Participants
Two research groups took part in the tests. The experimental group included eight female members of the Polish National Team in short track, with a mean age of 18.7 ± 2.9 standard deviation, mean height of 162.4 ± 2.4 cm, and mean body weight of 57.2 ± 5.9 kg. The control group included eight female students active in sports (but not in speed skating), with a mean age of 20 ± 0.9, mean height of 169.1 ± 4.1 cm, and mean body weight of 68 ± 4.2 kg. These students were randomly selected from among female students of physical education at Opole University of Technology. The research was conducted during the training cycle, after a weekend break in training, to avoid the short-term effect of fatigue accumulation due to the training. The participants were informed about the purpose and course of tests and signed a consent to participate in the tests. The tests were approved by the Bioethical Commission of the Chamber of Physicians in Opole, Poland. In interviews conducted before the tests, all the respondents declared they were right-handed and right-legged in daily and sports activities (e. g., tossing a ball, kicking a ball, supporting with a foot during swinging). Furthermore, a kick-a-ball test (with three attempts) confirmed all the participants were right-legged, while the modified Edinburgh questionnaire confirmed they were right-handed [21].
Procedures
The sEMG signal frequency in the gluteus maximus muscles was examined in an isometric contraction using the position from the Biering-Sorensen test [11, 14]. To avoid too high loads for the skaters, the tests were stopped after 60 seconds of the contraction, and they were not continued until the subject was unable to hold the position because of muscle fatigue. The effectiveness of the fatigue test during a 60-second contraction was confirmed by Mutchler et al. [22]. During the test, the subjects were lying on a horizontal table on the abdomen, with the iliac crests aligned to the edge of the table and the lower limbs attached to the straps around the ankle joints. They were instructed to hold the body (head, shoulders, and torso), without support, horizontally to the ground as long as they could, with the arms crossed at the chest (Figure 1).
The EMG measurement
In the test, a 16-channel EMG system (produced by NORAXON DTS) was used, which recorded signals with an accuracy of 16 bits at a sampling rate of 1500 Hz. The bio-electric test of activity of the right and the left gluteus maximus muscles was carried out by the SENIAM methodology [14, 23]. To improve the adherence of the electrodes, before the test, the hair was shaved and the skin was cleaned in the place where the electrodes were to be stuck. Surface electrodes (Ag/AgCl) were placed on the muscle between the movement point and the tendon attachment, along the longitudinal middle line of the muscle. Signal processing and EMG analysis were performed using NORAXON MR-XP 1.07 Master Editionx software.
Fatigue-related changes (frequency shift) in the frequency content were calculated for the raw EMG signal (Figure 2) obtained during a static contraction. Unfiltered raw sEMG was analysed step-wise in 1000 ms increments over the selected portion of the measurement (60 seconds in the Biering-Sorensen test). The mean frequency was calculated for each step using values based on the frequency power spectrum (calculated by a Prime Factor Fourier Transformation). A fatigue slope (being a regression coefficient from a linear regression line between the mean frequency and time) was estimated for each participant. The mean value of the slopes of these lines were analyzed with ANOVA with repeated measures taking into account the side factor (left/right muscle) and the studied group (experimental, control). sEMG frequency power spectrum is expected to shift to lower frequencies during fatiguing contractions, and the mean frequency analysis can be used to estimate the magnitude of that shift. This phenomenon is well established for static contractions at constant load levels and believed to reflect local fatigue (Figure 3.).
Technical specification of NORAXON DTS is as follows:
- basic noise of the device, below 1 uV RMS,
- input impedance above 100 Momh,
- CMR (common signal rejection factor) greater than 100 dB,
- sampling frequency 1500 Hz,
- gain500.
Statistical Analysis
The slopes representing the subjects’ fatigue were analyzed with ANOVA with repeated measures. The two groups (short track and control) constituted the between-subject factor, and the two sides (left leg and right leg) constituted the within-subject factor. Since the interaction was significant, Tukey’s post hoc tests were applied for pair-wise comparisons of the four factor combinations. For the analyses, a 0.05 significance level was used. All analyses were made in Statistica v. 13.1.