
I've been reading parts of Brian J. Sharkey's tome on
Fitness & Health. Now in its fifth edition, dated 2002, it presents the science of fitness and exercise physiology so a lay person can grasp it.
Sharkey tells me things which surprise me about aerobic fitness (he also covers muscular fitness and many other related topics). The main one is that aerobic training does not have all that great an effect on the heart.
There seem to be two schools of thought. The first (the one that Sharkey pooh-poohs) is that
aerobic exercise equals
cardiovascular exercise equals
cardiorespiratory exercise. What is being "trained," according to this school, is primarily the heart, lungs, circulatory system, volume of blood in the body, etc.
Sharkey goes with the second school of thought: what is being "trained" is mainly the specific muscles and muscle fibers used in the training exercise (as leg muscles for running). Also responding to the "training effect" are the nerves and nerve endings which feed those specific muscles.
So when training lowers the heart rate and increases the "stroke volume" for any given level of work effort, what's really happening is that the small nerve endings in the muscle tissue that is being worked learn to
... sense conditions in the muscle and modify the heart rate response to exercise via connections to the cardiac control center in the brain. ... [T]he reduced heart rate can be traced to the improved metabolic condition in trained muscle. When the heart beats more slowly, it has more time to fill, allowing an improved stroke volume. (p. 92)
"Improved metabolic condition" means that the muscle fibers and cells become more proficient at acquiring and using oxygen to burn fat and/or carbohydrate (CHO, from the symbols for carbon-hydrogen-oxygen). In so doing, they produce energy in the chemical form of adenosine triphosphate (ATP).
VO2max is a measure of how much oxygen the muscles are able to use, at the extreme of intensity or effort. This, the maximum volume of oxygen (O
2) the body can use to produce energy aerobically, is measured in ml/kg/min. That's milliliters of O
2, per kilogram of body weight, per minute. VO
2max is also called the "oxygen cost" or "maximal oxygen intake."
VO
2max goes up as aerobic fitness does. But it's really a measure of how much
intensity you are capable of for short periods of 5-15 minutes, not your
endurance for sustained efforts of over 15 minutes (see p. 77). To measure endurance, two thresholds below VO
2max are used.
The
aerobic threshold is the minimum level of exercise intensity, for a particular individual, which can produce significant changes in that person's fitness (see p. 101). It is also called the "minimum training threshold." It can be stated in terms of VO
2, the volume of oxygen being used. It is, of course, less than VO
2max.
The
anaerobic threshold or
lactate threshold is the maximum level of exercise intensity, for a particular individual, that avoids crossing into anaerobic exercise (see p. 102). In anaerobic exercise, oxygen is not used by the muscles in energy production. Energy is instead produced by means of chemical reactions based on glycogen, with lactic acid (lactate) as a byproduct. The anaerobic (lactate) threshold is also called the "minimum training threshold." Like the aerobic threshold, it can be stated in terms of VO
2, the volume of oxygen being used. The anaerobic threshold is greater than the aerobic threshold, but less than VO
2max.
Given that VO
2 and VO
2max are not easy to gauge, heart rate is used instead to define a
training zone for the individual. In theory,
HRmax, a person's maximum heart rate, is attained just when the person's oxygen intake is maximal, at VO
2max. At the anaerobic threshold, which may be 70% of VO
2max
for that person (see Fig. 6.1, p. 102), the heart rate may be 80% of HRmax. At the person's aerobic threshold, VO
2 may be at 55% of the VO
2max maximum and heart rate at 70% of
its maximum, HRmax.
These two thresholds change with training and consequent improved fitness. The more dutifully the person trains within his/her proper zone of exercise intensity — the training zone — the more the zone slides (gradually) upward.
Especially the lower boundary. This boundary, the aerobic threshold, rises with a sustained aerobic exercise regimen. High-fitness individuals get no benefit from the relatively low intensities which
do benefit low-fit individuals, for this reason (see p. 101).
How does this apply to me? Well, today I decided to try to find my aerobic fitness score by timing a run of 1.5 miles (2.4 km) and using Figure 4.4 on p. 84. The good news was that my score was off the chart. The bad news was that it was off the chart in the
wrong direction.
If you can run 1.5 miles in 9:35, your aerobic fitness score, VO
2max, is 55 (measured in ml/kg/min). A time of 11:40 corresponds to a 45 score. You get a 35 score if you run 1.5 miles in 14 minutes.
I took 22:10 to run 1.5 miles.
Actually, to run
and walk the distance.
I thought I was going to need — and would be able — to run faster than my usual jogging pace. So I started out too fast and had to subside into a walk after the first of the three 0.5-mi. circuits, which total 1.5 miles.
Bad strategy.
After I walked for a while, I began to run again, intermittently, with walking interspersed. Toward the end of the third and final lap, I ran as hard as I could for maybe a quarter mile, until I petered out a bit short of the finish line. I wound up with an abysmal time of 22:10.
Afterward, I realized I could have done better if I had jogged at my usual slow pace, using my heart rate monitor to position my intensity of effort in the low end of my accustomed zone at the beginning and then letting it rise gradually toward the high end of the zone as I continued to jog.
That would have been better because, as Sharkey emphasizes, I have been training specific muscle fibers in my legs to work efficiently at specific intensities. To change to a faster pace uses the muscles differently, in ways they're not accustomed to.
I think I could have run 1.5 miles in 20 minutes, that way. I've done it before, albeit five years ago, before I went back to being a couch potato.
20 minutes is still off the chart, though, in the wrong direction.
That means I have to increase my (as yet undeterminable) fitness/VO
2max score quite a bit by continuing to work out aerobically.
Specifically, I have to raise my aerobic threshold and maybe my anaerobic (lactate) threshold.
Which means I need to do work at both ends of my training zone, since, as Sharkey points out, exercise at the low end has different capacity, endurance, and performance benefits than exercise at the high end.
So what is my training zone? One thing I did manage to find out today is that my maximum heart rate, HRmax, is actually 170, not 160. 170 was as high as I could push my HR with what seemed to be my absolutely maximal effort.
So, based on HRmax = 170 beats per minute, here's a table I can use to figure my zone:
% of Hrmax | Rate (bpm) |
| 100 | 170 |
| 95 | 162 |
| 90 | 153 |
| 85 | 145 |
| 80 | 136 |
| 75 | 128 |
| 70 | 119 |
| 65 | 111 |
| 60 | 102 |
| 55 | 94 |
| 50 | 85 |
Sharkey (p. 102) says a person of low fitness (VO
2max under 35 ml/kg/min) should adopt a training zone of 60 to 75 percent of HRmax. Medium fitness (VO
2max between 35 and 45), 70 to 85 percent. High fitness (VO
2max over 45), 75 to 90 percent.
Since my fitness is super-low, maybe I ought to use 102 bpm (60%) to 128 (75%) as my training zone.
Up to now, I've been targeting a high-end heart rate of 136 (80%) and allowing overshoots to go over 140 — as if my fitness were "medium." (I was basing that on an assumed HRmax of 160, not 170).
So I'm going to have to think about dialing down my workouts, or at least some of them.
Aerobic workouts can fall in either of two categories, long-duration (or "long-slow") training or high-intensity training. The former involves staying in the lower part of the training zone for a relatively long time, which at least for me implies walking — not jogging, not running. The latter utilizes the upper portion 0f the training zone for necessarily shorter periods — for me, jogging, or combining jogging with walking, is high-intensity exercise. (Outright running is usually anaerobic for all individuals.)
Long-duration training increases the aerobic threshold. High-intensity training, on the other hand, pushes up the anaerobic (lactate) threshold. (See p. 80.)
Long-slow training "improves the ability of slow oxidative [muscle] fibers to use fat [rather than carbohydrate] as an energy source" (p. 97). That has benefits with respect to the lowering fats (lipids) and "free fatty acids" in the blood, among a number or other benefits. Turning the muscles into more efficient "fat-burning machines," it seems, pays off in many ways.
High-intensity (but still aerobic) training "recruits fast-twitch (fast oxidative glycolytic) [muscle] fibers" (p. 97). That's apparently why the lactate threshold goes up. Boosting the FOG fibers keeps the fast glycolytic (FG) fibers, which operate anaerobically, at bay.
High-intensity aerobic training "may also have more effect on the cardiovascular system" (p. 97).
So it seems to me that I ought to mix, or alternate between, long-slow and high-intensity training modes.