Land animals require more energy since they have to overcome greater inertia during locomotion. Land mammals have adopted many evolutionary mechanisms to support high energy demand.
Heart is four chambered instead of the usual two in aquatic animals like fishes. More number of divisions in heart increases the efficiency of blood circulation process which in turn increases the oxygen delivery rate to cells for producing more energy.
Land mammals also have developed lungs which increase the efficiency of gas exchange process which increases the output of respiration process.
In addition, land mammals also have a strong and developed musculoskeletal system which helps in locomotion.
The locomotion on land requires more energy than the locomotion in water which can be observed in the land animals like mammals.
In mammals, to move on the land, there should be a well-developed organ which could help in the circulation of blood to all the tissues and the mammals developed the well developed the four-chambered heart which required energy to pump the blood.
The mammals evolved the well-developed lungs to exchange the gases with the environment.
I would assume that because of not only sports but because of what americans do, like eat alot or smoke etc. it can effect organ systems because the heart has veins leading to every other organ, when the heart stops, the organs stops too.
