The Peripheral Heart Action (PHA) system alternates upper- and lower-body exercises to keep the blood circulating between the extremities, which also maintains an elevated heart rate. NASM notes that this method is ideal for clients with limited time who want variety, as it combines resistance and cardiovascular benefits.
The horizontal loading system trains all sets of one exercise before moving to the next, which is less time- efficient. The multiple-set system increases volume but doesn't necessarily save time. The superset system alternates two exercises with little rest, but doesn't inherently alternate upper and lower body. Thus, the PHA system is the most suitable for a time-limited client seeking variety.