The Acute Effects of Short- and Long-Duration CrossFit® Session on Tissue Damage, Autonomic Response, and Fatigue: A Crossover Study

<b>Background:</b> The main characteristics of CrossFit® are that it is constantly varied and of high-intensity. The planning of CrossFit® training sessions can be differentiated by modality, frequency, rest intervals, variation, and duration, which can elicit different responses. <b>Purpose:</b> The aim of the present study was to compare the acute effects of a short-duration (<i>Fran</i>) and long-duration (<i>Cindy</i>) session of CrossFit® on tissue damage (creatine kinase-CK), autonomic responses, and fatigue in experienced CrossFit® practitioners. <b>Methods:</b> Thirteen men (31.2 ± 3.2 years) with 5.72 ± 2 years of experience participated in a crossover study with a counterbalanced design, where they completed both the short-duration and long-duration CrossFit® sessions. Blood samples were collected to check tissue damage via CK, autonomic responses were verified through heart rate variability (HRV), and the fatigue by countermovement jump (CMJ) at the following moments: pre-workout, immediately after (IA), 24- and 48-hour after the training session. <b>Results:</b> A statistically significant increase in CK was observed for <i>Fran</i> at 24 hours versus pre-workout (<i>p</i> = .029), while for <i>Cindy</i> there was a statistically significant increase in CK IA (<i>p</i> = .002) and 24 hours versus at pre-workout (<i>p</i> = .002). HRV showed lower values (<i>p</i> < .05) IA for the time domain in <i>Fran</i> and for the frequency domain in both training sessions. HRV returned to pre-workout values at 24 hours. CMJ did not show significant differences between moments and training sessions. The present study showed that CK significantly increased from baseline just after the long-duration session and was increased 24 hours after both. HRV significantly decreased IA, except in the frequency domain for the shortest training session (<i>Fran</i>). CMJ did not change through 48 hours after training, regardless of the training session. <b>Conclusions:</b> Overall, these findings emphasize the importance of considering both the intensity and duration of CrossFit® workouts when assessing their impact on physiological responses and potential risks.