Anaerobic speed reserve and acute responses to a short-format high-intensity interval session in runners

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 × 15 s/15 s @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.