Weakening iliopsoas muscle in healthy adults may induce stiff knee pattern

OBJECTIVE: The goal of the present study was to investigate the relationship between iliopsoas muscle group weakness and related hip joint velocity reduction and stiff-knee gait (SKG) during walking in healthy individuals. METHODS: A load of 5% of each individual's body weight was placed on non-dominant thigh of 15 neurologically intact, able-bodied participants (average age: 22.4 ± 0.81 years). For 33 min (135 s × 13 repetitions × 5 s rest), a passive stretch (PS) was applied with the load in place until hip flexor muscle strength dropped from 5/5 to 3+/5 according to manual muscle test. All participants underwent gait analysis before and after PS to compare sagittal plane hip, knee, and ankle kinematics and kinetics and temporo–spatial parameters. Paired t-test was used to compare pre- and post-stretch findings and Pearson correlation coefficient (r) was calculated to determine strength of correlation between SKG parameters and gait parameters of interest (p < 0.05). RESULTS: Reduced hip flexion velocity (mean: 21.5%; p = 0.005) was a contributor to SKG, decreasing peak knee flexion (PKF) (−20%; p = 0.0008), total knee range (−18.9%; p = 0.003), and range of knee flexion between toe-off and PKF (−26.7%; p = 0.001), and shortening duration between toe-off to PKF (−16.3%; p = 0.0005). CONCLUSION: These findings verify that any treatment protocol that slows hip flexion during gait by weakening iliopsoas muscle may have great potential to produce SKG pattern combined with reduced gait velocity.