The processes of muscle protein synthesis (MPS) and muscle
protein breakdown (MPB) occur concurrently. This constant protein
turnover allows the muscle fiber to change its protein structure if
loading demands or diet changes. The plasticity of skeletal muscle
to respond to altered loading and contractile patterns is evidence
of the capacity for remodeling that a fiber can undergo. It is quite
well documented for example that mitochondrial content increases
with endurance-type work. In contrast, heavier loading leads to
less change in mitochondrial content, but increases contractile
or myofibrillar proteins. All of the aforementioned environmental
adaptations represent a re-patterning of the muscle’s genetic
expression patterns, protein translation, and processes for
breakdown of existing protein structures to ‘insert’ the new proteins.
A persistent muscle protein turnover also provides for a constant
mechanism of protein ‘maintenance’ by removing damaged proteins
and replacing them with new proteins. Damage to proteins can
come about through oxidation or simply mechanical damage due
to high forces during lengthening contractions. Regardless of the
mechanism, the balance between the processes of muscle protein
synthesis (MPS) and muscle protein breakdown (MPB) will determine
the net gain, loss, or no change of proteins in the myofiber. On a
gross level, protein balance determines whether muscle fiber size