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Table 1 Semiquantitative evaluation of the fast to slow twitch transition

From: Abnormal expression of p27kip1 protein in levator ani muscle of aging women with pelvic floor disorders – a relationship to the cellular differentiation and degeneration

Stage of muscle fiber Fiber Type II Type I Cytoplasmic Fiber size
(myotube) differentiation Typea stainingb staining p27  
1 - resting fast (young) fast +++ - - normal
2 - transitory 1 trans 1 +++ ++ - normal or increased
3 - transitory 2 trans 2 ++ +++ + normal or increased
4 - transitory 3 trans 3 - +++ ++ normal or increased
5 - resting slow (mature) slow - +++ - normal
6 - early deg (aging) deg 1–2 - +++ ++ normal
7 - late deg (regressing) deg 3–4 - +++ +++ fragmented
  1. a fast = fast twitch fibers; trans 1 = early, trans 2 = mid, trans 3 = late transition of fast- to slow twitch fibers; slow = slow twitch fibers; deg 1–2 = early, deg 3–4 = advanced degeneration (shrinking and fragmentation; see Fig. 3, D-F for examples, and Fig. 6 for staging). b Semiquantitative grading: -, none; +, weak; ++, moderate; +++, strong (see Fig. 3 to 6 for corresponding densities).