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2. TIP OF THE WEEK

With thanks to Discover magazine, here's a simple way to
determine whether your legs are loaded with fast-twitch or
slow-twitch muscle fibers.

First, why does it matter?

Simply put, if your parents endowed you mainly with
fast-twitch fibers, you'll have your best success when
explosive accelerations are necessary.

Conversely, if your muscle fibers are primarily slow twitch,
you're more suited for events requiring endurance.

So, if (a) you can't outsprint the kid delivering
newspapers, or (b) a century seems as tough as riding across
the country, you may be a "victim" of muscle fiber type, not
insufficient training. In fact, months of speedwork or
endurance rides may not appreciably improve your ability.

Experience may have already tipped you off to your fiber
type. Now here's a test to confirm it or, perhaps, tell you
something you didn't know:

---Stand flat-footed by a wall. Wet your fingertips, reach
as high as you can, and make a damp mark.

---Wet your fingers again. Bend your knees and spring up
while swinging your arm in a high arc. Do it several times
till you're sure that you can't make that second wet mark
any higher.

--Measure the distance between your standing mark and your
jumping mark. Then compare your number with the average for
your age:

MALES
15-19, 18.5 inches
20-29, 19.7 inches
30-39, 16.5 inches
40-49, 13.0 inches
50-59, 11.8 inches
60-69, 10.2 inches

FEMALES
15-19, 13.0 inches
20-29, 11.8 inches
30-39, 11.4 inches
40-49, 9.4 inches
50-59, 7.1 inches
60-69, 4.7 inches

The higher you leap over your age's average, the greater
your percentage of fast-twitch fibers. Leaps below the
average tell you just the opposite -- you have a higher
percentage of slow-twitch fibers.

If you're right on the average, you're balanced. If all wet
marks are in the same place, you have no-twitch muscle
fibers and welcome to the club.

Can the muscle twitch fiber classification change? In highschool I was an above average leaper from a a standing position, now the only way I can get any air is to climb up on the footstool and run off the edge.

Ideally, one should calibrate the jump-and-reach test according to one's height. (I already know I am definitely a slow-twitcher. I have no sprint, but I do well on long climbs, and my one big lifetime athletic achievement was a 12:18 double century.)

In partial answer to the other question, one can indeed train what one has, toward SLIGHTLY faster or slower twitchiness.

Definiate slow twitch muscles 13.5 inches instead of 18.5. I guess thats why i use to be a pretty decent long distance swimmer up untill i quit about 2 months ago. I wonder if the 20-30kms i use to swim a week have anything to do with it. I guess i've always know i was a slow twitcher, i could never sprint fast in the pool or on the track. Sprinting on bike is okay but i can only sprint my fastest for about 1-2 minutes before the latic acid starts to build up, probably has a bit to do with my fitness aswell. But i am good at giving a pretty constant power output over longer distances.

Stew

How can you figure in your body-fat percentage into the eqaution, afte all someone with 9% body fat is going out-jump someone with the same "twitch makeup" as someone with 30% body fat percentage. Right?

Ride the fat away, (I wish)
Pat

Gee, the accepted truth does change with time. Back when my primary exercise was running, it was gospel that sprinters had poor leaping ability.

The only way to conclusively determine whether you have a higher or lower proportion of fast-twitch or slow-twitch muscle fibres is with a muscle biopsy. This is painful and is done infrequently. The "Jump Test" is actuallky about as scientifically sound and accurate as observing whether you are stronger in sprints or in endurance riding or noting physiological appearance. The problem is that the jump test is dependent on a whole range of variables, including body size, weight, level of fitness, etc.

Having said that, your proportion of fibre types is genetically predetermined. You cannot grow more of one or the other. What you can do is train to use each type of muscle fibre more efficiently. In fact, you don't actually train the muscles per se, except in strength training [which trains both], you train the metabolic pathways and systems upon which these muscles depend -- aerobic/endurance training makes your body use its slow-tiwtch muscles more efficiently, while threshold training helps your body use the fast-twitch more efficiently.

In other words... whatever your muscle make-up, you should probably do the same kind of training. You'll just be better at some things than at others.

Agreed. I also think most experienced athletes instinctively know their dominant muscle type.

Velocipideo said it all! You are born with your genetic mix of fibers. You cannot change them, however you can enhance what you have through training.