Tension
For a fiber to adapt, it must be used during an activity (24). Recall from chapter 17 that
muscle fiber recruitment is primarily determined by the load which must be lifted. The minimum
tension considered to stimulate growth and strength gains is roughly 60% of 1RM. Recall also
that muscle fibers continue to be recruited up to about 80-85% of 1RM at which time further
force production occurs through rate coding. Therefore optimal Type II muscle fiber involvement
will occur with loads between 60-85% of 1RM (approximately 6-20 reps). However, simply
recruiting a fiber is not sufficient to make it adapt.
Metabolic work
Once a muscle fiber is recruited, it must do more work than normal for adaptations to
occur (40). Recent research has found that muscle growth is greater with longer sets and that
the metabolic changes (increased blood metabolites such as lactic acid, phosphate, etc) seen with
longer set times may be part of the growth stimulus (42-44). It has also been suggested that
increased levels of lactic acid may play a role in the growth stimulus possibly explaining why sets
of 20-60 seconds (in the anaerobic glycolysis range) seem to give better growth than shorter sets
(45).
Tension plus metabolic work: the time under tension hypothesis
Factors 1 and 2 combined make up the time under tension (TUT) hypothesis of growth
(39,46). TUT simply says that fibers must develop sufficient tension for a sufficient time period
to adapt. While the exact amount of time necessary to stimulate growth is not known and will
most likely vary from muscle fiber to muscle fiber, it has been suggested that set times between
20-60 seconds (corresponding with anaerobic glycolysis) be used for one or more sets (23,24,46).
Even within the context of high tension for sufficient time, growth is not guaranteed. We also
need to consider how the time under tension is spent. Although muscles only contract, depending
on the relationship between the force generated, and the load which must be lifted, one of three
types of muscle actions can occur.
The first is referred to as a concentric muscle action, where the muscle shortens while
contracting, lifting the weight. The second is isometric muscle action, where the muscle does not
change length while contracting, and the weight does not move. The third is eccentric muscle
action, where the muscle lengthens while contracting, and the weight is lowered.
Performing 40 seconds of pure concentric work is not the same as performing 40 seconds
of isometric work is not the same as performing 40 seconds of pure eccentric work. The third
part of the growth stimulus is thought to be the eccentric muscle action, which has different
characteristics than concentric or isometric actions.