Introduction. One of the major factors related to overuse injuries in runners is impact force resulting from the runner hitting the ground at heel strike. (Hreljac et al., 2000). This impact will increase and decrease based on the construction of the running shoe used as well as the type of surface being run on. The purpose of this research was to determine the effects that both midsole cushioning and surface stiffness have on this impact force.

Methods. Six recreation runners running at least 10 miles per week ran a 2.2 km course 3 times at a self selected pace. A variable cushioning running shoe (Adidas One^®) was used and set in a low cushioning, high cushioning, or self adjusting cushioning mode for each run. Accelerometers were attached at the leg and forehead to measure impacts and impact attenuation. Accelerometer data were sampled at 512 Hz. The course consisted of the following surfaces: cement, asphalt, dirt, gravel, woodchips, sand, grass, and a wooden suspension bridge. Cohen's D formula was used to calculate effect sizes comparing each shoe and each surface. The values 0.5 and 0.8 were used to determine medium and large effect sizes respectively.

Results. The soft shoe setting had a medium effect (ES=0.66) compared to the hard shoe setting. No medium or large effects were found in head acceleration or impact attenuation between shoes. Sand had the largest leg accelerations of 7.8 g's while the wooden bridge had the smallest (6.3 g's). Large effects were found in leg acceleration with sand and 3 different surfaces. Gravel had the largest head acceleration of 1.58 g's. Only medium effects were present between surfaces when comparing head accelerations. Sand had the highest impact attenuation of 83.8% while gravel had the smallest impact attenuation of 77.2%.

Discussion. Two of the softest surfaces (sand and grass) had the highest leg accelerations. This is believed to be due to changes made in kinematics while running on these two surfaces. Altering kinematics will change the effective mass of the body. The decrease in effective mass will increase leg accelerations even though the impact force may not have increased.