I just finished Morris’s Performance cycling and while I thought it was kind of unimpressive, it did get me to thinking about the use of heart rate as a training tool.

First, if we accept that by just riding moderately we train ourselves to within 5% of our VO2max and a few intervals here and there will get us to nearly 100% of our genetic ceiling, it seems that oxygen delivery is taken care of. That means that the training adaptations we are working so hard to get are primarily muscle-oriented. (i.e. related to oxygen use.)

As we all know, heart rate isn’t a great indicator of muscular work—this is why weightlifters don’t use it. It seems likely that a precise combination of resistance (wattage), duration, and frequency--irrespective of heart rate—would produce the most gains.

Some observations/questions:

1: If your heart worked as a simple mechanical pump, it would be easy to predict VO2max based solely on RHR and MaxHR, right? Essentially the size of your pump multiplied by its max RPMs (I’m assuming similar blood chemistry and the fact that most research suggests that lung volume isn’t a limiter.) I have not observed this to be true in people I’ve ridden with.

2: The heart is affected by many things beyond the demand put on it. At a constant power output on a Computrainer, mine varies almost 10bpm between watching a Discovery Channel tape and listening to Ministry. Where does all that extra cardiac output go?

3: Further, if I do two equally hard days in a row, my HR will be depressed during the second session. How am I creating the same power output at a lower HR? Is this really a sign of overtraining? I can do the work. Wouldn’t it be funny if we discovered this depression of the heart rate is a desirable attempt by the body to use the heart somehow more efficiently?

4: If you ride at a constant power output, your heart rate rises due to cardiac drift, forcing you to reduce power output to stay within your zone. This, however, affects the resistance level you’re training your muscles at. Would you be better off just holding the ‘optimal’ power output as long as possible instead of easing off? And why does drift occur? Increasing thermoregulatory demands? It seems to me that giving a narrow range of power output for a LSD ride is silly. Sure, 200w is easy for an hour, but kind of a grind for five.

5: Are slow twitch muscles trained by short intervals, or do our fast twitch motor units burn out and send us home before the slower ones even get warmed up?

I’d be interested to hear people’s thoughts, or if you could point me to any links/books on the subject, I’d appreciate it.

I was disappointed in the Morris book as well.

1-4. This is in part why state-of-the-art training seems to be moving toward power based training and away from HR based training. HR while roughly indicative of how much work you're doing isn't perfect and is highly variable, slow to respond to changes in power output, etc., power on the other hand is power and that is what matters.

5: Are slow twitch muscles trained by short intervals, or do our fast twitch motor units burn out and send us home before the slower ones even get warmed up?

Look up the Henneman Size Principle. You can't recruit your big (fast-twitch) MUs without first recruiting your small (slow-twitch) MUs. So if your riding along and increase you power output to do an interval you will have recruited additional large fast-twitch MUs most likely to give that extra power. I would guess that if you start fatiguing and can no longer produce a given power it would be most likely the large, fast-twitch motor units letting you down.

Where are you from in Wyoming, my wife is from Laramie?

I find the motor recruitment issue really interesting and felt like Morris hadn't completely thought it through.

In hard intervals, as you say, slow twitch units will be fully recruited and we're using fast twitch units to provide the additional power. These fast fibers get cycled out pretty quickly because they're quite prone to fatigue. So our inability to continue at a given power output is based on our fast twitch units becoming unavailable. At this point, though, have we stressed our slow twitch units enough to create a training response? And were the high lactic acid levels maintained counterproductive to the development of mitochondria?

On the other hand, LSD riding would tax our slow twitch muscles, eventually causing units to drop out and be replaced with fast twitch units which would get a good 'aerobic' workout at low lactic acid levels. Dunno....

I have to admit to not being an honest-to-God Wyoming native. Just one of many Jackson Hole transplants. After more than a decade here, though, I'm starting to think I need to go out and buy a cow...

I find the motor recruitment issue really interesting and felt like Morris hadn't completely thought it through.

In hard intervals, as you say, slow twitch units will be fully recruited and we're using fast twitch units to provide the additional power. These fast fibers get cycled out pretty quickly because they're quite prone to fatigue. So our inability to continue at a given power output is based on our fast twitch units becoming unavailable. At this point, though, have we stressed our slow twitch units enough to create a training response? And were the high lactic acid levels maintained counterproductive to the development of mitochondria?


They stressed too. But slow fibers require more work to make them better because they are more fatigue resistant.


On the other hand, LSD riding would tax our slow twitch muscles, eventually causing units to drop out and be replaced with fast twitch units which would get a good 'aerobic' workout at low lactic acid levels. Dunno....

I have to admit to not being an honest-to-God Wyoming native. Just one of many Jackson Hole transplants. After more than a decade here, though, I'm starting to think I need to go out and buy a cow...

It depends on LSD pace. If pace is too slow you can ride for long time without fatigue.
That's because slow fibers are trained well at aerobic threshold. This is a intensity level with all slow fibers recruited.