Previous epidemiologic and clinical evidence indicate that a poor cardiorespiratory fitness is a major risk factor for life-style related diseases (LSRD) such as obesity, hypertension, hypercholesterolaemia, arteriosclerosis and diabetes [1–4]. Moreover, low cardiorespiratory fitness has been found to be a predictor of cardiovascular disease (CVD) mortality, and all-cause mortality [5–8]. Thus, it is essential to maintain a high level of cardiorespiratory fitness to prevent LSRD.
Cardiovascular fitness is usually evaluated as the maximal oxygen uptake per body mass (V o2max, mL·kg-1·min-1). The Japanese Ministry of Health Labour and Welfare in 2006 proposed V o2max reference values for each age group to prevent LSRD . These V o2max reference values were determined by the "Committee for the Determination of the Recommended Exercise Allowance and Exercise Guide" established in August 2005, and were referenced in the "Exercise and Physical Activity Reference Quantity for Health Promotion 2006 (EPAR2006)". Originally, the "Recommended Quantity of Exercise for Health Promotion (1989)" had been formulated to mainly target the prevention of coronary artery disease. With the passage of more than 15 years following the establishment of this standard, the morbidity pattern of people has worsened and LSRD have increased in prevalence. In order to face this situation, the EPAR2006 was made based on the latest scientific evidence, and was designed to maintain and promote the health of people and prevent LSRD by improving their capacity for physical activity and exercise. These V o2max reference values proposed in the EPAR2006 were determined by experts through the systematic review of literature regarding the relationship between V o2max and LSRD such as obesity, hypertension, hypercholesterolemia, diabetes, cerebrovascular disease, CVD mortality and all-cause mortality.
It is well known that V o2max decreases with age [10–20]. It has been suggested that the age-related decline in V o2max is a consequence of attenuation of central and peripheral functions such as stroke volume, heart rate max (HRmax), peripheral O2 extraction, and lean body mass (LBM) or muscle mass [19, 21–25]. Among these determinants, reductions in HRmax and LBM or muscle mass have been suggested to be primary factors [26, 27]. While many studies on cardiovascular fitness have focused on cardiac measurements, it should be emphasized that muscle mass is one of the critical determinants of V o2max [13, 14, 19, 24, 26, 28–30] since the amount of tissue available to extract oxygen during maximal exercise, i.e., muscle, can directly contribute to the value of V o2max. For example, Sanada et al. reported the MRI-measured lower body skeletal muscle mass was closely associated to the absolute V o2max during running [28, 30]. Additionally, the age-related decrement in V o2max can be related to the age-associated muscle loss [24, 19]. Further, it is important to notice that LBM or muscle mass can be maintained to some degree by exercise training, while such training cannot prevent age-related declines in HRmax, [26, 27].
Therefore, we hypothesized that a certain level of muscle mass required to maintain sufficient cardiovascular fitness is present and that it could be a limiting factor of age-related V o2max attenuation. Based on this hypothesis, it is advantageous to Japanese women's health to propose such muscle mass required to maintain sufficient V o2max. Thus, the purpose of this study was to determine a required value of muscle mass to maintain the V o2max reference value determined by the Japanese Ministry of Health Labour and Welfare in 2006 (Ministry of Health, Labour and Welfare of Japan 2006).