Human athletic potential is a combination of nature vs nurture. Each individuals level of maximal performance has a certain genetic predisposition. That is you can teach a sprinter to run a better marathon, but they will never break the world record in the marathon and vice versa. That is because certain sports have specific physiological requirements for an athlete to succeed in them.
This is not all of the puzzle, may athletes have succeeded in their sport not because they were genetically gifted, but because they had the grit or determination to not give up. The right attitude can take you a long way. Passion, persistence and enthusiasm can take you a long way in this world and in sport.
You combine both genetic potential and the right attitude and you have a very scary and amazing, yet rare combination. Think Michael Jordan, Usain Bolt, or Chris Judd.
Outside of these factors you can not ignore the influence of sleep quality, daily nutrition, facility availability, access to good coaches, staying injury free over many years to build their athletic ability, keeping life simple by reducing stress and not spending too long on the studies or working to get by.
You can have the best training program in the world, but if any of these factors are not considered then performance will drop.
Performance enhancement = many factors considered to maximise the potential of each individual athlete.
Picture : Bazyler MA et al. Strength Training for endurance athletes: theory to practice. Strength and Conditioning Journal. 37(2). 2015.