Validity And Reliability Of Passive Measures Of Metatarsophalangeal Joint Stiffness In Runners

Stiffness of the metatarsophalangeal (MTPk) joint may influence running economy. Thus, it may also influence footwear effects on economy, as some elements of running footwear - such as stiffening agents embedded in midsole foam - affect MTP joint motion during gait. A reliable and valid method of assessing passive MTPk joint stiffness would be useful for individual footwear selection in runners. PURPOSE: 1) to assess the inter-day reliability of three different passive MTPk testing methods and 2) to assess the validity of different passive MTPk testing methods relative to MTPk while running. METHODS: Passive MTPk was assessed in 19 runners on two separate days using a custom pulley rig under three testing positions: lunge, seated, and standing. Dynamic MTP stiffness\ was assessed during barefoot running on an instrumented treadmill at 3.35 m/s. Inter-day reliability was assessed for passive stiffness using intraclass correlation coefficients (ICC) and Bland-Altman offset. Validity of passive MTPk measures were compared to dynamic MTPk using ICCs and Bland-Altman offsets. RESULTS: Passive measures were poorly correlated with dynamic MTPk (0.0018 ± 0.0010) (ICCs: lunge: 0.007 p = 0.48; seated: 0.027, p = 0.42; standing: 0.004, p = 0.48) and showed very large % differences compared to dynamic MTPk. All three positions for passive MTPk measures showed strong inter-day reliability (ICCs: lunge = 0.769 p = 0.002; seated: 0.732, p = 0.004; standing: 0.483, p = 0.092) and showed small overestimations compared to dynamic MTPk. CONCLUSION: The passive MTPk measures showed poor validity compared to dynamic MTPk but strong inter-day reliability. Thus, with improved validity, passive measures of MTPk would be adequate for repeated assessments.