To clarify the factors effecting aerobic (VO2max) and anaerobic (Pmax) power output in humans, we analyze the cross-sectional relationship between VO2max, load at VO2max, Pmax, load (P-load) or speed (P-speed) at Pmax, and lean body mass (LBM) in triathlon (TA) and American football (AFB) athletes, and control subjects (CON) during maximal cycling. VO2max and Pmax were greater in TA and AFB than in CON, and a significant correlation between VO2max and Pmax was found in CON, but not in TA and AFB. LBM was significantly correlated with Pmax or P-Load in all groups, and also correlated with VO2max in TA and AFB but not in CON. In AFB, a significant correlation between P-speed and Pmax was found. These results suggest that 1) increased LBM enhances maximal anaerobic and anaerobic power in trained athletes, 2) in untrained subjects, aerobic power is related with anaerobic power but not with LBM, 3) increased LBM is related with muscular power or speed in high power athletes. Thus, different factors affect aerobic and anaerobic power output between AFB, TA and CON.