Pre- and Post-Test Evaluation of a Periodized Off-Season Training Program in Professional Footballers

This study examined the physiological and performance adaptations of association football (soccer) players during a six-week transitional (off-season) period following the competitive season through a remotely supervised, periodized training program. Fifteen male players (19.57 ± 1.14 years; training experience: 13.60 ± 1.81 years) from the Greek Super League 2 completed pre- and post-intervention laboratory assessments, including anthropometry, cardiopulmonary function, isokinetic strength, and jump performance. The program integrated high-intensity interval training, aerobic conditioning, and individualized resistance training, adjusted according to test results. Anthropometric parameters remained stable. Maximal oxygen uptake (VO2max) increased significantly by 2.8% (56.31 ± 3.87 vs. 57.91 ± 3.02 mL/kg/min), while anaerobic threshold time and velocity declined by 6.2% (472.87 ± 35.06 vs. 443.33 ± 24.69 sec) and anaerobic threshold velocity fell by 6.1% (15.97 ± 1.17 vs. 15.00 ± 0.91 km/h), indicating a partial preservation of aerobic capacity but reductions in submaximal endurance. Isokinetic strength of the quadriceps and hamstrings was maintained across angular velocities, but squat jump height decreased significantly by 4.3% (p = 0.033), suggesting a loss of concentric-only explosive power despite stable countermovement jump performance. These findings indicate that targeted off-season training can maintain overall physical readiness, body composition, and VO2max, but may require additional endurance and concentric power elements to preserve all performance qualities essential for the competitive season. Overall, the program effectively preserved most performance variables with only minor decrements, representing a favorable outcome for the off-season; however, if greater asymmetries, deficits, or other training targets are identified, more specific adjustments to training intensities, such as the inclusion of additional endurance and concentric-only power elements, may be required to achieve significant improvements.