Answer to Question 1
You should be careful when selecting apps because many people are designing software applications with limited or no knowledge in the related field (e.g., health and fitness). When it comes to fitness, one size doesn't fit all, because fitness levels and needs vary widely among participants
If you are looking for apps, search for those designed by health and fitness experts who hold degrees and certifications in the related field. A good place to start is the government's app store (apps.usa.gov) that lists apps designed by experts.
As you search, take into consideration your desired activities and outcome, current fitness level, personal budget, and whether you wish to use a device other than your smart phone (e.g., wrist band, chest strap, clip on, arm band).
Answer to Question 2
Cardiorespiratory endurance, cardiorespiratory fitness, or aerobic capacity is determined by the maximal amount of oxygen the human body is able to utilize (the oxygen uptake) per minute of physical activity (VO2max). This value can be expressed in liters per minute (L/min) or milliliters per kilogram per minute (mL/kg/min). The relative value in mL/kg/min is used most often because it considers total body mass (weight) in kilograms.
VO2max is affected by genetics, training, gender, age, and body composition. Although aerobic training can help people attain good or excellent cardiorespiratory fitness, only those with a strong genetic component are able to reach an elite level of aerobic capacity (60 to 80 mL/kg/min). Further, VO2max is 15-30 percent higher in men. This is related to a greater hemoglobin content, lower body fat, and larger heart size in men (a larger heart pumps more blood, and thus produces a greater stroke volume). VO2max also decreases by about 1 percent per year starting at age 25 . This decrease, however, is only 0.5 percent per year in physically active individuals.