Relation between motor units and force

[Guest guest]

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