Recent changes in road-racing shoe construction has prompted research comparing the metabolic benefit of new shoe constructions compared to older styles (Barnes and Kilding, 2019; Hoogkamer et al., 2018; Hoogkamer et al., 2019; Hunter et al., 2019). Various shoe features in these new road racing shoes (such as low foam density, high longitudinal bending stiffness, and a high stack height) may alter metabolic benefit due to changes in kinematics and energy return (Hoogkamer et al., 2018; Hoogkamer et al., 2019; Hunter et al., 2019). All of the aforementioned racing shoe research has been done on level surfaces, preventing direct application to many road races which also include uphill and downhill conditions. However, one recent article found the Nike Vaporfly is not as effective on hilly courses compared with level (Whiting et al., 2021). Kinematic and kinetic differences between uphill and downhill running may affect the magnitude of benefit for this new style of racing shoe. Changes in surface grade influence running posture, as runners tend to lean forwards with uphill running and backwards with downhill running (Paradisis and Cooke, 2001). Such a change in posture can influence moment arms about lower limb joints and alter mechanical advantage (Roberts and Belliveau, 2005). The longitudinal bending stiffness of a shoe may also lead to changes in mechanical advantage about the ankle due to the center of pressure during toeoff being more anterior, however the increased bending stiffness does not appear to be the reason for improved running economy in this new style of shoe (Healey and Hoogkamer, 2021). Overall postural changes will also alter foot strike with a shift to midfoot foot strike when running uphill, and to a rearfoot foot strike when running downhill (Gottschall and Kram, 2005). Research on a recently popular racing shoe suggested that the energy savings caused by the shoe may be greater for rearfoot striking than midfoot striking runners (Hoogkamer et al., 2018). This suggests that the benefit of a performance shoe may be greater during downhill running due to the shift to a more rearfoot strike position. Surface grade may also alter the ground reaction forces experienced by a runner. Downhill running has been found to increase normal impact forces, which is hypothesized to be due in part to the shift to rearfoot strike landing (Gottschall and Kram, 2005; Telhan et al., 2010). These larger forces increase compression and elastic energy storage in the newer shoes that have a greater midsole thickness. It is important to note that results have been mixed on this topic, and with differing downhill grades and speeds used, other studies found no change or a slight decrease in ground reaction forces with downhill running (Snyder et al., 2012; Yokozawa et al., 2004). In uphill running at an equal metabolic effort, runners apply a lower peak normal force (Williams et al., 2020). Runners with a shorter ground time received a greater metabolic benefit associated with the new style of racing shoe (Hunter et al., 2019). This may indicate increased metabolic benefit when running in downhill conditions since running speeds are faster and hence ground contact times shorter during downhill running. Due to the biomechanical changes associated with downhill running, we hypothesized that new performance racing shoes would have a greater metabolic benefit when running downhill, than with level or uphill running. We also expected certain runners would obtain a greater benefit from the new style of running shoe due to the timing and amount of foam compression that would occur with a variety of running techniques. We anticipated differences in stride length, ground time, peak force and flight time would lead to variations in the amount of benefit obtained through wearing the new style of racing shoe at the various grades. |