Activin A, activin B, follistatin, and FSTL3 were measured in (1) 80 subjects divided according to age (young vs old) and fitness status (active vs sedentary) before and after exercise at 70% maximal oxygen consumption (VO2max), followed by 90% of VO2max until exhaustion; and (2) 23 subjects [9 healthy and 14 with metabolic syndrome (MetS)] who completed four sessions: no exercise, high-intensity interval exercise, continuous moderate-intensity exercise, and resistance exercise for up to 45 minutes.