Abstract
This study aimed to compare the effects of individualized versus non-individualized HIIT programming, based on players' locomotor profiles, on the magnitude of adaptations in aerobic, anaerobic, and neuromuscular capacities. A randomized, controlled, parallel-group design was conducted with 46 male youth soccer players (age: 16.5 ± 0.5 years), who were allocated into four groups: individualized HIIT (HIITind), long-interval HIIT only (HIITlong), repeated sprint training only (RST), and a control group that maintained regular training without any HIIT intervention. In the HIITind group, players were assigned to either HIITlong or RST based on their locomotor profile - endurance or speed-oriented - determined by the difference between maximal sprint speed (MSS) and maximal aerobic speed (MAS), respectively. In contrast, players in the HIITlong and RST groups followed the same protocol regardless of their profile. The training intervention lasted six weeks, with sessions conducted twice per week. Players were assessed at baseline and post-intervention for countermovement jump (CMJ), MSS over 30 meters (km/h), repeated sprint ability (RSA), and MAS, using the 5-minute running test. Significant improvements were found in all training groups compared to the control. RST showed greater improvements in CMJ (p < 0.001), MSS (p < 0.001), anaerobic speed reserve (ASR) (p < 0.001), and RSAmean (p < 0.001) compared to HIITind and HIITlong. No significant differences were observed between HIITind and HIITlong. Locomotor profiles influenced MSS (p < 0.001) and ASR (p < 0.001). These findings suggest that while both individualized and non-individualized HIIT protocols improve physical capacities, RST offers superior benefits for anaerobic and neuromuscular adaptations, whereas both HIITind and long are more effective than RST for enhancing aerobic capacity, with no significant differences observed between them.