Thanks to technology, when we lose something it can be replaced by a stand-in, as in the case of a prosthetic leg. Naturally, a prosthetic lacks many of the characteristics and functions of that which it is supposed to replace, while at the same time the prosthetic may have additional features, or properties that the original did not. For example, a prosthetic leg will not have any blood flow or muscle, and among other things will therefore no longer play a role in storing glycogen. However, this new leg may be capable of withstanding greater compression or torsion forces than the old one.
The use of prosthetics is not restricted to giving patients new limbs, or helping restore diminished senses in the myopic or hard of hearing, but extends outwards to anywhere something important has been lost. For example, social media is a prosthetic version of community and social contact in a pre-industrialised era. Sport and exercise are both replacements for the general movement we would experience when hunting, gathering, playing and travelling as a nomadic species. Due to the introduction of prosthetics, we may not even be aware that something is missing, but as in all cases, it only takes a more thorough inspection. The chances are, that the more widespread and universal some new thing seems, the more likely it is a prosthetic for something we have lost without realising it.
The difference between a prosthetic and a supernormal stimulus is that the superstimulus is simply a specific type of prosthetic that provokes a stronger than normal response than that which it replaces, by heightening certain features. Prosthetic legs would only be considered a superstimulus if we preferred them over real legs. A simplified way of distinguishing between the two is to determine whether something is simply being replaced, or whether it is being augmented and exaggerated at the same time. The two could also be placed on a spectrum based on our reactions to them when compared to their real counterparts; with lower-than-normal prosthetics on the left, and higher-than-normal, superstimuli on the right.
Some subjects often fall into both categories, as in the case of exercise. Walking and running in the park, are prosthetic versions of those same activities which were once necessary for travelling, hunting and gathering. We now have supermarkets, online shopping, and public transport systems which mean that our natural desire, and physiological need for movement must be fulfilled by other means. Walking or running on a treadmill, in a warm, air-conditioned gym, in front of a television screen, surrounded by others, is just one example of how a healthy activity is turned into a superstimulus.