2.1.2. The Cardiovascular System

It is obvious that a precise interplay between pulmonary ventilation, DO2 and the extraction of oxygen by the muscle tissue is a prerequisite for properly matching oxygen need and demand. The contribution of the cardiovascular system, in particular Qmax, is considered as the main determinant of VO2max in young people when including large skeletal muscle groups, e.g., leg cycling, running or cross-country skiing [18,19]. The demand of oxygen by these working muscles increases tremendously (about 10 to 20-fold, depending on the individual fitness) from rest to maximal work [10]. Consequently, a large amount of oxygenated blood has to be transported to muscles driven by the pumping heart. Thus, a 72% contribution of Qmax to VO2max changes has been demonstrated in sedentary young and older subjects of both sexes, and the Qmax increased to 81–89% in trained young and older subjects of both sexes [18]. The age-related decrease in cardiovascular function (particularly decline of Q which is HR x SV) will considerably affect VO2max. HRmax decreases according to the formula (208 (beats per min)—0.7 × age) in healthy sedentary and trained subjects, probably due to the decrease in intrinsic heart rate [20]. In addition, lower SV associated with reduced left ventricular (LV) compliance (diminished diastolic function) was shown in healthy sedentary people but seems to be preserved in masters athletes [21]. Impaired ability to modulate sympathetic vasoconstrictor activity (functional sympatholysis) and a reduced exercise hyperemia are also characteristics of aging. Again, regular PA was shown to offset these impairments [22].

## 2.1.3. Skeletal Muscle and Mitochondria

Capillarization of skeletal muscles and muscle oxidative capacity decrease likewise with aging at least in rather sedentary subjects [23,24]. However, long-term engaging in endurance sports can largely prevent the reduction in muscle capillarization and muscle oxidative enzymatic activity with aging, e.g., in 65- to 75-year-old athletes [25]. The oxidative capacity of skeletal muscles is not considered as an important limitation of VO2max because it exceeds the amount of oxygen consumed during whole body exercise [26]. Thus,

benefits of regular training on adaptations in the skeletal muscle rather promote improved submaximal exercise performance than VO2max. Such adaptations include increases in capillary supply and mitochondrial key enzyme activities favoring a higher rate of fat oxidation and a concomitant reduction in the glycolytic flux, as well as a tighter control of the acid-base status [10]. Consequently, the anaerobic threshold (submaximal endurance performance) declines at a slower rate with aging (especially in trained individuals) than VO2max [27,28]. A noteworthy observation is the inverse relationship between mechanical efficiency and VO2max (shown in elite cyclers), which was attributed to variations in the amount of efficient type I and less efficient type II fibers of working muscles [29].
