utilization of different fiber types (due to differences in physical characteristic) greatly affects the
fuel used, the adaptations seen to training, and the effects of a ketogenic diet. All of the
information which follows regarding exercise types and the necessity of carbohydrates or not is
ultimately tied into the issue of recruitment.
Recruitment and rate coding
The body generates force through one of two different mechanisms. It can either recruit
more fibers (called recruitment) or send more signals so that the fibers contract more strongly
(called rate coding). For large muscles, the body uses recruitment up to roughly 80-85% of
maximal force production at which point all fibers available have been recruited (6). Above this
point, force production is accomplished solely through rate coding. Untrained individuals may not
be able to recruit all of their Type IIb muscle fibers. With regular training, complete recruitment
can be developed (7,8,9).
Muscle fiber recruitment and the Size principle (2)
Fibers recruit from smallest (Type I) to largest (Type IIb) according to the Size Principle.
At low intensities (i.e. slow walking or about 20% of maximal force), only Type I fibers are
recruited. As intensity increases (i.e. jogging), more Type I fibers are recruited until they can no
longer provide sufficient force. At this point, some Type IIa fibers are recruited. As force
production requirements continue to increase towards maximal levels, Type IIb fibers are
recruited. Please note that recruitment is determined by force, not velocity. Near maximal slow
movements will recruit Type IIb fibers as long as force requirement are high enough (see below).
With regards to specific fiber types, Type I fibers are recruited from zero to about 60% of
maximum force production capacities. Around 20% of maximal force, some Type IIa fibers are
recruited and are maximally recruited around 75-80% of maximal force. Type IIb fibers do not
begin to recruit until about 60-65% of maximal force production and continue to be recruited up to
about 85% of max. as stated before. An overview of recruitment of different fiber types appears
in figure 2.
Figure 2: Relationship between force requirements and fiber recruitment% Max Contraction0 20 40 60 80 100
SO
FOG
FG
FG
% of Fibers used
0 20 40 60 80 100