Does Fatigue Depress Heart Rate?

Question: I race in the 45+ masters division and use daily bike commutes for training. After a day or two off the bike, I can stay above my lactate threshold heart rate for long intervals. However, after commuting 30 miles per day for a few days, it becomes increasingly difficult to achieve a high heart rate. Is this normal? Or is it old age? — Doug S.

Coach Fred Matheny Replies: It’s not old age!

It’s normal for heart rate to decline as riders get tired. We see this in stage racing where a rider can maintain a heart rate of, say, 185 in an early time trial. But a week later, because of accumulated fatigue, he can reach only 175 even though he has the same perceived exertion.

My three teammates and I noticed this phenomenon in the 1996 Team Race Across America. Each of us knew our lactate threshold heart rate, and we could maintain or exceed it through the first afternoon on the way from Irvine, CA, to Savannah, GA.

However, the next morning we found that our heart rates had dropped at least 15 beats per minute even though we were riding just as hard. For the next five days, we couldn’t raise our heart rates so we didn’t even bother wearing the monitors. But we still averaged the same number of miles each day (around 530).

Similarly, I suspect your heart rate is lower because of the accumulated fatigue from commuting.

Scientists aren’t quite sure why heart rate declines in these circumstances. It’s probably a combination of inadequate glycogen replacement and dehydration. Some exercise scientists have even theorizedthat the heart slows down to “protect” itself from what you’re asking it to do!

To keep your heart rate from sliding downward, pay close attention to your carbohydrate intake. You might need to consume a recovery drink right after your morning commute in addition to your normal eating. Also, sip water or sports drink all day at work to make sure you stay adequately hydrated.

If this doesn’t help, consider riding easier on an additional day each week. This will reduce fatigue so the workouts that count are high quality.