Introduction Advanced footwear technology such as the Vaporfly 4% significantly reduces energy cost and improves running performance due to more advanced midsole materials and higher longitudinal bending stiffness (LBS) (Hebert-Losier et al., Citation2020). Many studies have investigated the effects of running shoe stiffness on lower limb biomechanics, energy cost, and subjective perception. Evan found that the optimal running shoe stiffness depends on the running speed, and runners tend to choose the shoes with higher LBS when the running speed is increased. Keonyoung (Keonyoung & Sukyung, Citation2017) pointed out that higher LBS is beneficial to the improvement of running energetics, as long as it does not disturb the natural MTP joints flexion. However, most of the test shoes in previous studies used a plate underneath the insole to change the LBS of the running shoe, which is not consistent with the "sandwich construction" of racing shoes on the market today. Also, few studies have investigated the impact mechanisms of LBS on lower limb work patterns. Purpose of the study: This study aimed to investigate the effect of marathon racing shoes with different LBS on the elite runners` lower limbs biomechanics. The findings will provide guidance for the design of marathon racing shoes and the development of longitudinal bending stiffness standards. Methods: Fifteen male runners (height: 169.55 ± 4.77 cm, weight: 60.16 ± 5.22 kg, BMI: 21.54 ± 1.70 kg/m2) were recruited from a local running community. All participants were Chinese nationals, rearfoot strike, a minimum weekly running distance of 50 km, with the foot size of EU (41 ± 0.5) and the ability to complete a marathon in under 3 hours. This study adjusted LBS by altering carbon plate thickness: Soft longitudinal Bending Stiffness (SLBS) (0.31 Nm/deg), Medium longitudinal Bending Stiffness (MLBS) (0.40 Nm/deg), and Hard longitudinal Bending Stiffness (HLBS) (0.48 Nm/deg). Participants completed 5 valid trials on a 66-meter indoor concrete track at a speed within 4.8 m/s ± 5%. Ground reaction force (GRF) and kinematics were measured using force plates (AMTI Force & Motion, Watertown, MA, USA) and a 10-camera motion capture system (Vicon, Oxford, UK). Muscle activation was recorded using Noraxon Ultium EMG sensors (Noraxon, Scottsdale, AZ, USA). Kinematic, kinetic data and Surface electromyography signals were processed with Vicon Nexus 2.7 and Visual3D. Statistical analysis was performed using MATLAB, with Shapiro-Wilk tests for normality and paired t-tests for comparisons (a < 0.05). Results: As LBS increased, the hip joint`s contribution to the peak TM decreased, with the SLBS showing significantly higher contributions than others. In contrast, the ankle joint`s contribution increased initially and then decreased, with the SLBS having a significantly lower contribution than others (p < 0.05). The HLBS had significantly longer contact time than both the SLBS and the MLBS (p < 0.05). Push-off time was longer in the HLBS compared to the MLBS (p < 0.05). Vertical stiffness was significantly higher in the HLBS and MLBS than in the SLBS (p < 0.05). MLBS had the highest tibialis anterior/medial gastrocnemius co-activation (p < 0.05) Discussion and conclusion: We found that changes in LBS affected the work patterns of lower limb joints, with the MLBS and HLBS contributing a smaller proportion of the hip moment and the ankle joint contributing a higher proportion compared to the SLBS. In other words, increased LBS resulted in a significant increase in the role played by the distal joints during running. This conclusion was further supported by an increase co-activation of the TA and MG muscles. Muscle-tendon units around the ankle joint are more capable of storing and returning elastic energy, which is beneficial in reducing lower metabolic costs during long-distance running (Sasa et al., Citation2021). The increase in LBS did not necessarily lead to better running performance. There was no significant difference in the lower limb joint contribution between MLBS and HBS, and contact time and push-off time were even longer in HLBS. In addition, the BF and RF co-activation of HLBS increased, and the proximal joint muscle work contribution was higher, which proved that the running performance of HLBS was worse compared with that of MLBS, which may be due to the phenomenon of proximal joint compensation caused by the runner`s inability to adapt to the high stiffness. Therefore, there is a reasonable range of values for LBS of running shoes to enhance running performance.
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