Max HR vs LT Zones

From RoadBike Rider, March 10, 2016, by Coach John Hughes

Max Heart Rate Doesn’t Show Cycling Fitness

Imagine two club riders: Joan is 41, so using Dr. Milkin’s formulae her calculated max heart rate is 171 bpm. She started riding when she was 25.

Jim is 55, and his calculated max heart rate is also 171 bpm. He just started riding last year. On club rides Joan rides with the fast bunch, drops the guys on hills and holds her own in the sprints. Jim rides with the new riders’ slow group and is gradually getting fit enough to stay with the group.

Max heart rate doesn’t take into account differences in fitness. Joan is much fitter than Jim, and their heart rate responses are much different. Even if Joan and Jim tested their max heart rates, training based on their individual max heart rates still wouldn’t take into account fitness. Why not? Your cycling fitness is a result of:

1. How fast your heart can beat.
2. Your heart’s stroke volume – how much blood it pumps per beat.
3. Your VO2 max – how much oxygen your muscles use out of the oxygen delivered by your lungs.
4. How hard you can ride and how fast your heart beats before you start to go anaerobic.
5. How much power your muscles can deliver.
6. Your pedaling economy, how efficiently that power moves you down the road.

All six of these improve depending on how long you’ve been riding and the kind(s) of training that you do. Training based on your max HR only takes into account the first factor.

Why Lactate Threshold Is A Better Measure of Fitness

When Coach Dan Kehlenbach and I were writing our book Distance Cycling, we researched the literature on how to set training zones based on heart rate. Although some coaches use max heart rate, the consensus is that heart rate training zones based on a rider’s lactate threshold (LT) are better.

“Wait a minute,” I can hear you thinking. “Last week Coach Hughes wrote about “Why Your Lactate Threshold May Be Irrelevant.”

The point of that article was that you don’t need a heart rate monitor and you don’t need to know your LT in order to train effectively. You can use perceived exertion like I – and 49% of RBR readers – do.

A couple of weeks ago, I explained that your body has three different energy systems:

1. Oxidative aerobic system (low power / long duration).
2. Glycolytic anaerobic system (moderate power / short duration)
3. ATP-PC (high power / short duration)

To complicate matters further, your legs have three different types of muscle fibers:

1. Slow-twitch (low power, great endurance)
2. Fast-twitch IIb (moderate power and endurance)
3. Fast-twitch IIa (high power and shorter endurance)

Each energy system and fiber type responds to different kinds of training.

The key distinction for training purposes is between the oxidative aerobic system using your slow-twitch muscles and the glycolytic anaerobic system using your fast-twitch muscles. Your LT is the HR where you become increasing anaerobic. LT isn’t a fixed percentage of max heart rate but varies by fitness.

Our hypothetical rider Joan is very fit, and her LT is a much higher percentage of her max heart rate than Jim’s LT, who’s just starting to ride. Lactate threshold takes into account your fitness – and that’s why it’s a better way to set your training zones.

Estimating Your Own Lactate Threshold

If you spend several hundred dollars and go to a lab to measure your LT, you’ll ride an ergometer. Every few minutes the lab tech will draw blood, measure the amount of lactic acid and then increase the resistance of the ergometer. When your lactic acid reaches 4 mmol / L that’s your lactate threshold.

A lab test isn’t necessary unless you’re a serious racer, and if you are a serious racer, then a power meter will provide more useful data.

Each of my clients does a baseline time trial and from that I estimate his or her LT. The client repeats the baseline TT periodically to see if his or her time has improved, which means the rider is producing more power! If the client is fairly new to riding, then his or her LT may also shift up.

Your functional lactate threshold is the heart rate that you can sustain for a 1-hour time trial, but that’s a pretty painful test.

Here’s a less painful test to estimate your lactate threshold:

• Ride this solo rather than competing with anyone.
• Do the test after a very easy week when you’ve only ridden a few hours.
• Use a course that is flat or slightly uphill and will take you at least half an hour to ride going flat out.
• Do the test on a calm day.
• Warm up thoroughly for about half an hour.
• Ride as hard as you can for 20 minutes. (Use a course that takes longer so that as your fitness improves you can repeat the test on the same course.)
• Try to pace yourself so that your effort is pretty constant for the full 20 minutes.
• Record how far you rode, your average speed and average heart rate. (If your HRM doesn’t calculate average HR just eyeball it.)
• Your lactate threshold is about 95% of your average heart rate for the 20 minutes.
• Repeat the test every 4 – 6 weeks at the same time of day and under the same conditions.

If you go faster and farther, then your overall fitness has improved. If your sustainable average heart rate goes up, it may be because you’re getting better at pacing yourself through the time trial, or it may be because you LT has shifted up.