2.5. Physical Performance Measurements

Physical performance was assessed by determining muscular strength (absolute and relative hand grip strength test), muscular endurance (curl-up test), and aerobic capacity (20-m Progressive Aerobic Cardiovascular Endurance Run: 20-m PACER test, estimated maximal oxygen consumption: VO_2max). The National Fitness Award project test items for Korean adolescents showed a high consistency, with reliability ranging from 0.87 to 0.99 [21]. All physical performance assessments were conducted by trained experts, and the detailed evaluation methods are as follows:

Muscular strength

To assess the absolute hand grip strength (kg, %), the participants first extended both feet shoulder-width apart in an upright position. A hand dynamometer (GRIP-D 5101, Takei, Niigata, Japan) was adjusted to the second finger of the participants. Then, the arms of the participant were straightened down and kept 15° apart from the torso. At the signal “start,” the participant exerted maximum strength to hold the hand dynamometer for 5 s. The maximum value was recorded to the nearest 0.1 kg for the left then right hand. The relative hand grip strength (kg, %) was calculated using the formula: (absolute hand grip strength (kg)/body weight (kg)) × 100.

Muscular endurance

The curl-up test (number of times) was performed to assess muscular endurance. First, the participant was instructed to lay down with their knees bent, and their feet fixed to the floor about 30 cm away from their hips. At the signal “start,” the participant extended their arms forward until their fingertips touched their knees, then returned to the starting position. This movement was regarded as one curl-up. Each participant repeated this process. The number of curl-ups was measured and recorded.

Aerobic capacity

To perform the 20 m PACER test (number of times), the experts divided each lane into a 20 m course and drew a line with tape at both ends. Participants were required to run the 20 m back and forth across the marked track, keeping time with beeps. The “start” beep was signaled 5 s after the “ready” command, at which point the participants started to run the 20 m. If they reached the relevant line before the beep, they had to wait until the second beep before running toward the end of the opposite line. If participants did not reach the relevant line before the first beep, they would run in the opposite direction at the second beep. They were eliminated if they could not reach the relevant line before the second beep. In this way, participants must continue until the line is not reached before the second beep, and the eliminator must stay clear of the line. The maximum number of repetitions was recorded.

Another effective aerobic capacity measurement index is maximal oxygen uptake, VO_2max. However, it is costly and hard to measure VO_2max directly. Instead, VO_2max was estimated by the 20 m PACER using the quadratic model developed by Mahar et al. [22] to estimate VO_2max (mL/kg·min) in adolescents.