Participants
This study was approved by our institutional review board. All study participants provided written informed consent, and the rights of the subjects were protected. This study is a prospective, investigator-initiated trial. All data were managed in Excel files by a blinded author, and statistical analyses were done by a statistician. This prospective case-control study enrolled 112 patients with foot pain at our institute between July 2018 and November 2019. Physical examinations and evaluations of all images were independently performed by two experienced surgeons. Any disagreements on any diagnoses of PF were resolved by consensus. In this study, the inclusion criteria were PF patients with normal foot posture in terms of naviculocuboid overlap and talonavicular coverage angle on plain radiographs, without tightness of the GCM or hamstring muscles. We excluded 91 patients for the following reasons (Fig. 1): pain in both feet, metatarsalgia, Morton neuroma, calcaneal spur, pes cavus and pes planus, and tightness of the GCM and hamstring muscles in the Silfverskiold and popliteal angle tests, respectively. We also excluded patients who had received a steroid injection within 6 months or had undergone knee surgery within 1 year. Of the 112 patients, 91 were excluded; therefore, 21 patients were finally enrolled. The 21 normal control subjects selected from our database of volunteers had no history of lower extremity injury symptoms within 1 year and agreed to participate in the study.
Assessment of isokinetic muscle performances
Muscle strength of the quadriceps and hamstring
Isokinetic knee extension/flexion strength (concentric/concentric mode, Nmkg−1 × 100, Biodex Medical Systems, Shirley, NY) was measured in the sitting position with 90° flexion of the hips and knee joints on a dynamometer (Fig. 2A). Flexion and extension strength were considered to represent hamstring and quadriceps strength, respectively. Each test consisted of 5 repetitions of flexion/extension (ROM, 90° to 0°) for each leg at 60°/s.
Muscle strength of the GCM
Isokinetic GCM strength (concentric mode, Nmkg−1 × 100) was measured in a semi-seated position with 20° of knee flexion[16] on a dynamometer (Fig. 2B), and 5 repetitions of plantar flexion for each leg at 30°/s.
Assessment of the muscle reaction time (acceleration time)
Muscle reaction time was measured by the acceleration time (AT) during isokinetic strength testing. Muscle reaction time was defined as the time (ms) required to attain the pre-set angular velocity (60°/s for the knee joint and 30°/s for the ankle joint) during maximal muscle contraction. Lower AT values signify a rapid muscle reaction ability[17-19]. The AT was calculated automatically using the Biodex advantage software.
Assessment of the foot pressure and posture
Foot pressure was measured by pedobarography[5,20,21] (Tekscan, Massachusetts) during a 2-meter walk and recorded at 50 Hz. Based on a previous study,[21] the peak pressure and pressure–time integral were calculated for each of the 5 segments of the foot (Fig. 3): the medial forefoot (MFF), lateral forefoot (LFF), medial midfoot (MMF), lateral midfoot (LMF), and heel. These data were processed to yield the valgus/varus index (VV index, %), which is defined as ((MMF + MFF) - (LMF + LFF))/(MMF + MFF + LFF +LMF), with plus (+) and minus (-) values of the VV index indicating hindfoot valgus and varus, respectively.[21] The same peak pressure and VV index assessment processes were used for the normal controls.
Statistical Analysis
The sample size calculation for this study was based on a previous study of muscle strength in patients with lower extremity injuries,[19,22] and a muscle strength difference > 10% between the groups was considered significant. To determine the sample size, we conducted an a priori power analysis, with an alpha level of 0.05, and a power of 0.8. Effect size (Cohen's d: 1.00) was calculated using the mean and standard deviation from the results of a pilot study involving 5 ankles in each group; 17 ankles in each group were required to adequately identify a clinically meaningful difference of > 10% in muscle strength between the groups. The power necessary to detect differences in muscle strength was 0.813.
The Student’s t-test was used to compare the strength and reaction time of the quadriceps, hamstring, and GCM muscles, and the foot pressure and posture between patients with PF and normal controls. To determine whether a continuous variable followed a normal distribution, the Shapiro test was used. Correlations between the strength and reaction time of the quadriceps, hamstring, and GCM muscles were assessed using Pearson’s coefficient of correlation. Data were analyzed using SPSS software version 17.0 (SPSS Inc., Chicago, IL, USA). A value of p < 0.05 was considered statistically significant.