משהו ששמעתי על השריר סיבים ארוכים וקצרים

[kapulsky]

you have three types of muscle fibers,
lets call them 1.type A 2. type B and 3. type BII
now type A is more endurance ie. high reps, running, etc
the lower the reps are the more type B and BII you will use.
you shuld also know that type A has very little growth potential.
i know it very briff but that more than enough on that for basic understanding