Question: I like to do rides of 100 to 200 miles and find that as a long ride goes on, my hear rate increases even though my speed and perceived effort remain constant. What’s going on? — Macklin G.
Coach Fred Matheny Replies: What you’re experiencing is a phenomenon called “cardiac drift.”
If researchers put you on a stationary bike and set the wattage at a moderate level to elicit a heart rate of about 80% of max, after about 30 minutes your heart rate would creep up to 90% even though you were generating the same amount of power. On the road, you may see something similar in a time trial or in the ultra rides you’re doing.
This situation is caused by dehydration, but other factors are involved and science hasn’t quite figured them out yet. Merely drinking won’t eliminate the rising heart rate because you can’t absorb enough fluid from your stomach to keep up with the deficit.
Muscle fatigue contributes. As a time trial or long ride progresses, your heart must work harder to maintain the same power output from themuscles.
Try varying your weekly workouts so you aren’t always doing steady rides or time trial-like efforts. Instead, do two days quite easily at about 60-70% of max heart rate. Include one day of hard, steady riding at 85-90% of max. Add an interval day when you elevate your heart rate to 90% or above for short periods, alternated with easy cruising. Your week should also include one long endurance day and two days off the bike.
Also, swallow plenty of sports drink. The more you practice hydrating during hard efforts, the faster your stomach will empty and send the fluids to the cells, where they’ll help keep your heart rate down.
Finally, try to keep yourself under control early in the ride, especially if you have a tendency to go too hard on climbs. Aim for a heart rate no higher than about 85% of max. That should delay cardiac drift and help you finish stronger.