Muscle Fibers???

[**Middleweight**]

Can someone explain this?? I know when doing more reps you work different fibers as opposed to lower reps heavier weights.
I was reading a whole thread on this somewhere. All help is much appreciated!

[BigBad]

The easiest way to describe this is that higher reps with less resistance work more of your Type I (slow twitch) muscle fibers, while lower reps with heavier weight work your Type II (fast twitch) fibers.

The fast-twitch motor unit (Type II) develops force rapidly, and also relaxes rapidly, and hence has a short twitch time. In contrast, slow-twitch motor units (Type I) develop force & relax slowely, and have a long twitch time.

Type I fibers are generally fatigue resistant & have a high capacity to supply aerobic energy. Yet, these have limited potential for rapid development of force, as characterized by low actomyosin myofibrillar ATPase activity & low anaerobic power.

Type II fibers fatigue very easily, have low aerobic power, rapid force development, high actomyosin myofibrillar ATPase activity, and very high anaerobic power. Type II fibers are also broken down into Type IIa and Type IIb fibers.

Type IIa fibers have more capacity for aerobic metabolism and more capillaries surrounding them than Type IIb and therefore show more resistance to fatigue.

Hope this helped and wasn't too confusing. -BB

[**Middleweight**]

Wow!! Thanks for the info! As far as the outside appearance, how does the working the type 1 help you out? More striations?

[sin]

i think the physical appearance of the fibers is the same, except that the type 1 fibers are red due to the red blood cells, and the type 2 fibers are white, since they are anerobic. i just read a research paper that showed that it was possible to convert white fibers to red and vice versa. This puts a little dent in the idea of a genetic predisposition to be a runner, or weight lifter etc.