More synthesis than breakdown indicates an anabolic state that builds lean tissues, more breakdown than synthesis indicates a catabolic state that burns lean tissues.
"[3] Protein turnover is believed to decrease with age in all senescent organisms including humans.
Four weeks of aerobic exercise has been shown to increase skeletal muscle protein turnover in previously unfit individuals.
[4] A diet high in protein increases whole body turnover in endurance athletes.
However, if protein breakdown falls too low then the body would not be able to remove muscle cells that have been damaged during workouts which would in turn prevent the growth of new muscle cells.