Designed as a pair, the FIRM Strength and FIRM Cardio cross train by varying the sequencing of exercises: FIRM Strength shapes muscle by first working the upper body, then the entire lower body, and FIRM Cardio burns fat using PHA (Peripheral Heart Action) training to work the upper and lower body alternately. PHA sequencing also strengthens the heart by repeatedly making it pump blood back and forth between distant muscle groups. When used on alternate days they sculpt your body by preventing muscles from adapting to predictable exercise sequences, and maximize aerobic endurance and fat burning.