An eccentric muscle action
Numerous studies have have compared concentric only training to eccentric only training.
Most find that the eccentric training groups experiences more growth even when the total
number of repetitions (time under tension) performed by both groups is identical (2,47-51).
There are a number of physiological differences between the performance of concentric and
eccentric muscle actions, summarized below. In general, force capacity during an eccentric
muscle action is approximately 30-40% greater than that during a concentric muscle action (8,9).
That is, if 100 pounds can be lifted by a muscle, typically 130 to 140 pounds can be lowered.
Additionally, Type II muscle fibers (which show the greatest amount of growth) are
preferentially recruited during eccentric actions (8,9). As Type II fibers have a greater force
production capacity than Type I, this may partly explain the greater strength seen during
eccentric training.
During eccentric muscle actions, fewer muscle fibers are recruited (8,9). This means that
the fibers recruited receive more overload per fiber (54) which may explain the preferential
growth seen. Finally, eccentric but not concentric lifting stimulates protein synthesis (9).
If eccentric actions are the primary stimulus for growth, the question arises of why
perform concentric (lifting) muscle actions at all? First and foremost, concentric actions are
responsible for most of the metabolic work during training contributing 84% of the total metabolic
work (49). Additionally, concentric strength limits eccentric strength (55). That is, you can only
lower as much weight as you can lift unless you have partners lift the weight for you, so that it
can be lowered. This implies that periods of concentric only training (to improve concentric
strength capacity) may be useful so that more weight may be used during the eccentric portion of
the lift.
A final observation about eccentric training is that heavy eccentric loading is associated
with most of the muscle soreness from training (56). Twenty-four to thirty-six hours after
training, soreness occurs and is called delayed onset muscle soreness (DOMS). DOMS is thought
to reflect direct mechanical damage (small tears) in the muscle fibers (57). Following eccentric
induced trauma, the muscle undergoes an adaptation to prevent further damage and DOMS from
the same overload (58).
It has also been suggested that tears to the cell membrane allow calcium to flow into the
cell, activating enzymes which break down protein (59,60). Full recovery from this type of
eccentric trauma is completed with 4-7 days suggesting that the same muscle should not be
worked any more frequently than that, at least not with heavy eccentric contractions.
Another possible mechanism by which eccentric muscle actions may be involved in muscle
growth is through satellite cell proliferation (61). Satellite cells are a type of cell located on the
surface of muscle fibers involved in muscle cell regrowth.
In response to both hormonal and mechanical stimuli (such as muscle damage), satellite
cells become active to help with tissue repair. In animal models, satellite cell activity is involved
in muscle hyperplasia (generation of new muscle fibers). Although hyperplasia does not appear
to play a role in human growth (14), heavy eccentric muscle actions may have the capacity to
stimulate satellite cell proliferation by damaging the cell and causing a local release of insulin-like
growth factor 1 (IGF-1) (61-63).