Energy return in footwear – revisited

Advanced Footwear Technologies (AFTs) have enhanced running economy in the laboratory and improved performance in long distance events. Their resilient midsoles have renewed interest in 'energy return', the idea that elasticity in shoe soles could benefit athletic performance. This paper examines the concept of energy return, it's definition, measurement and interpretation. 'Energy return' itself is a misnomer. Running shoe cushioning is entropically elastic, not energetically elastic and a net dissipator of energy. The term is easily confused with 'rebound' and 'coefficient of restitution', but neither are equivalent. Energy return is not a material property but a nonlinear, system dynamic outcome that depends on inertia, initial conditions, geometry, pre-test conditioning and other factors. Consequently, measurements are sensitive to test protocol variations. Results from quasi-static compression tests and standard impact tests of identical ethylene vinyl acetate foam specimens give different results. Impact tests of 180 running shoe models reveal that energy return overestimates rebound height by 57%, on average. The difference is due to the potential energy deficit (Edef) created as the shoe sole is compressed, which accounts for 25–45% of total energy output. AFT shoes reduce energy expenditure by exploiting the known effects of shoe weight, cushioning and bending stiffness. The magnitude of energy return is small and differences among shoes even smaller. Any direct effects of 'energy return' on running economy remain unknown. Low density AFT foams store less energy per unit volume than conventional materials. They must be thicker and more compressible to accommodate the same input loads. Regardless of any effects on running economy, the higher energy return of AFT shoes is a necessary compensation for their inherently greater energy deficit.