To explore the effects of 8-week polarized training (POL), high-intensity interval training (HIIT), and threshold training (THR) interventions on the cardiorespiratory fitness (CRF) of untrained healthy young adults. This study recruited 36 young adults and randomly assigned them to POL, HIIT, THR, or control (CG) groups to undergo an 8-week training intervention. The training impulse applied to all three intervention groups was identical. The training intensity was divided into Zone 1, 2, and 3 (Z1, Z2 and Z3) on the basis of the ventilatory thresholds (VT). The weekly training intensity distribution for POL was 75% of Z1 and 25% of Z3; HIIT was 100% of Z3 and THR was 50% of Z1 and 50% of Z2. Each group underwent Bruce protocol testing and supramaximal testing before, during, and after the intervention; relevant CRF parameters were assessed. 8 weeks of POL and HIIT significantly increased VT₂ (p < 0.05); 8 weeks of POL, HIIT, THR and significantly increased VO_2max and TTE (p < 0.05). The effect size of POL in relation to VO_2max and TTE improvements was greater than that of HIIT and THR (g = 2.67 vs. 1.26 and 1.49; g = 2.75 vs. 2.05 and 1.60). Aerobic training models with different intensity distributions have different time effects on improving CRF. Relative to HIIT and THR, POL improved more variables of CRF. Therefore, POL is a feasible aerobic training method for improving CRF.

• The effect of POL on improving VO (12.9%) and time to exhaustion (12.0%) was greater than that of HIIT and THR.

• THR mainly comprises moderate-intensity exercises (Zones 1 and 2) and only improves aerobic metabolism (VT).

• Both POL and HIIT encompass Z3 exercises; this indicates that high-intensity training is conducive to enhancing the energy supply for aerobic metabolism and delaying complete reliance on anaerobic metabolism.