monitor?

I use a heart rate monitor with my Garmin computer. Before I got a power meter, my heart rate monitor was the only equipment I had to gauge my intensity. We also use heart rate monitors in my spin class, and so I’ve gotten very familiar with the levels of effort associated with the various heart rate zones.  This guide was developed by fitness expert Joe Friel:

Level                     Name                                    Maximum Duration

Level 1                  Recovery

Level 2                  Endurance                             All day

Level 3                  Tempo                                  1-5 hours

Level 4                  Subthreshold                         45-120 minutes

Level 5a                Superthreshold                      15-60 minutes

Level 5b                Anaerobic Endurance             3-7 minutes

Level 6                  Power                                   1 minute

Level 7                  Sprint                                    15 seconds

These zones correspond with a percentage of your threshold level, i.e., the highest power that you can maintain for one hour. With training you can increase your threshold level, which indicates better fitness.  Threshold level can be assessed quite accurately as 95% of average power in a 20-minute test.

It’s important to note that your threshold level is different from your maximum heart rate, which is genetic.  Furthermore, although the various zones are calculated as percentages of threshold power, they commonly are correlated to heart rate because heart rate is cheaper to measure than power.  (A heart rate monitor costs a lot less than a power meter.)  Evaluating intensity with heart rate works well, but it’s not foolproof, as I have discovered in recent weeks.

In spin class and at Tuesday Worlds, we ride a lot at Level 3 and Level 4 with occasional forays to Level 5a or even 5b. Additionally, in spin class I’ve learned to assess my effort level pretty accurately just by perceived exertion, which I can confirm with my heart rate monitor.  This ability has translated fairly well to riding on the road, too.

In the last several months it’s gotten harder for me to get my heart rate level up in spin class.  My 10-minute warm-up often isn’t enough even to get me to Level 2; only jumps or sprints will start elevating my heart rate. I haven’t thought too much about it, simply attributing it to the huge amount of endurance riding I’ve been doing for the last couple of years.  However, I’ve only just realized how this manifests itself on my road bike.

On a typical solo road ride in the 30-mile range, I usually spend most of my time in the upper endurance/low tempo zone, around Level 2.8 to 3.5.  In the last few weeks, however, I started noticing that my heart rate zone was usually around 0.7 to 1.8 – even though my perceived effort seemed much higher.  I told Robert that I thought my heart rate monitor was broken! He asked if I had looked at actual heart rate, not just the heart rate zone. Aha! Why didn’t I think of that? (That’s why I pay him the big bucks.) Sure enough, my heart rate has also been much lower. My heart rate monitor is working correctly after all.

Robert pointed out that when I’m riding my bicycle for 8 to 13 hours, as I do when I ride a 200-or 300-km  brevet, there’s no way my heart can sustain a high heart rate for that long.  Although that should be obvious from the zone chart above, it still doesn’t quite explain why my heart rate is lower even on the 30-mile rides. I was intrigued and decided to do a little research.

The average adult resting heart rate is about 70 beats per minute (bpm). Although I was already well aware that endurance athletes usually have a much lower resting heart rate (as low as 40 bpm or sometimes even less), I didn’t know that endurance training also decreases the submaximal heart rate. In other words, with a lot of endurance training, your heart is also going to beat more slowly while exercising.  One reason is because your body adapts to utilize more oxygen (VO₂max increases).  Also, the stoke volume of the heart (the amount of blood pumped per beat) increases.  These processes allow the heart and body to work more efficiently for a given effort.

Now I better understand the difference between power and heart rate.  While training with intensity increases threshold power, endurance training lowers the heart rate during exercise. Doing both types of training is important and results in better performance, whether you’re a sprinter or a randonneur.  For the past couple of years I’ve been very happy with the effectiveness of my training mix: the intensity of Worlds and interval training as well as long, slow centuries and brevets. I think I’m just now really seeing pronounced effects from my endurance training.

Yesterday I was glad to find that I still can get my heart rate up even during a shorter ride. I did one of my favorite routes, which is 31.3 miles. A typical ride on this route will yield data approximately as follows:

Average speed = 17 mph

Average power = 135 W

Intensity factor (= normalized power/threshold power) = 0.70 to 0.75 (Normalized power is slightly higher than average power.)

Now check out yesterday’s data:

Average speed = 18.4 mph

Average power = 176 W

Intensity factor = 0.884

I had already planned to do a longer than usual lunchtime ride yesterday because of an atypical schedule. Little did I know how therapeutic it would prove to be.  In the morning I went to a meeting that made me sick to my stomach (politics triumphed over science).  I worked out my frustration on the bike. Maybe I should use this training strategy more often: get mad before I ride.