The effect of deep posterior compartment muscle strength on foot kinematics, forefoot loading, and function in subjects with stage II posterior tibial tendon dysfunction

Purpose/hypothesis. The purpose of this study was to investigate how weakness is associated with foot mechanics and self-reports of function in subjects with stage II posterior tibial tendon dysfunction (PTTD).;Materials/methods. Thirty stage II PTTD subjects (age; 58.1+/-10.5 years, BMI 30.6+/-5.4) and 15 matched controls (age; 56.5+/-7.7 years, BMI 30.6+/-3.6) volunteered for this study. Strength testing was completed using a custom device that measured isometric forefoot adduction and ankle inversion. Strength ratios (involved/uninvolved) were calculated for each subject with strong and weak PTTD groups defined based on comparison to controls. Foot mechanics were assessed with two methods: (1) kinematic data were collected using a multi-segment foot model with an Optotrak Motion Analysis System (Northern Digital Inc, CAN) and Motion Monitor Software (Innsport Training Inc, USA), and (2) plantar loading data were collected using Pedar-X insoles (Novel Inc, St Paul, MN) taped to the bottom of the feet. Self-reported function was assessed using the revised Foot Function Index and the Short-Form Musculoskeletal Functional Assessment tools.;Results. The strength ratio averaged 1.06+/-0.1 (range 0.87-1.36) for controls and a 20% deficit (weakness) was used as a cut-off for weakness: PTTD strong (average 1.06+/-0.1, range 0.89-1.25), PTTD weak (average 0.64+/-0.2, range 0.42-0.76). Significant differences in foot mechanics were associated with strength. Subjects with PTTD who were weak demonstrated significantly greater flatfoot deformity and altered plantar loading patterns compared to subjects with PTTD who were strong and controls. Subjects with PTTD who were strong demonstrated greater flatfoot deformity compared to controls but no difference in plantar loading patterns. Strength was not associated with self-reported function or pain.;Conclusions. Deep posterior compartment strength is associated with foot mechanics as evidenced by greater flatfoot deformity and altered plantar loading patterns in subjects who are weak. Surprisingly, flatfoot deformity in subjects with PTTD who are strong suggests factors other than strength may also play a role in flatfoot deformity. A pre-existing flatfoot posture may be present without a strength deficit due to congenital flatfoot posture.;Clinical significance. Treatment outcomes may be different for subjects with stage II PTTD who are weak and those who are strong.