Can changes in midsole bending stiffness of shoes affect the onset of joint work redistribution during a prolonged run?

Objective: To investigate if changing the midsole bending stiffness of athletic footwear can affect the onset of lower limb joint work redistribu-tion during a prolonged run.Methods: Fifteen trained male runners (10-km time of<44 min) performed 10-km runs at 90% of their individual speed at lactate threshold (i.e.,when change in lactate exceeded 1 mmol during an incremental running test) in a control and stiff shoe condition on two occasions. Lower limbjoint kinematics and kinetics were measured using a motion capture system and a force-instrumented treadmill. Data were acquired every 500 m.Results: Prolonged running resulted in a redistribution of positive joint work from distal to proximal joints in both shoe conditions. Compared tothe beginning of the run, less positive work was performed at the ankle (approximately 9%;p0.001) and more positive work was performedat the knee joint (approximately 17%;p0.001) at the end of the run. When running in the stiff shoe condition, the onset of joint work redistri-bution at the ankle and knee joints occurred at a later point during the run.Conclusion: A delayed onset of joint work redistribution in the stiff condition may result in less activated muscle volume, because ankle plantarflexor muscles have shorter muscles fascicles and smaller cross-sectional areas compared to knee extensor muscles. Less active muscle volumecould be related to previously reported decreases in metabolic cost when running in stiff footwear. These results contribute to the notion thatfootwear with increased stiffness likely results in reductions in metabolic cost by delaying joint work redistribution from distal to proximal joints.