The influence of Maximalist and Minimalist midsole shoes on dynamic stability and complexity during running movement

Trail running is a rapidly emerging running discipline that requires mental and physical endurance. Most of the time, running takes place on unpaved surfaces, with stones, moss and grass. To minimise the risk of lower limb injury, the optimal choice of footwear needs to be well considered before starting training. Data suggests that footwear (insole, outsole and midsole) can play an important role in lower limb injuries. Shoes are usually divided into three main groups: "maximalist", "conventional" and "minimalist" shoes, which are chosen by runners according to their feeling and needs. A thin sole and no heel drop for minimalists; a thicker sole and a heel drop of more than one centimetre for conventional and finally, a usual cushioning of more than thirty centimetres with zero heel drop for maximalists. The efficiency of shoes is described by the influence they have on the variability of movement of the lower limbs during running. Over the years, the concept of variability has evolved from an idea of noise to an ideal movement pattern with deterministic characteristics according to dynamical systems theory. Deterministic systems can be solved using nonlinear dynamics: Lyapunov Exponents (index of stability/variability of running movement) and Sample Entropy (index of complexity of lower limb trajectory during running movement). Studies, as Frank et al. (2019) or Borgia et al. (2020) investigated the effects of minimalist and maximalist shoes, while we are not aware of any studies that would compare maximalist and conventional shoes, the subject of this investigation. Seventeen inertial measurement units (IMUs) attached to the entire body and Xsens Analyse® graphics software, in the Xsens 3D Motion Capture System, were used during four running sessions per participant to measure the kinematic properties of thigh, shank and foot. Four trials of five minutes each were performed alternating conventional shoes (shoe A, Altra® Olympus 4) to maximalist (shoe B, Adidas® Terrex Speed SG) and vice-versa, according to the random scheme ABBA. The midsole material is EVA, while the arch plantar is a polymeric blend of EVA/TPU (thermoplastic urethane) for both footwears. Later, the kinematic results were used to calculate the LyE and SampEn values for hip, knee and ankle joints. Only the knee joint movement running pattern was significantly affected by maximalist footwear (p < 0.05), which reduced the stability of the running movement; a strong trend was shown for the ankle, but both hip and ankle were not significantly affected by footwear. Finally, the Sample Entropy showed the hip to be the least complex joint, followed by the ankle and knee. In this case, we did not find a significant change in complex trajectory for any shoe (p > 0.05), and not a single lower limb joint was affected by shoes. In the end, different pronation/supination, plantarflexion/dorsiflexion and inversion/eversion of the ankle between maximalist and conventional shoe did not seem to have any influence on the stability and complexity of the running motion of the joints.