The effect of different midsole materials in safety shoes on perceived comfort, muscle activities, and biomechanical parameters during walking – a musculoskeletal modelling approach

In certain occupational environments, safety footwear is essential for mitigating the risk of foot, leg, or knee injuries. For employees, who have to stand or walk for extended periods in such footwear, ergonomic considerations and comfort play a key role in maintaining health and work capacity. Therefore, this study aimed to investigate the subjective perception, muscle activities, and biomechanical parameters of the lower extremities during walking in safety shoes equipped with different midsole materials characterized by varying deformation and energy return properties. A prospective, randomized, single-blind crossover study was conducted with 24 healthy males wearing safety shoes in their daily work environments. Two S2 safety shoes with a comparable structural design but differing midsole materials (polyurethane (PU) vs. expanded thermoplastic polyurethane (eTPU)) were evaluated during walking with regard to subjective comfort, gait parameters, muscle activities as well as joint angles, joint moments, and muscle forces of the lower extremities calculated using a musculoskeletal multibody simulation. Both safety shoes received favourable comfort ratings from the participants. However, a significant majority expressed a preference for recommending the eTPU shoe model to their colleagues (eTPU: 14; PU: 5; p = 0.039). The musculoskeletal multibody simulation revealed only small midsole-related differences in upper ankle joint angle (1.24° ± 0.11°; p < 0.01) but no variations were observed in joint moments. However, the analysis identified lower muscle forces in the gastrocnemius medialis muscle, while wearing the safety shoes with an eTPU midsole (-1.05 ± 0.04 N/kg; p < 0.001). In conclusion, the midsole material had a low impact on subjective comfort as well as lower extremity joint angles and moments. However, the eTPU midsole may provide support to the plantar flexors during stance.