Hematological, Biochemical, and Performance Adaptations in Amateur Soccer Players Following a 4-Week Preseason Training Period.

<b>Background</b>: We examined changes in hematological, biochemical, and hormonal biomarkers, along with endurance and explosive performance indices, in amateur soccer players over a 4-week preseason period. <b>Methods</b>: Thirteen players (age: 19.7 ± 2.0 years; body mass: 73.0 ± 6.8 kg; height: 180 ± 0.1 cm; body fat: 8.6 ± 3.5%) were monitored during a 4-week preseason program, which included 21 training days, three friendly matches, and four days of rest. Before and after this period, endurance capacity was evaluated using the Yo-Yo IR1 test, and leg power was assessed using the CMJ. Blood samples were collected for three consecutive days in week 1 and after week 4 to assess hematological and biochemical parameters. Internal load during all weeks was assessed with session RPE (sRPE). <b>Results</b>: There was a 25.5% increase in Yo-Yo IR1 distance (2123 ± 413 vs. 1560 ± 356 m, <i>p</i> = 0.002), with the estimated VO₂max and the speed associated with VO₂max (vVO₂max) improving by 8.7% (49.5 ± 3.0 to 54.2 ± 3.5 mL/kg/min, <i>p</i> = 0.002) and 5.3% (16.0 ± 0.7 to 16.9 ± 0.6 km/h, <i>p</i> = 0.002), respectively. In contrast, CMJ performance in weeks 2-4 declined by 13.4-21.0% relative to baseline, while sRPE peaked during week 3 (4011 ± 440 AU). Hematological variables were mostly stable except for small increases in MCV and MCH (1.5-1.8%, <i>p</i> < 0.001), while there were significant reductions in urea (12%), uric acid (6.2%), and erythropoietin (33%). <b>Conclusions</b>: A 4-week preseason program substantially improved aerobic capacity yet compromised leg power. Changes in biomarker profiles suggest that the training load maintained an appropriate balance between overload and recovery. These findings provide valuable guidance for coaches seeking to optimize training protocols while minimizing the risk of overtraining and preventing injuries during the competitive season.