Twenty lower limbs of 10 healthy university students who habitually participated in recreational sports activity more than 2 days per week were enrolled in this study. Inclusion criteria were subjects without any history of pain in the leg and foot while running. Two of the subjects could not finish the running task and were excluded from the study. Thus, 16 lower limbs of the remaining 8 subjects (age, 21.4±0.5 years; height, 162.4±9.2 cm; body weight, 54.1±8.3 kg; 5 males) were analyzed. The study was approved by the Medical Ethics Committee of the authors' institution (2018–103), and all the subjects provided informed consent for study participation.
All subjects performed the same running task on a treadmill (Adventure3, Johnson Health Tech, Tokyo, Japan) set at an incline of 7% and a running speed of 8 km/h for 60 min after 3 min of warm up. Warm up started with level walking (4 km/h) for 90 s, followed by level jogging (6 km/h) for 90 s. After the warm up period, the incline was set at 7% and the running task was commenced (8 km/h). The foot contact pattern of the runners was not restricted.
Muscle stiffness was assessed using ultrasound shear wave elastography (Aplio 300, Canon Medical Systems, Tokyo, Japan) and a 5–14 MHz linear probe. A higher shear elastic modulus indicates an increase in the stiffness of the tissue. The shear elastic modulus (kPa) of each muscle was calculated automatically from 3 circular regions of interest (ROI) in the color mapped area of elastographic images of each muscle. Assessment of the shear elastic modulus after running was completed within 10 min after the running task. The shear elastic modulus of the longitudinal axis of the MG, LG, FDL, and abductor hallucis (AH) muscles were measured both before and after running for 60 min.
The LG and MG were measured at the proximal 30% of the lower leg (from the popliteal crease to the lateral malleolus). The FDL was measured at the proximal 50% of the area between the cleavage line of the knee and the medial malleolus [16, 17]. The AH was measured at the medial border of the foot below the navicular bone. The FDL muscle was identified using a B-mode ultrasound image during passive motion of the toe. In addition, we marked the measurement site with cotton bandages so that measurements both before and after running could be recorded at the same site.
Ten lower limbs of 5 participants were included in a reliability study. Muscle stiffness was assessed by a single tester who had practiced ultrasound imaging for 2 h per week for 2 months before the study. The tester performed assessments 3 times as the first session, and repeated the evaluations 3 times on another day as the second session. Assessment of intra-tester reliability of the measurement of muscle stiffness was performed using the intra-tester correlation coefficient [ICC (1,k)], and the standard error of the mean (SEM) was calculated from the mean value of the first and second sessions. Minimum detectable change with 95% confidence intervals (MDC95) was calculated using equation (1).
The structure and flexibility of the medial longitudinal arch was assessed using the modified arch height index (AHI)  and arch height flexibility (AHF) , respectively. Images of the medial side of the foot at 10%, 50%, and 90% body weight (BW) loading were captured by a digital camera (EOS Kiss X7i, Canon Inc., Tokyo, Japan) positioned at a constant distance (90 cm) from the foot. The dorsal arch height (DH) at 50% of the foot length divided into the truncated foot length in the 10%, 50%, and 90% body weight-loaded positions were calculated as AHI10, AHI50, and AHI90, respectively. The differences between AHI50 and AHI10, and AHI90 and AHI10 were assessed as AHF50 and AHF90 by equations (2) and (3):
Difference in AHI and difference in AHF (DAHI, DAHF) were defined as the difference in each parameter before and after running. A higher DAHI indicated a decrease in arch height after running, and a higher DAHF indicated an increase in arch flexibility after running.
Muscle stiffness, arch structure, and arch flexibility before and after running were compared using the Wlicoxon test, and the correlation of arch structure, arch flexibility, and the difference in each parameter with muscle stiffness before and after running were assessed using Spearman’s rank correlation coefficient. All statistical tests were performed using SPSS statistics version 25.0 (IBM Corp., Armonk, NY, USA), with p < 0.05 considered significant.