Objectives This study aimed to assess relationships of acute responses to short-format high-intensity interval training (HIIT) with the anaerobic speed reserve (ASR) of adolescent runners. Design Pre-post intervention design. Methods Eighteen highly-trained youth runners (15.83±0.86 years) underwent maximal sprinting speed (MSS) and maximal aerobic speed (MAS) assessments to determine ASR (MSS minus MAS) and a standardized HIIT protocol (2×(20×15s/15s @110% MAS)) was administered. Pre/post-HIIT assessments included biochemical (i.e., creatine kinase (CK)), neuromuscular (countermovement jump, CMJ; reactive strength index, RSI), cardiac (i.e., heart rate recovery (HRR)), and athlete-reported outcome measures (e.g., single item for fatigue). Pearson's r was calculated to assess relationships between acute responses and ASR, MSS, MAS, and relative intensity of the HIIT (%ASR). Results Athletes' ASR and %ASR were significantly associated with the pre/post difference of CK (r = -0.75; p<0.001; r=0.74; p<0.001, respectively), CMJ height, and RSI (r = 0.69; p = 0.002; r= -0.49; p=0.04, respectively). However, HRR did not correlate significantly with ASR or %ASR (r=0.37, p=0.131, r= - 0.31; p=0.22, respectively). The pre/post difference of RSI correlated with MAS (r= -0.54; p=0.02), and the pre/post difference of CK (r= -0.50; p=0.034) and of CMJ height (r=0.76; p<0.001) with MSS. Regarding athlete-reported measures, ASR and %ASR showed significant associations with most fatigue and recovery variables (r=0.57; p=0.014, r=0.57; p=0.013, respectively). The pre/post difference of the single item for fatigue showed a positive relationship with MSS (r=0.49; p=0.037). Conclusions Acute biochemical, neuromuscular, and athlete-reported responses to short-format HIIT showed strong relationships with ASR and MSS, indicating higher internal load in athletes with a lower ASR and MSS by using a higher %ASR, compared to athletes with a higher ASR and MSS. These findings can help to tailor training programs to individual needs and avoid possible overload. Practical implications • Coaches and practitioners should be aware that runners with a lower ASR (and MSS) show a higher acute internal (objective and athlete-reported) response to a high-intensity exercise, even when this exercise is normalized on their MAS but demanding a higher %ASR during the exercise compared to athletes with a higher ASR (and MSS). This can be of high relevance for athletes and coaches for explaining differences in acute responses and to adjust the training cycle and stimuli to best meet an athlete's requirements. • We recommend that a different form of short-format HIIT prescription (e.g. based on %ASR) might be more suitable for the athletes with a lower ASR and would overall lead to more homogenous responses in a group of athletes with different ASR types. • Higher volumes and lower intensities might be best for endurance-oriented runners or lower volumes and higher intensities speed-oriented runners.
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