We investigated 24 right ankles from healthy adults. We recruited individuals with no low-back or lower limb pain and no history of lumbar disc herniation. Three ankles were excluded because the TN stiffness was too great to be measured by SWE. Therefore, we included 21 right ankles (10 men and 11 women; mean age, 20.8 ± 0.5 y; height, 164.2 ± 8.6 cm; weight, 58.8 ± 12.5 kg).
Ethical approval was granted by the ethics committee of Morinomiya University of Medical Sciences (#2021-069), and all procedures were performed in accordance with the Declaration of Helsinki. Informed consent was obtained from all participants before testing.
In all participants, the ankle plantarflexion angle at rest and the maximum ankle dorsiflexion angle (Max-df) were measured to determine the range of angles useful for analysis of TN excursion and stiffness. The first angle was measured using a goniometer when both feet were lowered from the edge of a bed starting from a supine position. The Max-df was measured using a robotic dynamometer (Biodex System 4; Biodex Medical System Inc., Shirley, NY, USA), with the participant’s trunk and neck in the intermediate position, the hip in 90° flexion, and the knee in 30° flexion.
TN stiffness in ankle dorsiflexion
TN stiffness was assessed using the SWE mode of an ultrasound imaging system (Aplio 300, Canon Medical Systems, Tokyo, Japan) with a 10-MHz linear transducer (PLT-1005BT, Toshiba, Tokyo, Japan). Shear-wave velocity (SWV) was measured at two ankle positions, 25% and 75% of the dorsiflexion angle (25% df-SWV and 75% df-SWV, respectively), which were calculated from the total ankle range of motion based on the sum of the rest plantarflexion angle and the Max-df. During measurement, these ankle positions were set using the Biodex 4, with the trunk and neck in the intermediate position, the hip in 90° flexion, and the knee in 30° flexion. Images were acquired at 1 cm proximal to the superior edge of the medial malleolus. Long-axis images of the TN were acquired in imaging mode (Figure 1a) and particle velocimetric mode (Fig. 1b). The measurement site was marked with an oil-based pen. The SWV (m/s) was measured at three regions of interest randomly set in the TN, and the average value was calculated to represent the TN stiffness. To validate this measurement, six participants (five men and one woman; age, 21.0 ± 0.0 y; height, 168.2 ± 8.4 cm; weight, 60.2 ± 12.1 kg) were measured twice with an interval of one week between measurements. The stiffness intraclass correlation coefficients (ICCs) and 95% confidence intervals for the minimum detectable change (MDC95) were then calculated for each measurement condition.
TN excursion during ankle dorsiflexion
Using the ultrasonography system described above, we recorded movies of TN movement in real time. During measurement, the linear transducer was fixed to the measurement site, which was the same as in the stiffness evaluation, with a thermoplastic fixture and an elastic bandage. The Biodex System 4 was used to passively move the ankle from 20° plantarflexion to the Max-df while the participant’s neck and trunk were in the neutral position, the hip was in 90° flexion, and the knee was in 30° flexion. The ankle was moved at a constant velocity of 30°/s; 0.67 s was allowed for each of dorsiflexion and plantar flexion, and 0.33 s was allowed for switching the direction of movement.
The movies were analyzed by particle image velocimetry, an optical technique for measuring the displacement of a particle pattern, using the application software Flow PIV (Library, Inc., Tokyo, Japan) in accordance with a previously verified method. Flow PIV can isolate the relative pixel motion between successive frames of an ultrasound movie to visualize flow direction and velocity. Five regions of interest (21 × 15 pixels each, 0.0165 s per frame, 2-frame intervals) were set in the TN for tracking nerve motions (Figure 1c). The framewise mean of the flow velocity value and its time series were recorded (Figure 2). The maximum flow velocity values observed in the time-series data during three dorsiflexions of the same nerve were averaged and used as the maximum flow velocity value for analysis (Max-FV). Additionally, the distances obtained by multiplying each flow velocity value by a frame interval of 0.033 s were summed over a flexion cycle and the average of this sum over three successive flexions was defined as the TN excursion distance per dorsiflexion (TN-ED). The validation group comprised eight participants (three men and five women; age, 21.0 ± 0.0 y; height, 163.9 ± 9.5 cm; weight, 59.9 ± 12.5 kg), who were measured twice with an interval of 1 week between measurements. As before, the ICCs and MDC95s were calculated from these data for each measurement condition.
SPSS Statistics 24.0 for Windows (IBM, Armonk, NY, USA) was the statistical software. The distributions of the 25% df-SWV, 75% df-SWV, Max-df, Max-FV, and TN-ED consistently passed the Shapiro–Wilk normality test. Therefore, all data are presented as mean ± standard deviation (SD). A two-tailed paired t-test -was performed to examine the differences between the 25% df-SWV and the 75% df-SWV with significance level p < 0.05. Additionally, a single-regression analysis was performed with Max-FV and TN-ED as the dependent variables and 25% df-SWV, 75% df-SWV, and Max-df as the independent variables, to clarify the influence of changes in the TN stiffness on its excursion. The statistical significance level of this test was also p < 0.05. The ICC[1,2] and MDC95 were calculated to measure the reliability of the TN stiffness and excursion determinations. The MDC95 was calculated as: MDC95 = SEM × √2 ×1.96.