Importantly, studies using the spring-mass model have shown that running humans maintain the same bouncing movement of the body’s CoM across surfaces with different stiffnesses by adjusting their leg stiffness during the stance phase 22, 23. The elastic leg behaviour during running can be described as a simple spring-mass system, where a leg-spring supports the point mass representing the runner’s centre of mass (CoM) (Fig. This cyclic behaviour permits efficient force production through a stretch-shortening muscle action 17 and is essential for avoiding mechanically costly high-energy impacts during foot–ground contact 18. During running, the leg undergoes compression in the first half of the stance while gradually decelerating the body and then recoils in the second half of the stance to reaccelerate the body. Previous research 16 has identified elastic leg behaviour as critical to terrestrial locomotion. Our goal in this paper is to shed new light on understanding the mechanisms that might be responsible for countering the impact attenuation effect of extra shoe cushioning during running. These findings counters the impact attenuation theory 14 and the results of in vitro mechanical impact tests 15, both of which indicate a significant reduction in impact loading with increased cushioning.
In fact, some studies have noted even a slight increase in impact loading when running in shoes with a compliant versus a hard midsole 10, 11, 12, 13. The explanation for this counterintuitive finding may lie in the well-recognized, but poorly understood phenomenon that highly cushioned shoes have a limited ability to reduce impact loading 6, 9. However, studies show no evidence of reduced running injury rates with increasing amounts of cushioning 5, 6, 7, 8. In order to reduce the risk of running-related injuries, running shoe manufactures have added cushioning to shoe soles aimed at reducing impact loading. In particular, when the foot hits the ground, the magnitude of the vertical ground reaction force impact peak (IP) and loading rate (LR) have been linked to the risk of running injuries 3, 4, so the study of running injury prevention has primarily focused on the management of impact loading. However, each year between 37% and 56% of runners worldwide incur injuries 2 that typically result from repeated loading of the musculoskeletal system. Running, a popular exercise across the world, offers significant cardiovascular and other health benefits 1. These discoveries may explain why shoes with more cushioning do not protect against impact-related running injuries. We attribute the greater impact loading with the maximalist shoes to stiffer leg during landing compared to that of running with the conventional shoes. This surprising outcome was more pronounced at fast running speed (14.5 km/h), where ground reaction force impact peak and loading rate were 10.7% and 12.3% greater, respectively, in the maximalist shoe compared to the conventional shoe, whereas only a slightly higher impact peak (6.4%) was found at the 10 km/h speed with the maximalist shoe. We found that highly cushioned maximalist shoes alter spring-like running mechanics and amplify rather than attenuate impact loading.
To better understand the shoe cushioning paradox, we examined impact loading and the spring-like mechanics of running in a conventional control running shoe and a highly cushioned maximalist shoe at two training speeds, 10 and 14.5 km/h. However, despite decades of shoe technology developments and the fact that shoes have become increasingly cushioned, aimed to ease the impact on runners’ legs, running injuries have not decreased. Running shoe cushioning has become a standard method for managing impact loading and consequent injuries due to running.
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