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

Abstract

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.

1. Introduction

Professional football is defined by sustained high-intensity physical demands throughout the year, with multiple energy systems being continuously recruited [1]. In this context, the ability to maintain and improve key physical qualities—such as maximal oxygen uptake (VO₂max), sprint capacity, agility, and muscular strength—is vital for preserving optimal performance across the competitive season [1,2,3].

The “off-season” period, which follows the conclusion of the competitive phase, represents a strategically important window not only for physiological recovery but also for preventing adaptation losses caused by training cessation [1]. If this phase is not properly structured, players may experience significant declines in performance levels. In the literature, training losses during the off-season are typically described in two stages: complete training cessation and a period of low-to-moderate-intensity individual training [4]. In both cases, a reduction in training load can lead to the onset of detraining (DT) effects.

A reduction or complete cessation of training during the off-season can lead to noticeable physiological declines in athletes within a short period. Research has shown that even just 2–4 weeks of reduced activity can result in significant decreases in VO₂max, diminished repeated sprint ability, and losses in lower-limb muscular strength [5,6]. In addition to such short-term DT effects, extended periods exceeding four weeks—referred to as long-term detraining—have been associated with more pronounced reductions in endurance and muscular strength [4]. These performance regressions, commonly observed in football, are directly linked to the inability to maintain neuromuscular adaptations and the insufficient stimulation of the anaerobic energy system [7].

The physiological changes that occur during this period are known to negatively impact not only performance but also injury risk. In particular, a decline in VO₂max, when combined with insufficient recovery capacity, can hinder an athlete’s ability to readapt to the demands of competitive play [1]. Indeed, a rapid increase in training volume and intensity at the start of the season may exacerbate this lack of adaptation, thereby elevating the risk of injury [4,8]. Therefore, the off-season should be considered not only a time for rest but also a preparatory phase for gradually reintroducing training loads.

Hamstring and adductor muscle group injuries are among the most commonly reported during the off-season period [9,10]. Insufficient strength levels and limitations in range of motion have also been associated with these types of injuries [11,12]. Such declines are critical, as entering pre-season with reduced performance capacity may compromise players’ ability to withstand the elevated training demands of this phase, thereby heightening the risk of overreaching and injury. According to Gabbett’s [8] training–injury prevention paradigm, the appropriately graded prescription of high training loads can enhance fitness and act as a protective factor against injury. Therefore, the assessment of neuromuscular performance, functional strength testing, and load monitoring are of great importance during this phase.

A characteristic of the off-season period is that many athletes reduce or discontinue structured training. This insufficient stimulation of the energy systems and limited activation of the musculoskeletal system often lead to a decline in key conditioning parameters. In a previous study [13] reported significant reductions in aerobic endurance, in semiprofessional footballers during this period. Moreover, hormonal changes—such as decreased testosterone levels and elevated cortisol concentrations—have also been observed, and these biological alterations have been linked to stress responses associated with the cessation of training [14,15].

Research indicates that when physical activity drops to minimal levels during the summer break, athletes typically require a reloading period of 4 to 8 weeks to return to their pre-training performance levels [13]. This highlights that the pre-season should not be viewed merely as a “preparatory phase”, but also as a critical window for injury prevention strategies. Accordingly, the literature emphasizes the importance of maintaining a minimum level of fitness during the off-season in order to mitigate the effects of abrupt training load increases and reduce the risk of load-related injuries in the early pre-season phase [4].

Off-season practices supported by planned and periodized training protocols have the potential to prevent such physiological declines. In this context, high-intensity interval training (HIIT) has been shown to elicit significant improvements in VO₂max, repeated sprint performance, and time-to-exhaustion tests—even with relatively short training durations [16,17]. Due to its time efficiency, HIIT is recommended as an alternative to traditional endurance training during the off-season [18]. Similarly, resistance training performed in-season contributes to sustained performance by enhancing muscular strength and endurance [19,20]. Such protocols are also employed to manage adverse changes in body composition, such as increases in fat mass and losses in lean muscle mass [4].

Equally important to maintaining performance is understanding the individual responses of players to training loads. For this reason, global positioning system (GPS) data, rating of perceived exertion (RPE) scores, countermovement jump tests, functional strength assessments such as the Nordic curl, and measurements of hip adduction/abduction strength all play a critical role in individualizing training programs based on players’ performance profiles and in optimizing load management strategies [21,22].

Nonetheless, a notable gap in the literature remains. There are a limited number of structured studies employing a pre-test → off-season training → post-test design that comprehensively assesses multidimensional motor qualities—such as aerobic power, anaerobic capacity, agility, and strength—through ergometric testing [1,13]. Moreover, the lack of direct comparisons between groups undergoing off-season training programs and those remaining completely detrained leaves a significant gap in understanding the full extent of detraining effects [4].

The primary aim of this study is to evaluate the physiological adaptations occurring in professional footballers between the end of the competitive season and the off-season period and to analyze the effects of a periodized training protocol delivered via remote monitoring. In this context, the effects of the training intervention were objectively measured through ergometric performance tests, aiming to contribute to the development of evidence-based strategies applicable in high-performance football settings.

2. Materials and Methods

2.1. Study Design

Following the completion of the team’s final official match, the players maintained team-based training for approximately 40 days. Within three days after the competitive season concluded, they attended the laboratory to undergo a comprehensive ergometric testing battery, which included assessments of anthropometric characteristics, aerobic capacity, jump performance, and lower-limb isokinetic torque. Tests were performed in the morning (08:00–12:00) under standardized laboratory conditions (temperature ~20 °C; relative humidity 40–55%)

During the first three days of the pre-season preparation period, the same testing battery was administered to quantify changes that had occurred during the off-season. In this study, the period between the two testing sessions, lasting approximately six weeks, was defined as the off-season phase.

The first 10 days of the off-season were devoted to passive rest, followed by seven days of active recovery, during which the players participated in recreational sporting activities. Over the subsequent three weeks, they engaged in individualized training programs performed independently. These programs were designed to preserve physical conditioning, address previously identified performance deficits, and optimize readiness for the upcoming pre-season. Targeted strength training interventions aimed at correcting lower-limb muscular imbalances were also incorporated (Figure 1).

2.2. Participants

The study included a total of 15 professional male football players competing in the “Greek Super League 2,” a second-tier professional league. Inclusion criteria comprised: absence of any injury for at least one month before the initial assessment and throughout the transition period, no use of medication, attendance at a minimum of 90% of the prescribed individual training sessions—ensuring consistent exposure to the intervention—and refraining from participation in any other sporting activities during the intervention period. The participants had a mean age of 19.57 ± 1.14 years and a mean training experience of 13.60 ± 1.81 years. Players represented five outfield positions: central defenders (CD; n = 2, 13.3%), central midfielders (CM; n = 3, 20.0%), side/full-backs (SD; n = 5, 33.3%), side midfielders/wingers (SM; n = 3, 20.0%), and forwards (F; n = 2, 13.3%); the sample comprised outfield players only.

All participants and their legal guardians were fully informed about the study procedures and provided written informed consent. The study was conducted according to the principles of the Declaration of Helsinki and received approval from the institutional ethics committee.

2.3. Anthropometric Measurements

Body weight was measured to the nearest 0.1 kg using a TANITA DC-360 digital scale (Tanita Corporation, Tokyo, Japan). Body fat percentage was estimated with the same device using bioelectrical impedance analysis. Standing height was measured to the nearest 0.1 cm using a Seca 220e stadiometer (Seca, Hamburg, Germany).

2.4. Isokinetic Strength Testing

Isokinetic strength of the hamstrings and quadriceps was measured using a Cybex II dynamometer (Lumex Inc., Ronkonkoma, NY, USA). For each angular velocity, the highest torque value was recorded after correcting for limb mass and equipment resistance.

Participants sat in an adjustable chair with straps across the trunk, hips, and thighs to limit extra movement. The tested limb was positioned at 90° knee flexion (0° = full extension). The knee joint axis was aligned with the dynamometer’s lever arm at the distal lateral femoral condyle. Lever arm length and pad placement were adjusted individually so the pad rested just above the medial malleolus.

The protocol included knee extensions that started from 90° of flexion and knee flexions that started from full extension. Each movement was performed through the full range of motion with maximal effort, as quickly and forcefully as possible, while the arms were crossed over the chest. At each velocity, participants completed three maximal repetitions. The highest peak torque was used for analysis. Rest periods were 30 s between repetitions and 1 min between changes in velocity. Standard verbal encouragement was provided.

Testing was performed at angular velocities of 60°/s, 180°/s, and 300°/s. Peak isokinetic torque was the highest torque recorded across the full range of motion. The hamstring-to-quadriceps (H:Q) ratio was calculated by dividing peak concentric hamstring torque by peak concentric quadriceps torque, following the method of Mandroukas et al. [23].

2.5. Laboratory VO₂max Measurement

Maximal oxygen uptake (VO₂max) was measured on a motorized treadmill (Pulsar; h/p/Cosmos, Nussdorf-Traunstein, Germany) using a continuous incremental protocol. The test started at 8 km/h with a 0% gradient. Speed increased by 2 km/h every two minutes until 12 km/h. From 14 km/h, the gradient was set to 2%. Speed then rose by 1 km/h each minute until the participant reached exhaustion.

Oxygen uptake and breathing variables were measured with a Quark cardiopulmonary exercise testing (CPET) system (COSMED, Rome, Italy) in breath-by-breath mode. The gas analyzer was calibrated before each test using a 2 L syringe and reference gases. VO₂max was the highest value recorded over at least five consecutive breaths in steady-state conditions. Heart rate was tracked continuously with a Polar Team Pro device (10 Hz, Polar electro, Kempele, Finland).

Validity followed the criteria of Mortensen et al. [24] and Kenney, Wilmore, and Costill [25]. A test was valid if the final-minute heart rate exceeded 95% of the predicted maximum (220-age), a VO₂ plateau occurred despite higher workload (ΔVO₂ < 150 mL), the respiratory exchange ratio was ≥ 1.10, and the participant stopped the test despite encouragement.

2.6. Countermovement Jump and Squat Jump

Vertical jump performance was evaluated using the countermovement jump (CMJ) and the squat jump (SJ). Each test was performed twice, with the highest value retained for analysis. In the CMJ, athletes began from an upright stance with their hands placed on the hips, executed a self-selected countermovement to a comfortable depth, and then performed a maximal vertical leap. In the SJ, participants started from a static squat position at approximately 90° of knee flexion, avoiding any preparatory movement, and attempted to achieve maximal jump height.

Invalid trials were repeated after a one-minute rest period. Between repeated attempts, a recovery period of 30 s was allowed, while three minutes of rest were provided between the two jump protocols. All measurements were obtained using the OptoJump optical system (Microgate, Bolzano, Italy), an instrument with demonstrated strong concurrent validity and high test–retest reliability for estimating vertical jump height [26].

2.7. Transitional Phase Training Program

The transitional phase training program focused on the development of aerobic capacity and muscular strength. Following the conclusion of the competitive season, the players first completed a 10-day period of passive rest, which was followed by 7 days of recreational activities and alternative sports, an approach reported in the literature to support psychological regeneration and maintain overall physical activity [1,27]. During the subsequent three weeks, they undertook a total of 14 training sessions, comprising seven strength-oriented sessions and twelve aerobic running sessions. On strength training days, low- to moderate-intensity aerobic running was also integrated to promote balanced adaptations across energy systems. The overall structure and specific training prescriptions are described in detail below.

The program followed a combined structure, integrating field-based aerobic conditioning and high-intensity interval training with individualized strength training. Aerobic training sessions were prescribed at intensities determined by percentage of maximal heart rate (HRmax) and included continuous running, long intervals, short intervals, and mixed fartlek protocols. This progressive approach aimed to stimulate VO₂max and enhance aerobic power capacity.

Strength training targeted both upper- and lower-limb muscle groups, focusing on the development of maximal strength, explosive power, and core stability. Lower-limb exercises were individualized according to the athletes’ ergometric and isokinetic test results. Players identified with significant inter-limb strength asymmetries in testing were prescribed an additional 1–2 unilateral sets on the weaker limb. This approach was employed to enhance strength symmetry, improve joint stability, and reduce the risk of injury.

Strength training interventions were individualized based on data obtained from ergometric and isokinetic assessments. In key multi-joint free-weight exercises—such as the front squat, landmine press, and dumbbell row—one-repetition maximum (1RM) tests were administered to determine the players’ maximal strength capacity.

The 1RM tests were conducted following standardized protocols in which the load was progressively increased until the participant could complete only a single repetition with proper technique. Adequate rest periods of 3–5 min were provided between attempts [28,29]. Experienced strength and conditioning coaches supervised all tests to ensure both safety and correct lifting technique.

In situations where direct 1RM testing was not feasible, submaximal testing was performed using loads corresponding to approximately 70–85% of the estimated 1RM. The 1RM was then calculated from the maximal number of repetitions completed using validated prediction equations such as the Epley and Brzycki formulae [30].

An overview of the off-season program structure is presented in Figure 2, while the detailed content of the strength training program is illustrated in Figure 3.

2.8. Statistical Analysis

A priori power analysis was undertaken using G*Power software (version 3.1; [31,32]) to establish the minimum sample size necessary to detect significant within-subject differences via a two-tailed paired t-test. The calculation assumed a large, expected effect size (d = 0.80; [33]), an alpha level of 0.05, and a desired statistical power of 0.80. This analysis indicated that at least 12 participants were required; the final cohort comprised 15, thereby exceeding the calculated threshold.

All statistical procedures were conducted using JASP (version 0.19.3; [34]), Jamovi (version 2.6; [35]), and IBM SPSS Statistics (version 29.0.2.0; [36]). For each primary outcome, descriptive statistics—including means, standard deviations, and 95% confidence intervals—were computed. The assumption of normality was evaluated using the Shapiro–Wilk test, supported by visual inspection of histograms and Q–Q plots. Paired t-tests were employed to assess within-subject changes. Effect sizes were expressed as Cohen′s d [33] and interpreted using modified thresholds: small (<0.20), moderate (0.20–0.79), and large (≥0.80). Statistical significance was accepted at p < 0.05.

3. Results

All 15 participants completed the study with a mean adherence rate of 97%, surpassing the predefined inclusion threshold of 90%.

Table 1. Baseline and Post-Program Anthropometric and Physiological Indicators with Statistical Comparisons.

Cohen’s d	p	t	Change (%)	Post (Mean ± SD)	Pre (Mean ± SD)	Variable
−0.540	0.055	−2.092	0.6	1.78 ± 0.05	1.77 ± 0.05	Height (m)
0.196	0.460	0.760	−0.6	72.21 ± 5.71	72.68 ± 6.44	Body weight (kg)
0.323	0.232	1.249	−1.0	22.89 ± 1.29	23.13 ± 1.54	BMI
0.408	0.136	1.582	−8.6	8.09 ± 1.99	8.85 ± 2.31	Body Fat (%)
0.084	0.750	0.325	−0.3	63.01 ± 4.32	63.19 ± 4.62	Muscle mass (kg)
0.351	0.195	1.360	−9.2	5.90 ± 1.77	6.50 ± 2.58	Fat mass (kg)

BMI = body mass index.

presents the pre- and post-intervention descriptive statistics for the participants’ anthropometric and physiological characteristics, along with the results of paired-samples t-tests and corresponding effect sizes (Cohen’s d). No statistically significant changes were observed in body weight, body mass index, muscle mass, and fat mass throughout the transitional phase. Notably, body fat percentage demonstrated a small, nonsignificant decrease over time. These findings suggest that the implemented intervention program effectively maintained the participants’ anthropometric and physiological profile during the transitional phase, with a modest, yet statistically nonsignificant, improvement in body composition indicators.

Table 2. Descriptive and Comparative Analysis of Cardiopulmonary Function Pre- and Post-Intervention.

Cohen’s d	p	t	Change (%)	Post (Mean ± SD)	Pre (Mean ± SD)	Variable
−0.435	0.114	−1.686	6.7	60.13 ± 8.12	56.33 ± 8.09	Resting HR (bpm)
0.649	0.025 *	2.513	−3.9	118.9 ± 6.1	123.7 ± 10.3	Systolic Blood Pressure (mmHg)
0.249	0.351	0.965	−1.9	62.7 ± 5.7	63.9 ± 5.8	Diastolic Blood Pressure (mmHg)
0.969	0.002 *	3.754	−6.2	443.33 ± 24.69	472.87 ± 35.06	Anaerobic Threshold Time (sec)
−0.301	0.264	−1.164	0.7	180.27 ± 5.09	178.93 ± 5.19	Anaerobic Threshold HR (bpm)
0.959	0.002 *	3.713	−6.1	15.00 ± 0.91	15.97 ± 1.17	Anaerobic Threshold Velocity (km/h)
−0.452	0.102	−1.751	3.4	50.91 ± 3.25	49.22 ± 3.75	Anaerobic Threshold VO2max (ml/kg/min)
0.310	0.249	1.202	−1.5	546.73 ± 25.96	555.07 ± 31.08	Maximal Exercise Time (sec)
−0.394	0.149	−1.527	1.3	195.27 ± 7.40	192.80 ± 6.65	Maximal HR (bpm)
0.422	0.125	1.633	−2.1	18.37 ± 0.83	18.77 ± 1.03	Maximal Velocity (km/h)
−0.585	0.040 *	−2.266	2.8	57.91 ± 3.02	56.31 ± 3.87	VO2max (ml/kg/min)
−0.488	0.080	−1.889	2.3	4171.67 ± 267.16	4079.07 ± 325.84	VO2max (ml/min)
−0.183	0.491	−0.708	0.9	1.18 ± 0.02	1.17 ± 0.03	Respiratory Exchange Ratio (RER)

* p < 0.05; HR = heart rate; VO₂max = maximal oxygen uptake.

presents the descriptive statistics and paired-samples t-test results for cardiovascular and cardiorespiratory variables at pre- and post-intervention assessments conducted during the transitional (off-season) phase. A statistically significant reduction in systolic blood pressure was observed. Furthermore, significant decreases in anaerobic threshold time and anaerobic threshold velocity were observed, as determined with the V-slope method for ventilatory threshold assessment, indicating a reduction in submaximal aerobic performance despite the implementation of a structured training program. Conversely, VO₂max (ml/kg/min) demonstrated a small but statistically significant improvement from pre- to post-intervention, suggesting a partial enhancement or maintenance of VO₂max (Figure 4). No significant differences were noted in resting heart rate, diastolic blood pressure, maximal exercise time, maximal velocity, or respiratory exchange ratio (p > 0.05), indicating that while certain cardiovascular markers improved—likely in response to the training stimulus—elements of submaximal performance may have been insufficiently targeted or deconditioned due to the specific design and load characteristics of the off-season training program.

Table 3. Descriptive and Comparative Analysis of Isokinetic Strength and Jumping Ability Pre- and Post-Intervention.

Cohen’s d	p	t	Change (%)	Post (Mean ± SD)	Pre (Mean ± SD)	Variable
−0.275	0.305	−1.064	2.8	264.40 ± 34.26	257.13 ± 35.47	Right Knee Extensors 60°/s (Nm)
0.075	0.777	0.289	−0.5	192.20 ± 17.13	193.07 ± 16.84	Right Knee Extensors 180°/s (Nm)
−0.060	0.820	−0.232	0.4	150.40 ± 15.05	149.80 ± 14.39	Right Knee Extensors 300°/s (Nm)
−0.057	0.830	−0.219	0.7	163.53 ± 25.14	162.40 ± 25.97	Right Knee Flexors 60°/s (Nm)
0.295	0.273	1.141	−3.3	126.27 ± 15.36	130.53 ± 20.36	Right Knee Flexors 180°/s (Nm)
−0.093	0.723	−0.362	1.3	104.40 ± 16.94	103.07 ± 14.18	Right Knee Flexors 300°/s (Nm)
0.149	0.573	0.577	−1.7	245.13 ± 35.85	249.47 ± 44.36	Left Knee Extensors 60°/s (Nm)
0.170	0.520	0.659	−1.8	184.33 ± 26.26	187.80 ± 27.72	Left Knee Extensors 180°/s (Nm)
0.080	0.761	0.311	−0.7	144.73 ± 20.05	145.80 ± 20.12	Left Knee Extensors 300°/s (Nm)
−0.300	0.265	−1.161	2.9	157.07 ± 20.96	152.60 ± 22.47	Left Knee Flexors 60°/s (Nm)
−0.098	0.710	−0.380	0.9	125.80 ± 20.05	124.67 ± 19.00	Left Knee Flexors 180°/s (Nm)
−0.006	0.983	−0.022	0.1	101.60 ± 17.24	101.53 ± 19.92	Left Knee Flexors 300°/s (Nm)
0.23	0.393	0.881	−2.1	61.95 ± 6.12	63.25 ± 6.31	Right Knee H/Q Ratio at 60°/s (Nm)
0.26	0.323	1.024	−2.5	65.71 ± 5.61	67.42 ± 7.02	Right Knee H/Q Ratio at 180°/s (Nm)
−0.07	0.794	−0.266	1.0	69.50 ± 9.72	68.83 ± 6.88	Right Knee H/Q Ratio at 300°/s (Nm)
−0.47	0.088	−1.831	4.2	64.60 ± 8.13	61.97 ± 8.80	Left Knee H/Q Ratio at 60°/s (Nm)
−0.30	0.262	−1.168	3.1	68.94 ± 11.34	66.85 ± 8.33	Left Knee H/Q Ratio at 180°/s (Nm)
−0.08	0.757	−0.315	1.3	70.77 ± 11.29	69.89 ± 11.80	Left Knee H/Q Ratio at 300°/s (Nm)
−0.065	0.804	−0.252	0.4	41.91 ± 3.97	41.74 ± 4.14	CMJ (cm)
0.609	0.033 *	2.358	−4.3	37.69 ± 4.87	39.40 ± 4.93	SJ (cm)

* p < 0.05; CMJ = countermovement jump; SJ = squat jump; SD = standard deviation.

presents the descriptive statistics and paired-samples t-test results for isokinetic strength and jump performance assessments conducted before and after the intervention during the transitional (off-season) phase.

For the right lower limb, a small numerical increase was observed in knee extensor torque at 60°/s (from 257.13 ± 35.47 Nm to 264.40 ± 34.26 Nm), as well as at 300°/s (from 149.80 ± 14.39 Nm to 150.40 ± 15.05 Nm); however, neither change reached statistical significance (p > 0.05). Right knee flexor strength showed minor reductions at 180°/s and 300°/s, but these were also non-significant.

For the left lower limb, knee extensor torque at 60°/s decreased slightly from 249.47 ± 44.36 Nm to 245.13 ± 35.85 Nm, with similarly small non-significant reductions at 180°/s (from 187.80 ± 27.72 Nm to 184.33 ± 26.26 Nm) and 300°/s (from 145.80 ± 20.12 Nm to 144.73 ± 20.05 Nm). Conversely, left knee flexor torque at 60°/s increased modestly from 152.60 ± 22.47 Nm to 157.07 ± 20.96 Nm, with small, non-significant improvements also observed at 180°/s and 300°/s.

In terms of jump performance, countermovement jump (CMJ) height increased slightly from 41.74 ± 4.14 cm to 41.91 ± 3.97 cm, though this change was not statistically significant (Figure 5). In contrast, squat jump (SJ) height demonstrated a statistically significant reduction from 39.40 ± 4.93 cm to 37.69 ± 4.87 cm (d = 0.609), suggesting a decline in explosive concentric lower-limb power during the off-season period (Figure 6).

4. Discussion

This study investigated the physiological and performance adaptations of professional football players during a six-week transitional (off-season) period following the competitive season, implemented through a remotely supervised, periodized training program. The program effectively preserved the players’ anthropometric profile, with only minimal changes in body composition, while cardiorespiratory and neuromuscular outcomes showed mixed responses.

Throughout the transitional period, anthropometric parameters, including body mass, body mass index, muscle mass, and fat mass, showed no statistically significant changes. A small, non-significant reduction in body fat percentage was recorded, suggesting that the intervention was successful in preventing the typical increases in fat mass reported during detraining phases [4,13]. Previous research has consistently shown that unstructured off-season periods are often associated with unfavorable changes in body composition, particularly increases in adiposity and reductions in lean tissue [1,5]. In the present study, the combined use of resistance training and aerobic conditioning appears to have contributed to the preservation of muscle mass and prevention of excessive fat accumulation.

Cardiorespiratory outcomes revealed a notable divergence between maximal and submaximal aerobic measures. VO₂max showed a small but significant increase, consistent with evidence that high-intensity interval training (HIIT) enhances central cardiovascular adaptations (e.g., stroke volume and cardiac output) that support maximal oxygen uptake [17,18,37]. In contrast, anaerobic threshold time and velocity declined, likely due to insufficient peripheral adaptations—such as capillarization and mitochondrial enzyme activity—that are better sustained by continuous or tempo-based running [38]. Consequently, while maximal aerobic power was maintained, submaximal endurance capacity diminished, a change that may impair match fitness early in the competitive season when prolonged moderate-intensity efforts are required. This pattern aligns with previous findings in adolescent soccer players, where HIIT mitigated but did not fully prevent declines in intermittent endurance [39].

Neuromuscular performance and isokinetic strength outcomes indicated no statistically significant changes in quadriceps or hamstring strength across angular velocities, suggesting that the strength training component of the program was effective in preventing detraining-induced losses. The maintenance of muscle function during the off-season is particularly important given the well-established association between reduced eccentric hamstring strength and increased risk of hamstring injury [11,12]. The inclusion of individualized unilateral work for players with pronounced inter-limb asymmetries may have contributed to this stability. Concerning explosive power, CMJ height was maintained; however, SJ height declined significantly. This reduction in SJ performance—despite stable CMJ values—may reflect a decrease in concentric-only explosive strength, potentially due to reduced neuromuscular drive or insufficient concentric-specific plyometric stimulus within the program [40]. These findings suggest that, while stretch–shortening cycle efficiency was preserved, pure concentric power capacity may require targeted intervention during transitional periods. However, in a previous study conducted on adolescent soccer players, a five-week intervention was followed by improvements in performance across all evaluated jump tests (drop jump, SJ, and CMJ) [39]. Similar outcomes were observed in the study of Michailidis et al. [41], where overall muscle strength was preserved during a six-week off-season intervention, yet concentric-specific power showed declines.

These findings are consistent with the review by Van Hooren and Zolotarjova [42], who noted that the performance advantage of the CMJ over the SJ is mainly explained by stretch–shortening cycle (SSC) utilization, elastic energy storage, and optimized muscle–tendon prestress. They also highlighted that a greater CMJ–SJ difference can indicate increased muscle slack or reduced concentric force generation capacity, rather than superior performance. In our context, the preserved CMJ alongside the decline in SJ suggests that SSC efficiency was maintained, whereas concentric-only power was compromised, reinforcing the need for concentric-specific training stimuli during transitional phases. Building on these findings, inter-limb asymmetries also warrant attention, as they are closely linked to both injury risk and performance limitations in football. Recent evidence in elite youth players [43] has demonstrated significant associations between single-leg countermovement jump asymmetries and isokinetic strength imbalances, particularly at lower angular velocities, underscoring the importance of identifying residual asymmetries to guide individualized training prescriptions, with a particular emphasis on concentric-specific or unilateral strength interventions.

Compared with studies of longer off-season durations, adaptations appear time- and content-dependent. For example, protocols emphasizing prolonged tempo/interval volumes preserved or improved submaximal markers to a greater extent [44], whereas our program prioritized HIIT and produced small VO₂max gains but declines in the anaerobic threshold. Moreover, interference phenomena may explain the SJ performance decrease when concurrent endurance volume is relatively high; endurance-dominant programming can blunt strength–power adaptations [45], aligning with the selective reduction in concentric power (SJ) despite stable CMJ.

From an applied standpoint, these findings underscore several considerations for practitioners when designing off-season training programs for elite footballers. First, the composition of the training load should be balanced to preserve both maximal oxygen uptake and submaximal endurance performance. Second, the decline in SJ performance suggests a need for concentric-specific power training to maintain full neuromuscular function. Finally, routine monitoring of body composition during the off-season remains important to ensure that fat mass is controlled and muscle mass is preserved, given their implications for both performance and injury prevention.

The outcomes of this study align with those reported by Ispirlidis et al. [13] and Clemente et al. [4], who emphasized the role of structured off-season programs in mitigating the effects of detraining. However, the observed reduction in anaerobic threshold parameters contrasts with findings from interventions placing greater emphasis on prolonged interval training [44], highlighting the critical role of training specificity in the preservation of different physiological qualities. The maintenance of isokinetic strength is consistent with prior research indicating that low-volume, high-intensity resistance training can attenuate strength losses during reduced-load phases [45].

A principal limitation is the absence of a control group, which constrains internal validity by limiting our ability to attribute the observed changes solely to the intervention rather than natural seasonal variation or unmeasured covariates. To contextualize the magnitude of our effects, we compared our results with studies using a control condition. For instance, Chatzinikolaou et al. [39] reported smaller decrements in intermittent endurance in the intervention group compared to the controls, suggesting that structured off-season programming can buffer detraining. Our findings are directionally consistent with that pattern (VO₂max maintained/increased; submaximal indices declined), yet the lack of a contemporaneous control in the present study warrants cautious interpretation. Another limitation is the relatively small sample size (n = 15). Although the a priori power analysis indicated that this number of participants was sufficient to detect meaningful within-subject changes, the limited cohort still constrains the generalizability of the findings to broader populations of professional footballers. Nutritional intake was also neither monitored nor standardized; players did not receive dietary recommendations during the off-season, and their food choices were left entirely to individual discretion. This constitutes a further limitation, as uncontrolled dietary variation may have influenced body composition and physiological adaptations. Future studies with larger samples and control conditions are therefore warranted to strengthen both internal and external validity. To preserve the performance variables maintained in the present study, and to enhance or recover those that declined, upcoming research could refine off-season training protocols by adjusting training intensities (e.g., varying load, volume, and frequency) and incorporating broader monitoring approaches, such as cognitive function, mental fatigue, sleep quality, nutritional intake, and psychosocial well-being, using validated metrics from the literature, thereby more comprehensively addressing the multidimensional demands of high-performance football.

5. Conclusions

This study demonstrates that a planned and individualized six-week transitional period training program can be effective in maintaining body composition and overall physical capacity in professional football players. Throughout the program, fat mass did not increase, muscle mass was preserved, and VO₂max was sustained. However, the observed reductions in specific endurance markers, along with the decline in explosive power, indicate that off-season training may not preserve all performance components to the same extent. This highlights the need for a transitional period aiming to integrate a broader spectrum of training loads and carefully targeted content. Overall, the findings suggest that well-structured off-season training programs can enhance the competitive readiness of professional footballers, enabling them to commence the season in an optimized physical condition.

From a practical perspective, coaches are encouraged to complement high-intensity interval training with sufficient submaximal running volumes to preserve lactate threshold performance and to integrate concentric-specific power exercises (e.g., squat jump variations, and concentric plyometrics) to prevent reductions in explosive strength. Although this study was conducted in professional players, the principles of the program can be adapted to semi-professional and amateur levels using simplified monitoring tools, making the approach broadly transferable across football contexts. Future research should explore how varying program duration and training load composition influence the balance between maximal and submaximal adaptations during the off-season.