Effects of Individualized Low-Intensity–Based Exercise Training on Metabolic Flexibility and Aerobic Performance

INTERVENTION: Behavioral : Participants will be randomly assigned to the RZ1, LT1, or POL group and will complete three supervised treadmill exercise sessions per week for eight weeks. Exercise intensity will be individualized based on the lactate threshold test conducted before the intervention. The RZ1 group will exercise at approximately 72% of the individual LT1 intensity, consisting of two 60‐minute sessions and one 90‐minute session per week. The LT1 group will exercise at the individual LT1 intensity using the same weekly frequency and duration. The POL group will perform a weekly alternating combination of low‐intensity exercise at RZ1 and high‐intensity interval training (HIIT). One weekly schedule will consist of one 60‐minute RZ1 session, one 90‐minute RZ1 session, and one HIIT session, whereas the alternate weekly schedule will consist of one 90‐minute RZ1 session and two HIIT sessions. Before HIIT, participants will perform 5–15 minutes of self‐selected low‐intensity warm‐up according to their individual condition and prescribed exercise intensity. This will be followed by five 4‐minute intervals at Zone 3, interspersed with four 3‐minute active‐recovery periods at RZ1 intensity, and a 5‐minute cool‐down. The prescribed HIIT duration, excluding the warm‐up, will be 37 minutes. The warm‐up period will not be included in the prescribed training‐volume calculation. Heart rate and rating of perceived exertion will be monitored during all exercise sessions. CONDITION: Not Applicable PRIMARY OUTCOME: Metabolic flexibility and aerobic performance: fat oxidation rate (FATox), carbohydrate oxidation rate (CHOox), respiratory exchange ratio (RER), relative energy contribution from fat and carbohydrate (%), running speed at Lactate Threshold 1 (LT1), running speed at Lactate Threshold 2 (LT2), and running speeds (km·h?¹) corresponding to blood lactate concentrations of 1.5, 2.0, 2.5, 3.0, 3.5, and 4.0 mmol·L?¹ derived from the graded lactate threshold test, as well as maximal oxygen uptake (VO2max) measured during the maximal exercise test SECONDARY OUTCOME: Absolute and relative contributions of the oxidative, glycolytic, and phosphagen energy systems calculated from oxygen uptake measured during and after the maximal oxygen uptake test and blood lactate data Heart‐rate responses measured during the lactate threshold and maximal oxygen uptake tests: heart rate at each stage of the lactate threshold test and exercise heart rate and peak heart rate during the maximal oxygen uptake test INCLUSION CRITERIA: Men aged 25 to 45 years Recreational runners who have performed regular running exercise for at least the previous 6 months Individuals who run at least three times per week and at least 15 km per week Individuals who are provisionally randomized to the RZ1, LT1, or POL group and meet the following group‐specific blood lactate response criterion during the baseline lactate threshold test: RZ1 and POL groups: blood lactate concentration must decrease from the resting level during exercise at the individualized RZ1 speed LT1 group: blood lactate concentration must decrease from the resting level or, if increased, the increase must be less than 0.2 mmol/L above the resting level Individuals who understand the purpose and procedures of the study and voluntarily provide written informed consent Individuals who are able to participate in the 8‐week exercise intervention and assessments at baseline (Pre), after 4 weeks (Post 1), and after 8 weeks (Post