Eccentrics, And
Their Connection To mTOR
Activation of the
mTOR pathway is the switch that triggers protein synthesis, and as far as
mechanical tension is concerned, this switch is activated almost exclusively
during muscle lengthening (eccentric) actions.
It was once thought
that because eccentric loading inflicts the greatest micro-tearing to the
muscles, muscle damage must be the main stimulus for growth, but this isn’t the
case. In fact, excessive micro-trauma to the muscle fibers might even retard the
growth process (partly because eccentrics reduce muscle insulin sensitivity).
It’s the cell
signaling, via several pathways (ex. EPK and PKB pathways), as well as the
activation of mTOR and the production of local growth factors (like IGF-1) in response
to eccentrics which is the main stimulus for muscle growth. The micro-tears to
the muscle are just a consequence of training, and are not necessary for growth
to occur.
The external
resistance needed to optimally activate mTOR is only 60% of maximum (a weight
allows for roughly 20 reps to be performed), as it's the act of stretching the
muscle under load which is responsible for the effect (and this effect is
further magnified if the muscle being stretched is ‘pumped’, because performing
a loaded stretch on a pumped muscle stretches the surrounding fascia, and
increases the sensitivity of the IGF-1 receptors). Eccentrics performed in this
manner, with such loads, aren't damaging and won't impair recovery, but will
activate one of the main pathways involved in muscle growth.
Along with the cell
signaling effect that low load, slow eccentrics have, is the occlusion effect (depriving
the muscle of oxygen while it's doing mechanical work) that they can create, as
long as constant tension is maintained – when a muscle is contracted, blood
can't enter the muscle so oxygen isn't delivered, and if a muscle never relaxes
during a set, blood will remain outside the muscle, and the muscle will go into
an hypoxic state, increases the release of IGF-1. All of this positively affects
both protein synthesis, and muscle growth.