Several studies have shown the conversion of type II fibers to type I.
Under conditions of chronic electrical stimulation the Extensor Digitorum Longus in rats (which is primarily fast twitch) becomes almost entirely slow twitch, indicating a considerable conversion of type II to type I.
Also, hyperthyroidism in rats can convert the almost entirely slow twitch soleus muscle in rats to primarily fast twitch. If we immobilize a limb we can also observe a conversion of fast twitch fibers to slow twitch fibers. In fact, there are a host of different treatments (drugs, metabolic perturbations, hormones, activity patterns) that can enduce a large scale muscle fiber type conversion.
The fact that we can shift a muscle from 98% type I to better than 50% type II indicates that some fibers are shifting from type I to type II.
The conversion probably proceeds from type IIb to type IIa to type IIx then type I. In fact, evidence now indicates that the fast twitch contractile proteins (type IIa and probably also IIb) are constitutively expressed (expressed by default in the organism) and that it takes some kind of stimulus to express the type I contractile proteins.
This is supported by the numerous studies on muscle adaptions to disuse. If muscles are not allowed to contract (eg, by severing the nerve or physically immobilizing the limb) the muscles begin to express the fast contractile proteins. The longer the immobilization, the more fast twitch the muscles become.
Dr. Giarnella is correct to the extent that it has been difficult to demonstrate a conversion of type I fibers to type II due to exercise training. The current theory proposes that type IIb fibers are converted to type IIa during strength training and type IIb are converted to IIa and type IIa to type I during endurance training programs.
Years ago it was assumed that muscle fiber type was fairly stable and genetically determined. This has been disproven in recent years by literally hundreds of studies indicating that muscles posses an incredible degree of plasticity.
Mike Prevost, Ph.D.
Although i am a little peeved that he needed someone else to confirm my post… hmm… ;p