The Actual Rate of Decline is faster than the Best Possible Rate of Decline, that is the rate of decline due solely to the ageing process and the difference  between the two is the Fitness Gap. Both the point at which physical decline starts, and the rate at which it proceeds is for the first few decades determined by loss of fitness, and loss of fitness is determined by social factors, namely by the decisions people make about their life and the pressures which influence them decreases

Fitness has four aspects, all beginning with the letter S, and what we call the fifth S,  namely the pSychological benefits of exercise