
The reason for this is that while foams, polyurethanes, gels and airbags can be tuned to provide a softer ride (for lighter runners), doing so not only reduces stability but also causes them to break down (wear out) much sooner. A soft shoe would be even more problematic for a heavier runner, since the greater forces would impact the material more rapidly, greatly diminishing the cushioning comfort, responsiveness and overall useful life of the shoe.
If these same materials are tuned to provide a firmer ride, in an effort to accommodate a heavier runner, the shoe would provide greater durability and stability, but the added firmness would provide a harder, less comfortable and less responsive feel.
As indicated, the solution is in the springs of DSS. A steel coil spring is strong, durable and extremely versatile. Similar springs have been tested for 1,000,000 cycles with minimal reduction of function. More importantly, a coil spring can be designed to provide uniform levels of cushioning and responsiveness for a broader load range (weight range) than any other material, providing everyone with a “custom” ride.