Re: Relation between motor units and force

[Guest guest]

, I am an exercise physiologist and hope that maybe I can help

you out some. Unfortunately, the differences between the fiber types

is a little more complex than what you are considering. Remember

contractile surface area is only one of the differences between the

fiber types, not the only difference. Fortunately though, what you

were looking at, in regards to the two texts you read from, is

simpler than you made it. First off it seems you might have slightly

misunderstood the texts some. This is what you said:

" I align what Z & K say ( " The force per unit of fast and slow motor

fibers is similar " ) to S & O's example: " ...if each fiber can produce 5

g of tension. " The statements both imply that the the cross sectional

area of a single FT fiber is similiar to that of a ST fiber; so the

force individual FT and ST fibers exert is similar. "

Neither text really " implied " that a single FT fiber and ST fiber

have a " similar " cross-sectional area. Z & K noted that one of the

biggest differences between the fiber types is cross-sectional area,

and also said that the " force " created by a single unit (myofibril)

is similar between ST and FT (this is where I think you

misunderstood). And all S & O stated was that the larger the cross-

sectional mass the larger the force of contraction.

So using the anatomy of the muscle we know that each individual fiber

contains many myofibrils (the contractile units). So the larger the

fiber the more contractile units. The more contractile units the

larger the force of contraction. FT fibers are known to be

considerably larger then ST fibers, and thus a motor-unit that

stimulates FT fibers may be recruiting a larger cross-sectional area

(more contractile units) then a motor-unit stimulating ST fibers even

if there are more ST fibers being recruited. This is just one of the

reasons for the difference in force production. Many other things

could possibly contribute to the force differences observed, but one

that is fairly well believed to exist is that FT fibers have a higher

capability for electrochemical transmission of action potentials and

a rapid level of calcium release and uptake by the sarcoplasmic

reticulum resulting in contraction and the development of tension at

2-3 times the rate of the ST fibers (in other words, they can

contract faster).

It has been theorized that we are born with a

specific percentage of the various fiber types and that we cannot

change that no matter how we train (I personally believe that

hyperplasia can occur in humans like it does in other animals and

thus I don't completely adhere to this theory). For example, if

someone has say 30% FT and 70% ST fibers throughout all of the

muscles in the body they will most likely not excel at a high-

velocity, high-force producing activity like Olympic-lifting or power-

lifting, but may instead do well at slower longer duration

activities. If only it were this simple though. Fast twitch fibers

and slow-twitch fibers are only a " grouping " of the various kinds of

fibers, and as research continues we continue to find more

differences between the fibers, thus creating more fiber-types within

each class and more complexities when it comes to understanding how

are muscles respond to the various forms of training.

But when it comes to training, as the late great creator of this

yahoo group, Dr. Mel Siff stated, " It is relatively meaningless to

discuss muscle action without considering the role played by the

connective tissues associated with the muscle " (Supertraining p.40).

So in other words you have to research a lot more than just muscle

fiber-types to understand all the dynamics of muscle movement and how

are muscles respond to the various forms of training. I would

suggest you read " Supertraining " by Dr. Siff to give you a better

understanding of this in addition to other texts. I personally like

some books by Tudor Bompa as well as Dr. Kraemer. There are

many great authors though, especially any books by scientists from

the old Soviet Union (not so easy to find though). Hope this sheds a

little light on the subject for you.

Chad Scheitel, MA, CSCS

Minneapolis, MN

>

> I'm hoping that someone can help me make sense of the relation

between motor

> units and force. As I see it, the key distinction is between

individual

> muscle fibers and motor units, comprised of many individual fibers.

>

> First, from Zatsiorsky and Kraemer, *Science and Practice of

Strength

> Training *(2nd Ed) p. 61:

> " The force per unit of fast and slow motor fibers is similar, but

the FT

> motor units typically possess larger cross sections and produce

greater

> force per single motor unit. "

>

> And, from Stone and O', *Weight Training: A Scientific

Approach*, p.10

> :

> " Generally, a larger cross-sectional mass of contractile tissue

will exhibit

> greater contractile force independent of the number of fibers (28).

In

> addition, the number of fibers within a motor unit may vary from as

many as

> 500 fibers to as few as 25 (20). Theoretically, if each fiber can

produce 5

> g of tension, the tension within a single motor unit can vary

from .125 kg

> to 2.5 kg. "

>

> I align what Z & K say ( " The force per unit of fast and slow motor

fibers is

> similar " ) to S & O's example: " ...if each fiber can produce 5 g of

tension. "

> The statements both imply that the the cross sectional area of a

single FT

> fiber is similiar to that of a ST fiber; so the force individual FT

and ST

> fibers exert is similar.

>

> If an individual muscle fiber's cross sectional area of contractile

proteins

> is the primary determinant of the force it can exert, and if the

force per

> unit of ST and FT fibers is similar, then, if " the FT motor units

typically

> possess larger cross sections and produce greater force per single

motor

> unit " (ZK), it would be because a FT motor unit includes more

fibers than a

> ST motor unit.

>

> On the other hand, to say " a larger cross-sectional mass of

contractile

> tissue will exhibit greater contractile force *independent of the

number of

> fibers* " (S & O), implies that different individual fibers are not

similar.

> They differ in cross sectional mass and, consequently, in the force

they can

> generate. Therefore, a motor unit with fewer but larger individual

fibers

> could have an overall cross sectional mass that exceeds the cross

sectional

> mass of a motor unit with a greater number of individual fibers

which,

> individually, have smaller cross sectional areas. If the cross

sectional

> area of individual fibers differ, then the cross sectional mass of

the motor

> unit would be independent of the number of fibers that comprise the

motor

> unit.

>

> Obviously, I'm not an exercise physiologist. My purpose is to come

to a

> much more precise understanding of those factors that have a

practical

> impact on the way I design my workouts. This strikes me as one of

those

> issues. If anyone can " unconfuse " me regarding the relation

between motor

> units and force, I would greately appreciate your comments.

>

> Thanks,

>

> Pitruzzello, Ph.D.

> Chicago, IL

>