There are certain immutable truths concerning the performance of the human body as it ages, particularly as the athlete reaches age 40. The physical peak for most humans, in most sports, is between 25 and 35 years of age; during this peak period, the well-conditioned athlete can create a confluence of muscular strength, peak cardiovascular and oxygen transport, speed and reaction time, and mental capabilities (including the ability to deal with competitive pressures), all bound together by a desire to succeed.
The heart, as with every other human muscle, will gradually lose efficiency and power over time. A typical rule-of-thumb calculation to confirm the physiological fact of reduced heart capacity is that utilized by the American College of Sport Medicine: in males, 220-age = maximum heart rate (beats/minute); in females, 226-age = maximum heart rate.