What's the relationship between VO2 max and slow-twitch muscles?

Factual Questions
1

World-class marathoners and elite high-altitude mountain climbers are reknowned for their high VO2 max. What is the relationship between V02 max and slow-twitch muscles? Don’t those who perform at world-class levels in the endurance events usually have both a high VO2 max and a high level (85%+) of slow-twitch muscles?

Below are excerpts from an interesting article I ran across. Thought you might enjoy it too. Thanks for your assistance in advance–and have a great weekend.
http://cbshealthwatch.medscape.com/cx/viewarticle/230791_print

“No matter what you do, for instance, your “VO2 max,” or maximum aerobic capacity–the maximum rate at which the heart, lungs, and muscles can burn oxygen to make energy–declines with age. If you don’t do regular aerobic exercise, it falls 10% per decade after age 25, several studies have shown. If you do, it declines at half that rate.”

“People who don’t keep their muscles strong by lifting weights lose about one-third of a pound of muscle per year, says Miriam Nelson, an exercise physiologist at Tufts University. Researchers are still trying to figure out how much of this loss can be offset by regular weight lifting.”

“And there does appear to be a biological wall: We are born with all the muscle fibers we’re ever going to have. Muscles can get bigger with use, even among people 100 and older, studies since 1990 have shown. But the number of fibers doesn’t increase.“

“There is immutable decline” in part because of a decrease in the number of motor nerves that activate muscles, says John Faulkner, a muscle physiologist at the University of Michigan who, at age 76, still runs 4-6 miles a day.
In fact, it’s atrophy of muscles that is the biggest factor in the decline of athletic performance with age. In other words, inactivity, more than age itself, is the great enemy of fitness and health."

“One Dallas study showed that college students forced to stay in bed for 3 weeks wound up with changes in fitness and muscle mass comparable to 20 years of aging. Scientists are also finding that some types of muscles age faster than others. Fast-twitch muscles, which are light colored, are the type used for power (speed). These fatigue more quickly and decline more rapidly with age.”

“Slow-twitch fibers, however, which look red because they are rich in myoglobin, are for endurance, and they seem to decline more slowly with aging. This may explain why older athletes can’t run or swim as fast as young ones but can do quite respectably in endurance events like ultramarathons.”

2

I’m not a physiologist, but I play one on TV…

I think that the answer to your second question is yes, and to your first question the relationship is a bit more complicated than I understand at the moment, but all muscle contributes to the use of oxygen, and apparently slow twitch(aka tonic) fibers do so disproportionately:

from An Ohio University lecture

This is another page about muscle fiber types you may find useful

3

You’ve overlooked, altho your links have shown, that there are more than two types of twitch muscles. Type II (fast twitch) has two types and according to Wevets’ first link, there are three types of slow twitch (Type I):

I’ve been under the impression that although fast twitch cannot become slow twitch and vice versa, you can “teach” slow twitch to become faster by developing them by exhausting the fast twitch. That is, endurance running will help you become faster because when the fast twitch muscles are depleted, the slow twitch, which have switched over to a faster version, take over.

As far as muscle atrophy accounting for the slow-down with age, I don’t buy it. I’ve been working out with weights almost as long as I’ve been running, almost 30 years. I’m stronger than I’ve ever been, but slower. I read a study, but I can’t find it now (I had copied it but I think I accidentally deleted it) that the reason one slows down with age is the confounding of the twitch muscles. In other words, fast twitch become more like slow twitch. I’ll try to find the article.

4

While apparently the fibers don’t change, the proportion of muscle characterized by each type can change:

From Schmidt-Nielsen, Knut. 1990. Animal Physiology. Cambridge U. Press, p.411.

I have no idea about the aging issue.

5

From * Scientific American * September 2000: We have two different types of fast twitch, Types 2a and 2x. (Slow twitch fibers are known as Type 1.) These are determined by a component of the myosin molecules in a muscle known as the “heavy chain.” The difference is in the way they break down ATP (adenonine triphosphate) to derive the energy needed for contraction. Slow fibers rely more on aerobic metabolism, whereas fast fibers depend more on anaerobic metabolism. Thus, slow fibers are important for endurance activities, whereas fast fibers are the key to sprinting.

As people age, they lose muscle fibers, and never gain new ones. Individual fibers, however, can become thicker by additional myofibrils (a constituent of a fiber involved in contraction).

The mechanical stresses that exercise exerts on tendons and other structures connected to the muscle trigger signaling proteins that activate genes that cause the muscle fibers to make more contractable proteins, chiefly myosin and actin, which are part of a myofibril.

Muscle fibers have multiple nuclei, but the nuclei cannot divide. New nuclei are donated by stem cells, which have one nucleus each, but can replicate by dividing. They proliferate in response to the wear and tear of exercise. With these additional nuclei, the fiber is able to create more myofibrils, and with the increased production of proteins, the muscle becomes thicker.

Exercising the muscle fibers converts Type 2x into Type 2a. In a study, when exercise was stopped for three months, the amount of 2x was not only regained, but also doubled. Thus, sprinters should provide for a tapering period leading up to a major competition, which is what they do without understanding the physiology. Recent experiments indicate that a rigorous exercise regime could convert slow fibers to fast 2a fibers. It is not known if 2a can be converted to 1, but if it can, the time required is quite long. Elite endurance athletes generally have a remarkably high proportion - up to 95% - of type 1 in their major muscle groups, such as the legs, indicating that they can.

Since new muscle fibers are constantly being lost (all cells die) and cannot be replaced, by age 50, muscle mass is often reduced by 10%, and by age 80 by 50%. By thickening the individual fibers, weight lifting can stave off the loss of mass from the muscle as a whole.

Before fibers die, they change shape and appearance, becoming more rounded. In addition, in younger people, the fast and slow fibers are distributed in a checker fashion, whereas in aged muscle the fibers cluster in groups of either slow or fast cells. It appears that as we age, some of our motor nerves die and the nerves’ muscle fibers also would atrophy and die, unless they are reinnervated by another motor nerve, which could be a motor nerve from another type of muscle fiber. Thus, there occurs an obfuscation of the border between slow and fast fibers, so that increasingly as we age more and more fibers are neither strictly slow nor fast, but somewhere in between.

The link previously noted says there are 3 types of fast twitch and 2 slow twitch, while Scienfic American says 2 and 1. I suppose it has to do with nomenclature since I did not read the actual material in the outline.