Some Info on Muscle fiber conversion
The topic of fiber type conversion is complex and not fully explored in all details. The conversion of IIb fibers is possible through endurance training; they can fully disappear. The transformation is a result of both functional adaption and morphological changes (s. Hoppeler, 1992).
Strength training can result in fiber conversion. Staron et. al (1991) and Adams et al. (1993) found that a 7 week training program for cross section enlargement resulted in a considerable reduction of IIb count and an increase in IIa count. After 13-20 weeks of hypertrophy training no IIb fibers could be found. The percentage of IIb fibers decreased from 16% to 0.9%. Another study by Staron et al. (1994) showed transformation of fiber types from IIb to IIa after 2 weeks of training in female participants. In male subjects, a significant decrease in IIb fibers was observed after 4 weeks of training. After 8 weeks, a reduction of IIb countfrom 21% to 7% was observed.
The study by Staron et al. (1991) also demonstrated that short detraining periods resulted in reconversion of fiber types. In addition, speed strength training can reverse the transformation as well.
Transformation of fiber types appear to be related only to changes within a fiber type (see reviews by Kraemer et al. 1996). Conversion of type II fibers to type I is doubtful (Kraemer et al. 1995). Exercise induced morphological changes of slow twitch fibers into IIb fibers couldn't be verified; However through appropriate training the speed of muscular contraction can be increased as a whole.
Submitted by Nizar Abu-Hamdeh, May 13, 2006
References:
Adams, G.R., et al.: Skeletal muscle myosin heavy chain composition and resistance training. 1993. Journal of Applied Physiology. 74: 911-915.
Hoppeler, H.: Strukturelle und funktionelle Grundlagen der Schnelligkeit. Vortrag >Trainerseminar Schnelligkeit <. 3.-5.4. Stuttgart 1992
Kraemer, W.J., Flexk, S.J., and Evans, W.J. 1996. Strength and Power Training: Physiological mechanisms of adaption. In: Exercise and sport sciences reviews, edited by J.O. Holoszy, 363-398. Baltimore: Williams & Wilkins.
Kraemer, W.J, Patton, J., Gordon, S.E., Harman, E.A., Deschenes, M.R., Reynolds, K., Newton, R.U., Triplett, N.T., and Dzaidos, J.E. 1995. Compatability of high intensity strength and endurance training on hormonal and skeletal muscle adaptions. Journal of Applied Physiology 78: 976-989.
Staron, R.S., Leonardi, M.J., Karapondo, D.L., Malicky, E.S, Falkel, J.E., Hagerman, F.C., and Hikida, R.S. 1991. Strength and skeletal muscle adaptions in heavy resistance-trained women after detraining and retraining. European Journal of Applied Physiology 70: 631-40.
Staron, R.S., Karapondo, D.L., Kraemer, W.J., Fry, A.C., Gordon, S.E., Falkel, J.E., Hagerman, F.C., and Hikidi, R.S. 1994. Skeletal muscle adaption during the early phase of heavy resistance training in men and women. Journal of Applied Physiology 76: 1247-1255.