Interestingly, at birth most muscles are composed
of slow (type I) muscles and only as the body matures does the
final proportion of fast and slow muscles emerge. It appears very
likely that the innervation determines fibre type. Although there
may be a trophic effect of the nerve itself (perhaps a substance
or antigenic quality), there appears to be a strong influence
of firing pattern in determining many of the characteristic properties
of muscle.
In the laboratory, if a slow muscle (whose motor
nerve generally fires at a low frequency over a long period of
time) is stimulated with periodic high frequency bursts of
activity (more akin to the activity seen in fast fibres)
the muscle begins to show many of the properties of a fast fibre.
Moreover, if a slow fibre is denervated and subsequently re-innervated
by a fast motor (innervating fast fibres) the fibre will
go on to express all the characteristics of fast fibres.
