2.4. Measurements

The simulated taekwondo matches were conducted under the format used in Hong Kong competitions (two rounds of 2 min each). The competition area had an octagonal shape with a diameter of 8 m and a side length of 3.3 m. The matches were governed by qualified referees and the points were awarded according to the Olympic rules: 1 point for a valid punch to the trunk protector, 2 points for a valid kick to the trunk protector, 4 points for a valid turning kick to the trunk protector, 3 points for a valid kick to the head, 5 points for a valid turning kick to the head, and 1 point to the opponent for every penalty (known as gam-jeom). The remaining time and live match scores were shown adjacent to the competition area on iPad screens and could be observed by the participants during a match. The performance in the simulated match was evaluated in terms of the total score and winning ratio obtained by each participant.

In the main examination session, the participants were required to perform 30 s of all-out cycling (Excalibur Sports Cycling Ergometer, Lode, Groningen, The Netherlands), and all participants executed nonstop locomotion until the end of the exercise. Before the test, the seat was adjusted according to the height of participants, and participants were required to complete 1 min of warm-up. The load of the cycling ergometer was set at 0.090 kp/kg body weight. The lactate concentration was measured within 1 min after the Wingate test using one portable lactate analyzer (Lactate Plus Meter, Nova Biomedical Corp., Waltham, MA, USA). The performance indicators included rate of fatigue, total peak and mean power, and total and mean power (wattage) per unit weight, which were recorded using a computer.

A battery of computer tests, including the Eriksen flanker test (EFT), Stroop test (ST), and rapid visual information processing test (RVIPT), were performed via a laptop computer to measure the cognitive function of the participants [36]. The order of these three tests was the same for all participants. The duration of each test was approximately 5 min, and both the accuracy, i.e., the proportion of correct responses made, and the reaction time (in ms) were used to assess the cognitive function performance. To avoid any potential learning effects, all participants were required to practice the cognitive function tests in the preliminary trial. In each main trial, participants were also asked to do several practical sessions before real tests.

In brief, the EFT assesses attention with two levels, congruent and incongruent. For this test, five arrows were shown on the computer screen in the same (congruent level) or different (incongruent) directions. Participants needed to select the arrow key pointing in the same direction as the central arrow. On both congruent and incongruent levels, the arrows were presented in green on a black background with a varied delay of 400 to 4000 ms. The arrows remained on the screen until the participant responded. The ST consisted of two levels of colors and word tests in which the target position was counterbalanced for the left and right side. The baseline level contained twenty stimuli in which the test word was placed in white in the center of the screen. The target and distractor were presented randomly on the left or right of the test word, and participants were required to respond as quickly as possible using the right or left arrow keys. The choices remained on the screen until the participants responded with an inter-stimulus interval of 1-sec. The complex level contained forty stimuli, where participants selected the color of the test words rather than the actual words. The RVIPT assesses sustained attention capacity. In this test, participants were asked to monitor consecutive sequenced numbers in a stream (100 digits per min) and tap the space bottom if three successive even (e.g., 3–5–7) or odd numbers appeared (e.g., 2–4–6). There were 8 target sequences per minute, and numbers 2–9 appeared on the screen for the duration at 600 ms intervals. Responses would be recorded within 1500 ms after the last digit sequence.

During the cognitive function tests, a multi-channel fNIRS system using two wavelengths of infrared light (751 and 839 nm) was adopted to scan cortical areas underlying the forehead for determining the prefrontal cortex (PFC) oxygenation levels of participants. The fNIRS (OctaMon System, Artinis Medical Systems, Gelderland, The Netherlands) device had two sets of 1 × 4 multichannel probe holders, which consisted of 8 light- emitting sources and 2 light-absorbing detectors arranged alternately at an inter-probe distance of 3.5 cm. This configuration thus resulted in 4 channels per set (a total of eight channels). Optode position was set and covered the participants’ prefrontal brain area according to the international 10–20 system with FPz located between optodes 3 and 6 (Figure 2). Through the application of light intensity on the differential path-length factor (DPF) and age-dependent DPF [37], the fNIRS device can calculate relative changes to baseline levels of H₂Ob concentration. Before each cognitive function tests were conducted, the data of baseline condition was collected and lasted 30 to 60 s. During the fNIRS data collection process, participants were instructed to sit quietly, breathe smoothly, and avoid head movement and swallow. The HR and blood pressure were not controlled.

Optodes position of fNIRS device. 1–8 are channel numbers.

A Polar HR sensor (Polar Vantage NV, Polar, Kempele, Finland) was used to monitor the HR of the participants throughout the experiment. A 0.5-μL capillary blood sample was obtained through a finger prick at different time points to measure the blood lactate concentrations by using a portable blood lactate analyzer (Lactate Plus, Nova Biomedical Corp., Waltham, MA, USA).

The RPE scale was used to measure the combination of all sensations and feelings of physical stress and general fatigue experienced by the participants during the different stages of the experiment [38]. The VAS-MP, which is a continuous scale, was used to measure muscle pain intensity [39].