According to erikson, the stage of intimacy versus isolation

The theory of Erik Erikson says that there are eight stages in the psycho-social development of a healthy person from infancy to late adulthood.
According to Erikson, the stage of intimacy versus isolation takes place during young adulthood between the ages of approximately 19 and 40. Intimacy versus isolation is the sixth stage of his theory in which young adults face the challenge of forming close relationships with others. If this stage is successful it will result in strong relationships, while failure results in loneliness and isolation

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The person who utilize the progression principle will keep adding the difficulty of the exercise that he's doing, while the person who does not, will only stay stagnant.  the person that utilize progression principle, will get a stronger endurance because now his body is conditioned to do harsher routines.{the answer was already on verified)