Participants
This study includes 16 participants (8 men age 26.3 ± 3.7 yrs.; body height 1.81 ± 0.05 m; body mass 82.4 ± 11.0 kg, and 8 women age 29.0 ± 6.7 years old; body height 1.76 ± 0.69 m; body mass 61.6 ± 16.5 kg). In this measurement process, all the participants were injury free. Before the measurements were performed, the protocol was explained to them, as well as their rights regarding their participation in the study, which were in accordance with the Helsinki Declaration.
Protocol
First, measuring equipment was placed on the participants: surface electrodes, IMU, accelerometer, and a miniature laptop as a data logger. After the equipment placement, the measurements of maximal voluntary isometric contractions (MVIC) were performed (plantar flexion, dorsal flexion, ankle eversion and inversion), which were used for normalizing EMG signal. All MVIC were performed against resistance. Participants were verbally encouraged to give maximal effort and hold the contraction for approximately five seconds. With the first isometric device, ankle eversions were measured for normalization of m. peroneus longus (PL) and m. peroneus brevis (PB). The subjects were sitting on chairs with adjustable height so that the angle of their knees and hips was at a 90°. The knees were fixed to exclude any hip movement. With the second isometric device, the subjects performed plantar and dorsal flexion of the foot for normalization of m. soleus (SOL) and M. tibialis anterior (TA). The subjects were sitting with their knees fixed, and the angle of their hips and knees was 90°. To determine MVIC for m. gastrocnemius medialis (GM) and lateralis (GL), a stationary barbell was used for the participant to push up during standing isometric plantarflexion, as according to Hebert-Losier [26]. The measurements were performed in random order. Every MVIC measurement was performed twice with a two-minute rest period between test.
Walking measurements
Participants performed three different types of walking at their self-selected walking speed: walking on a flat surface (NORM), walking on a medial incline ramp with a 30° inversion (FULL) (Fig. 1a), and walking on a medial incline ramp with a 30° inversion with additional instruction, that the participants maintain the lateral part of the foot elevated, supporting themselves only with the medial part of the foot (LAT) (Fig. 1b). These three walking types were always performed with both legs (left and right), but only the right leg was recorded. The participants underwent measurements of walking in random order. A single walking task was 33-m long and consisted of four lengths on the incline ramp. The incline ramp was 8.25 m long, made of wood, and lined with sandpaper, which prevented the foot from sliding off (Fig. 2). A 30-s rest period was provided between each task repetition.
Kinematic recordings
During walking, a 3-D sensor for measuring acceleration is taped to the heel of the participant (Biovision, Wehrhein, Germany), to measure the contact time. To control the foot position and the measurements of the lengths of strides, Panasonic DMC-FZ200 camera (300 fps; Panasonic UK, Great Britain), was used. It was placed in a sagittal position to the participant. To measure the gait speed, expressed in m/s− 1, Stalker’s type hyper-frequency radar (Stalker Professional Radar, Radar Sales, Plymouth, MA, USA) was used. It was placed in a frontal position in relation to the direction of walking. The participant was tracked with a camera and radar from the start to the end of the measuring interval.
The foot position during walking was controlled and monitored with inertial measurement unit (IMU) (MTx, Xsens Technologies B.V., Enschede, Netherlands), which was placed on the dorsal side of the right forefeet. Foot position was observed in the x-axis (Roll) and was expressed in degrees (°).
Surface electromyographic recordings
First, points for the EMG electrodes placement were determined according to the SENIAM [27]. Bipolar Ag/AgCl surface electrodes (Kendall, Neustadt/Donau, Germany), 10 mm in diameter and with a 20 mm inter-electrode distance were placed on the belly of next muscles: peroneus longus, peroneus brevis, tibialis anterior, soleus, gastrocnemius medialis, and gastrocnemius lateralis of the right leg. To minimize any electrode movement, they were fixed using adhesive tape. A sixteen-channel EMG system with a differential amplifier (Biovision, Wehrhein, Germany), was used to record muscle activation. The EMG and acceleration data were transmitted to a UMPC miniature notebook computer (Viliv, Yukyung Technologies Corp., South Korea) where the analogue data were sampled at 2000 Hz (16-bit resolution) and stored for analysis.
Data reduction
For processing the EMG signal, Labchart 7 was used (AD Instruments, Dunedin, New Zealand). In processing the data of MVIC, the signal was digitally filtered (20 Hz/500 Hz, bi-directionally) to remove a base line shift. Later, a full-wave rectified signals were taken and smoothed with a median filter (Window width 501 samples). In each of the six muscles, the maximal EMG signal value was taken and used for the normalization of the walking. The same procedure of processing the EMG signal in walking was used (filtering, full-wave rectified and smoothing).
The average of 15 correctly performed measurements of strides made with the right foot was analysed. The average EMG amplitudes during the stance phase (from the moment of foot strike to the moment the foot left the surface) were used for statistical tests. Contact time was also measured (expressed in seconds). Only strides with full heel-toe technique were used for further analysis. The contact time for each step was read from the acceleration signal of the right leg. Heel strike was detected as the time of occurrence of the nearest main peak of the vertical heel acceleration. Toe-off was recognized as when the acceleration happened after raising the heel from the floor. The accuracy of contact time achieved from the acceleration signal was also monitored by counting photos from the high-speed cameras during the foot’s contact with the ground. An accuracy control was made for the two analysed steps in each condition. The steps for control were chosen randomly from analysed steps. The contact times of all analysed strides in three types of walking were averaged, and the standard deviation was calculated for each participant. The stride length, expressed in meters, and considered to be two consecutive steps, from heel to heel (2-D kinematics), was analysed with Kinovea 0.8.15 software (Joan Charmant & Contributors, Bordeaux, France).
Statistical analysis
For all parameters (EMG, kinematic parameters), average values were calculated, and the Shapiro-Wilk test was used to check the normal distribution. At normally distributed variables (p > 0.05), the parametric ANOVA test for repetitive measurements was used. Post-hoc tests, using a Bonferroni correction, were used to determine pairwise differences when the ANOVA showed an overall significant F statistic. If the data were not normally distributed, a non-parametric Friedman test was used. To compare two non-parametric parameters, a Wilcoxon signed ranks test was used when necessary. Statistical significance was accepted at an alpha level of 0.05, except in the case of multiple Wilcoxon test, for which the alpha level was adjusted to 0.017 to avoid inflation of Type I error.