Gygax looked into the real world research, when he created Character Ability scores. The clue is in his mention in DMG of STR being measured by the amount of weight the character can lift in a Military Press exercise. That is straight from the Human Performance research of his day. It moved on since then, and got MORE interesting. You picture of the Gymnast's strength is the perfect example.
Human abilities are grouped into two distinct categories, which behave differently: Physical Abilities and the five forms of measurable Human Intelligence. Human abilities can all be improved by training and exercise and can all be brought up to be "All 18's". Human Intelligence consists largely of latent talent, influenced by one's genes, and also required constant practice for improvement, but presence of latent talent makes a quantum leap in how much the intelligence ability can be developed, and also, Human Intelligence is more of a Zero Sum structure - you CAN NOT get "All 18's" with the Human Intelligence abilities, though fully developed Renaissance Men have existed and were clearly exceptional in two or three areas. The abilities themselves are as follows:
Physical abilities are STRENGTH, FLEXIBILITY, and ENDURANCE. STR is the amount of weight one can dead lift off the ground and over one's head. Men can get stronger in this than Women. FLEX is the ability to do splits and to contort one's body. Women are can be superior to men in this and there are male dancers who take female hormones to match women in FLEX. END is how heavy a burden one can carry on the back and for how long, also how long one can run or stay in a physical fight before getting winded and exhausted.
Human Intelligence abilities are as follows: Verbal/Arithmetic Intelligence, measured by the IQ test; Artistic Intelligence, ability to draw a 3D picture and correctly maintain the proportions of the objects in the drawing; Mechanical Intelligence - Manual Dexterity, ability to take apart an engine or a watch mechanism, and put it back together again without relying on schematics; Tumbling Intelligence, Agility - awareness of one's body in 3D, allowing one to do complex choreography in dance, gymnastics, and acrobatics; and Emotional Intelligence - allowing the person to read facial expression, pick on non-verbal behavioral cues, and successfully interact with people. So, ART, DEX, INT, CHA and all forms of Intelligence. AGILITY is the most complex of human abilities. It is a function of human intelligence, but requires high physical development in the areas of STR and FLEX to be developed. The minimal requirement for gymnastics is the STR to lift one's own weight, and women need less of it since they are typically smaller and lighter than men, and exceed them in FLEX.
This leaves two other abilities, Constitution and Wisdom. WIS is Gary Gygax own brilliant design. He defines it as the inherent discipline to do what is good for that person. Not smoking on the emotional level, because it is not good for you. This would be crucial for survival among various Saddam Husseins and other fanatic tyrants, as well as in the universe where exist many fickle and cruel gods. There is a large amount of literature written in Philosophy, and Gygax's contribution in on the level.
Constitution is in a class by itself. Human Endurance is not really the CON as defined by Gygax. However, there is a Genetic Trait called HARDINESS. A while back Cattell proposed that human behavior is the expression of people's psychological traits, and later it was proposed that some traits are learned while other traits are genetically inherited. Later still, some psychologists set out to find and define Genetic Traits, most were too abstract and too removed from everyday life to be readily thrown into a game or a self improvement book, but one such trait is called Hardiness. HRD is the ability to resist infection and how fast one recovers from injury. That sounds like CON to me. As a side note, people with exceptional Hardiness tend to be optimists, with a sense of humor, and they can not hold their alcohol. They suffer more acutely from hangovers, and they start feeling it sooner than the rest of us. For that reason, they typically are non-drinkers. My father ones said that they are extremely healthy individuals. Another side-note is that aspirin is the chemical agent that boosts Hardiness, speeding up certain metabolic processes, that produce high Hardiness, The Anti-Aspirin is the ARACHIDONIC ACID (AA), an Omega-6 fatty acid typically found in meat and egg yolks. For it to be bad, you have to mix it with the frying oil or grease, in which case the AA is bad for you, promoting inflammation. Moral of the story - Eggs are healthy, but fried eggs in excess are bad for you, especially if you got arthritis, asthma, allergies, or other inflammation based diseases.