https://www.peakendurancesport.com/endurance-training/techniques/running-efficiency-increase-running-speed-power-reducing-energy-cost-movement/Andrew Hamilton:
Generally, larger animals (like humans) tend to be a little more efficient during running than small animals like mice, for reasons that are not exactly clear. The real paradox is this, however: when muscles are stripped from the legs of freshly killed mice and tested in the laboratory, they operate with an efficiency of 25%. That same experiment can obviously not be carried out with humans, but it is known that when humans ride bicycles, their efficiency is also about 25%. Why is running such an inefficient process for mice and so efficient for humans? When mice run marathons, their efficiency of movement drops from 25% to just 3%, while for humans efficiency during running soars from the 25% achieved during biking to 50%! The answer to the mouse question is unknown, but the key to the human running response is obvious. Each time a human foot hits the ground while running, energy is stored as ‘elastic strain energy’ by the key ‘springs’ in the human leg – mainly the connective-tissue strips which run along the bottom of the foot, the Achilles tendon and its associated muscles, the relevant muscles and tendons around the knee and the relevant muscles and tendons surrounding the hip. All of these structures are stretched when the foot hits the ground, and this stretching process stores energy – i.e. increases the potential energy of the leg. When the structures recoil elastically during toe-off, they manage to return about 90% of the work required to stretch them (with only 10% lost as heat). If the tendons and muscles of the leg were not able to store energy during impact with the ground, the muscles would have to increase their work output and energy expenditure dramatically. In fact, Alexander estimates that when humans run at middle-distance speeds, the spring-like properties of the Achilles tendon and the arch of the foot alone cut the work the leg muscles have to do by half. Here lies the answer to our paradox: human leg muscles are still working with only 25% efficiency during running; they do not really become more efficient just because running is the chosen sport. If the mechanical cost of movement is two joules per kg body weight per minute, half of this cost is furnished almost for free by the legs’ springs. Thus the muscles cough up four joules per kg per metre to provide the other joule per kg/metre of mechanical cost – with an efficiency of 1/4 = 25%.
The bottom line for you as an athlete who uses running in your chosen sport is that the best way to decrease your cost of running – and thus run faster and longer – is by enhancing the function of your leg springs. Since they are able to store energy more effectively when the foot hits the ground and then release this energy more fully and in a more timely fashion during push-off, your metabolic cost of running at a specific speed will drop, and you will be able to move up to higher speeds during training and racing.