单选题 .  The fitness movement that began in the late 1960s and early 1970s centered around aerobic exercise. Millions of individuals became     21    in a variety of aerobic activities, and     22    thousands of health spas     23    around the country to capitalize on his     24    interest in fitness, particularly aerobic dancing for females. A number of fitness spas existed     25    to this aerobic fitness movement, even a national chain with spas in most major cities. However, their     26    was not on aerobics,     27    on weight-training programs designed to develop muscular mass,     28    , and endurance in their primarily male     29    . These fitness spas did not seem to benefit     30    from the aerobic fitness movement to better health, since medical opinion suggested that weight-training programs     31    few, if     32    , health benefits. In recent years, however, weight training has again become increasingly     33    for males and for females. Many     34    programs focus not only on developing muscular strength and endurance but on aerobic fitness as well.     35    , most physical-fitness tests have usually included measures of muscular strength and endurance, not for health-related reasons, but primarily     36    such fitness components have been related to     37    in athletics.     38    , in recent years, evidence has shown that training programs designed primarily to improve muscular strength and endurance might also offer some health     39    as well. The American College of Sports Medicine now     40    that weight training be part of a total fitness program for healthy Americans.21.