When people run, their movement can by modelled as if they are bouncing along like a pogo stick. Runners convert kinetic and potential energy into elastic energy stored in their tendons and muscles as they fall, which is then converted back into kinetic and potential energy as they bounce up; just like the energy stored and released from the springs in a pogo stick. Knowing that muscles' properties change as we age, Giovanni Cavagna, Mario Legramandi and Leonardo Peyré-Tartaruga decided to compare the mechanical work done by elderly and young runners to see how the natural asymmetry in running energy transfer changes with age (p. 1571). They found that the asymmetry was exacerbated in the older runners because their muscles generate less force while shortening (before take-off) then they do when stretching after landing. Given the differences in the runners' performances, the team suspect that the physiological properties of our muscle–tendon units are responsible for the natural asymmetry of energy transfer during the take off and landing phases of a running stride.