Investigation of the acute effect of linear and direction changing high intensity interval training on reactive strength index and maximum lactate levels in U-17 elite male football players

The aim of this study is to comparatively examine the acute neuromuscular and metabolic effects of two different high-intensity interval training (HIIT) protocols (linear and change of direction) in football players. Football is a complex sport that involves change of direction, sudden stops, accelerations, and high-speed actions, requiring the effective use of both aerobic and anaerobic energy systems. Therefore, in training planning, it is crucial to individually evaluate sport-specific physiological and neuromotor parameters. 24 licensed male athletes from the Ispartaspor U-17 football team voluntarily participated in the study. The participants had an average age of 16.20 ± 0.77 years, no health problems, and regular training backgrounds. Within the scope of the study, each player performed two different HIIT sessions—one linear and one change of direction—on separate days. Before the sessions, baseline Reactive Strength Index (RSI) and Drop Jump tests were conducted. Following each training protocol, RSI testing and maximum blood lactate measurements were repeated. RSI data were recorded using the Smart Speed Drop Jump Test device, and RSI values were calculated based on ground contact time, flight time, and jump height. In addition, to individualize training intensity, each athlete's maximal aerobic speed (MAS) was determined using the 30-15 Intermittent Fitness Test (IFT), and training loads were adjusted accordingly. Data analysis was performed using IBM SPSS 26.0. Repeated Measures ANOVA was used to assess RSI and Drop Jump data, while Tukey post-hoc tests identified the source of significant differences. A Paired Samples t-test was applied to compare the effects of the two HIIT protocols. Statistical significance was set at p < 0.05. According to the results, a significant increase in RSI values was observed following the change of direction interval training (p < 0.05), whereas no significant change was found after the linear protocol. Moreover, post-exercise blood lactate levels were higher following the change of direction training compared to the linear protocol, indicating greater metabolic stress in the former. In conclusion, change of direction high-intensity interval training induces greater neuromuscular activation and higher metabolic load compared to linear training. These findings suggest that incorporating change of direction exercises into training programs may be a more effective strategy for enhancing neuromuscular adaptations and performance in football players.