VO2 kinetics and performance in soccer players after intense training and inactivity

Assessment of Aerobic Fitness and Repeated Sprint Ability in Elite Male Soccer: A Systematic Review of Test Protocols Used in Practice and Research

BACKGROUND

Soccer requires players to cover distances around 10-12 km, with numerous consecutive sprints throughout the 90-min game. As such, aerobic fitness and repeated sprint ability (RSA) are crucial physical qualities for the modern soccer player to cope with the demands of the game. However, a comprehensive and systematic search of aerobic fitness and RSA assessment procedures in elite soccer has yet to be conducted.

OBJECTIVES

The aims of this systematic review were to (1) identify the tests and outcome variables used to assess aerobic fitness and RSA of elite male soccer players, (2) provide normative values for the most common tests of aerobic fitness and RSA across different playing levels, and (3) report the reliability values of these aerobic fitness and RSA tests.

METHODS

A systematic review of the academic databases MEDLINE, CINAHL, SPORTDiscus, Web of Science, and OVID for studies published until August 2023 was conducted, following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Studies were eligible for inclusion if (1) they were original research studies, published in a peer-reviewed journal, and written in English language; (2) they had the primary aim of assessing aerobic fitness and/or RSA; (3) players were male and older than 17 years of age (i.e. mean age of the group); and (4) their playing level was defined as 'professional', 'international', or 'elite'.

RESULTS

For aerobic fitness testing, 124 studies and 35 different tests were identified. Of those, 26 tests (74%) were field-based, whereas only nine (26%) were laboratory-based tests. The incremental treadmill test to exhaustion was the most commonly used aerobic fitness assessment method (56 studies, 45%), with maximal oxygen consumption ( V ˙ O2max) (mL/kg/min) being the most prevalent outcome variable (49 studies, 87%). The YYIR1 and YYIR2 were also commonly used tests, identified in 22 (18%) and ten studies (8%), respectively. The most frequently reported outcome variable in both tests was distance in metres, reported in 20 studies (91%) for YYIR1 and in all ten studies (100%) for YYIR2. For RSA testing, 27 studies and 18 different tests were identified. Substantial variability in the identified RSA testing protocols was observed in terms of direction (linear vs. multidirectional), sprint repetitions (6-15), sprint distance (20-40 m), type of recovery (active vs. passive), and recovery duration (10-30 s). The 6 × 40-m shuttle sprint protocol with a 180° change of direction and 20 s passive recovery was the most common RSA test, employed in eight studies (29%).

CONCLUSIONS

This systematic review provides a comprehensive overview of the testing methods used to assess aerobic fitness and RSA in elite male soccer players. A total of 35 different aerobic fitness tests and 18 RSA tests were identified, highlighting the diversity in methodologies used. The most prevalent aerobic test was the incremental treadmill testing to exhaustion, with a median V ˙ O2max value of 58 mL/kg/min. Field-based tests were preferred due to their practicality, cost-efficiency, and ability to assess multiple athletes simultaneously. A substantial variability in RSA testing protocols was identified in terms of sprint directions, distances, repetitions, and recovery types. Future research should focus on establishing the diagnostic accuracy of the most commonly used aerobic fitness tests to inform their utility in practice and bridge the gap between current testing practices and optimal fitness evaluation.

Quantifying the Effects of Detraining on Female Basketball Players Using Physical Fitness Assessment Sensors

This study leverages physical fitness assessment sensors to investigate the effects of a brief in-season break (detraining period) on the physical performance of female basketball players. Sixty-seven players (Senior (n = 19), U18 (n = 19), and U14 (n = 29)) were evaluated before and after a 3-week break using sensor-derived data from a countermovement jump (CMJ), an Abalakov jump (ABK), a linear speed test (20 m sprint), a seated medicine ball throw test (SMBT), and a Basketball-Specific Agility Test (TEA-Basket). The Total Score of Athleticism (TSA), computed as the mean Z-Score across tests, served as a composite indicator of physical fitness. Data obtained from performance sensors revealed significant interactions between time and category for the CMJ, ABK, 20 m sprint, and SMBT, while TEA-Basket measurements showed no significant changes. Time and baseline fitness level interactions were also significant for the CMJ, ABK, and SMBT but not for sprint time or the TEA-Basket. Despite observed declines in explosive strength, speed, and upper-body power across all groups, TSA scores remained stable. These findings underscore the utility of sensor-based evaluation methods in highlighting the adverse effects of short-term detraining and emphasize the necessity of tailored training strategies during competitive breaks.

Safety and efficacy of aggressive exercise therapy for patients with unilateral early-stage spondylolysis: A retrospective comparative cohort study

INTRODUCTION

Bony union in patients with early-stage spondylolysis can be achieved using a rigid corset and avoiding sports activities. However, prolonged absence from sports activities may decrease physical fitness. This study aimed to investigate the safety and efficacy of aggressive exercise therapy for patients with early-stage spondylolysis (ESS).

METHODS

Thirty-one patients (aged <19 years; 26 males) diagnosed with unilateral ESS were divided into light intensity (L group; n = 16) and aggressive (A group; n = 15) exercise groups. All patients wore a hard corset and received light intensity (L group) or aggressive (A group) exercise therapy for 2-3 months. After treatment, the bony union was assessed using magnetic resonance imaging and computed tomography. The mean interval between the initial and second computed tomography examinations, including treatment drop-out rates, was determined.

RESULTS

Second computed tomography examination results showed that bony union was either improved or achieved in both groups. The mean interval between the initial and second computed tomography examinations was 78.1 ± 19.7 and 78.1 ± 17.4 days in the L and A groups, respectively, with no significant difference. The L group had a higher drop-out rate (n = 5; 31.3%) than the A group (n = 1; 6.7%), albeit not significant.

DISCUSSION

Both groups showed bony union after treatment, suggesting that aggressive exercise therapy was safe and effective for patients with unilateral ESS. Moreover, the lower drop-out rate in group A suggests that aggressive exercise therapy maintained the patients' adherence to long-term treatment.

CONCLUSIONS

Aggressive exercise therapy was effective-to-safe and maintained good motivation in patients with ESS.

Testing in Intermittent Sports-Importance for Training and Performance Optimization in Adult Athletes

ABSTRACT

Performance in intermittent sports relies on the interplay between multiple physiological systems determining the capacity to perform short explosive efforts as well as repeated intense actions with limited recovery over the course of an entire game. Testing should reflect these demands to allow for sport- and position-specific capacity analyses that eventually may translate into optimized training and improved performance. This may include individual load management and optimized training prescription, intensity targeting for specific positions or individual athletes, as well as benchmarking for monitoring of training progression and enhanced engagement of athletes. This review provides an overview of available tests in different exercise domains identified as relevant (from assessment of single explosive actions to intermittent endurance capacity), forming the basis for recommendations on how to compose a comprehensive yet feasible test battery that may be integrated into the seasonal competition and training plan. The test procedures should cover the performance spectrum of relevance for the individual athlete-also in team sports to account for positional differences. We emphasize the benefits of sport-specific tests, highlight parameters of importance for test standardization, and discuss how the applied test battery may be supplemented with secondary tests directed toward specific energy systems to allow for more in-depth analyses when required (e.g., in terms of an underperforming athlete). The synergy between testing and tracking of match performance (utilizing time-motion or global positioning systems) is highlighted, and although tracking cannot substitute for testing, combining the tools may provide a comprehensive overview of the physiological demands and performance during competition contextualized to the athletes' maximal exercise capacity.

Effects of padel activity and proprioception training on soccer players in an off-season period

BACKGROUND

Off-season periods imply considerable changes in the fitness status of soccer players. So far, no studies evaluated the effects of proprioception-focused training during soccer off-season periods. In this work, we assessed how much some players' abilities (static and dynamic balance, reaction times, quickness, strength, and technical skills) were affected by proprioception training and padel activity during an off-season period of 12 weeks.

METHODS

Twenty-eight non-professional adult male soccer players were organized into three groups: a group carried out regular padel activity, ~2 h once a week. Another group underwent a regular proprioception training program, ~ 20 min, twice a week. The third group did not perform any specific activity (control). Static and dynamic balance, reaction times, quickness, strength, and technical skills were evaluated at three time-points: before starting, after 6 weeks, and after 12 weeks.

RESULTS

Both padel activity and specific proprioception training carried out for 12 weeks significantly improved players' monopodalic static balance with eyes open and dynamic balance. No significant effects of these training regimens were found on monopodalic static balance with eyes closed, visual and acoustic reaction times, acyclic quickness, and strength. Furthermore, proprioception training considerably improved technical skills.

CONCLUSIONS

Coaches may use padel activity and proprioception exercises for off-season programs featured by ease of execution, low training volume, and high compliance.

Adding LIT to HIIT: Is Low-Intensity Training Vital for Endurance-Trained Athletes during a 7-day HIIT Shock Microcycle?

PURPOSE

This study aimed to investigate the effects of a 7-d high-intensity interval training shock microcycle (HIIT-SM) with or without additional low-intensity training (LIT) on aerobic fitness and endurance performance compared with a control group (CG).

METHODS

Thirty-three endurance-trained athletes (7 women, 26 men, mean ± SD: age, 30.2 ± 6.9 yr; maximal oxygen uptake (V̇O 2max ), 59.8 ± 4.9 mL·min -1 ·kg -1 ) performed exercise testing at T1 and were randomly assigned to one of three groups: i) HSM, 10 running-based HIIT sessions (5 × 4 min at 90%-95% maximal heart rate) over 7 d; ii) HSM + LIT, equal to HSM with additional 30 min of LIT after each HIIT; iii) CG, regular training. Exercise testing was repeated 3 d (T2), 7 d (T3), and 14 d (T4) after the intervention. A 5-km time trial (TT 5km ) was performed 3-4 d before T1 and 10-11 d after the intervention. Data were analyzed by two-way repeated-measures ANOVA.

RESULTS

No interaction effect was found for V̇O 2max ( P = 0.170, pη2 = 0.09), peak power output ( P = 0.734, pη2 = 0.04), and work economy ( P = 0.804, pη2 = 0.03). There was an interaction for velocity at lactate threshold ( P = 0.006, pη2 = 0.18) with increased velocity in HSM at T2 (3.2%, P = 0.030), T3 (4%, P = 0.006), T4 (4%, P = 0.003), as well as in HSM + LIT for T2 (3.2%, P = 0.011), whereas CG showed no change. There was an interaction for TT 5km ( P = 0.044, pη2 = 0.19), with HSM improving 2.7% ( P = 0.003) and HSM + LIT 2.3% ( P = 0.010), whereas CG was, on average, -0.3% ( P = 0.821) slower.

CONCLUSIONS

HIIT-SM with or without additional LIT has negligible effects on V̇O 2max but improves other key endurance variables in endurance-trained athletes. No superiority of either intervention group was demonstrated. Therefore, additional LIT during HIIT-SM is not beneficial.

Supervised Offseason Training Programs are able to mitigate the Effects of Detraining in Youth Men Soccer Players Physical Fitness: A Randomized Parallel Controlled Study

This study aimed to analyze the effects of three off-season training programs on the aerobic capacity, countermovement jump (CMJ), and linear sprint performance of young male soccer players. The study employed a randomized multi-arm design, consisting of three experimental groups: i) a high-intensity interval training (HIIT) group; (ii) a plyometric jump training (PJT) group; and (iii) a HIIT+PJT group; and an inactive control group. Fifty-eight under-19 male soccer players (aged 17.6 ±0.6 years) were randomly assigned to participate in a 3-week offseason training program exclusively performing HIIT, PJT, or a combination of both, while the fourth group remained inactive. Players underwent assessments twice, using the Yo-Yo Intermittent Recovery Test - Level 1 (YYIRT), CMJ, and 30-meter linear sprint. Significant interactions between time and groups were found in CMJ (p<0.001), YYIRT (p<0.001), and 30-m sprint (p<0.001). Group*time interaction revealed that the control group was significantly different from HIIT (p<0.001), PJT (p<0.001), and HIIT+PJT (p<0.001) considering the CMJ. Moreover, the control group was significantly different from HIIT (p=0.037) in YYIRT. Finally, the control group was significantly different from HIIT (p=0.024), PJT (p<0.001), and HIIT+PJT (p=0.021) considering the 30-m sprint. In conclusion, off-season training programs are effective in significantly reducing declines in CMJ and sprint performance compared to maintaining training cessation. However, in the YYIRT, only HIIT seems to be significantly superior to maintaining inactivity. To mitigate aerobic performance declines, incorporating HIIT sessions twice weekly during the offseason is advisable. To enhance or maintain jump performance, integrating at least one session of PJT weekly is beneficial.

The physiology of ice hockey performance: An update

Ice hockey is an intense team sport characterized by repeated bursts of fast-paced skating, rapid changes in speed and direction and frequent physical encounters. These are performed in on-ice shifts of ~30-80 s interspersed with longer sequences of passive recovery, resulting in about 15-25 min on-ice time per player. Nearly 50% of the distance is covered at high-intensity skating speeds and with an accentuated intense activity pattern in forwards compared to defensemen. During ice hockey match-play, both aerobic and anaerobic energy systems are significantly challenged, with the heart rate increasing toward maximum levels during each shift, and with great reliance on both glycolytic and phosphagen ATP provision. The high-intensity activity pattern favors muscle glycogen as fuel, leading to pronounced reductions despite the relatively brief playing time, including severe depletion of a substantial proportion of individual fast- and slow-twitch fibers. Player-tracking suggests that the ability to perform high-intensity skating is compromised in the final stages of a game, which is supported by post-game reductions in repeated-sprint ability. Muscle glycogen degradation, in particular in individual fibers, as well as potential dehydration and hyperthermia, may be prime candidates implicated in exacerbated fatigue during the final stages of a game, whereas multiple factors likely interact to impair exercise tolerance during each shift. This includes pronounced PCr degradation, with potential inadequate resynthesis in a proportion of fast-twitch fibers in situations of repeated intense actions. Finally, the recovery pattern is inadequately described, but seems less long-lasting than in other team sports.

Impact of the COVID-19 Lockdown on the Body Composition and Physical Performance of Athletes: A Systematic Review with Meta-Analysis and Meta-Regression

Sporting events were cancelled, and sports training was banned to prevent the spread of COVID-19. These changes during the COVID-19 pandemic decreased the physical activity levels, increased sedentary time, and also impaired the mental health of elite and sub-elite athletes. The impact on body composition and physical performance is not clear, however, especially considering a systematic review with meta-analysis. Thus, our objective was to conduct a review in accordance with the PRISMA Statement studies published in scientific journals (PubMed, Web of Science, or Scopus databases) that investigated the effect that social distancing during the COVID-19 pandemic had on the physical performance (muscle power, cardiorespiratory capacity, and sprint) or body composition (body weight, percentage of fat, fat mass, and fat-free mass) of athletes. Data from 24 studies indicate that, throughout the global lockdown, the athletes maintained muscle power, cardiorespiratory capacity, and sprint, and prevented significant changes in fat mass and fat-free mass. However, the total body weight (meta-analysis with 18 studies), showed a significant increase (p = 0.006), with a small ES = 0.12; 95% CI = 0.04 to 0.21. Furthermore, the time of follow-up, level of training, and the age of the athletes were possible moderators of these effects. The data reinforce the importance of general strength and endurance exercises sessions to maintain physical fitness during non-competitive periods or due to the mandatory lockdown.

Linear and Change of Direction Repeated Sprint Ability Tests: A Systematic Review

ABSTRACT

Kyles, A, Oliver, JL, Cahill, MJ, Lloyd, RS, and Pedley, J. Linear and change of direction repeated sprint ability tests: a systematic review. J Strength Cond Res 37(8): 1703-1717, 2023-The ability to repeatedly sprint is important in many sports, but there is no established protocol for measuring repeated sprint ability (RSA). The purpose of this review was to identify overground RSA protocols previously reported in the literature and to recommend standardized protocols. A systematic review of the literature was used to identify studies that have used an RSA test, with data describing protocol design extracted. One hundred eight studies were included in the review, across which 47 unique protocols were identified. Eighteen protocols included at least one change of direction (COD), and this increased mean sprint time compared with linear RSA tests (7.26 ± 1.84 vs. 4.48 ± 1.02 seconds). There was considerable variability across protocols regarding sprint distance (20-40 m), sprint repetitions (3-15), recovery duration (10-60 seconds), recovery type (active vs. passive), and work-to-rest ratio (≤1:1.4-19.2). Separate protocols are needed for linear and COD tests, and these should reflect the brief nature of intense periods of competition and stress the ability to recover. Based on data across studies for protocol design and to ensure a demanding work-to-rest ratio, it is suggested that a linear RSA should comprise 6 × 30 m sprints separated by 15 seconds of active recovery. To provide some parity to linear tests, to keep work duration brief and to maintain a demanding work-to-rest ratio, a COD RSA should comprise 6 × 30 m shuttle sprints (15 + 15 m), providing one change of direction (180° COD), and maintaining a 15-second active recovery.

VO2max (VO2peak) in elite athletes under high-intensity interval training: A meta-analysis

Consensus is lacking regarding whether high-intensity interval training (HIIT) effectively improves VO₂max (VO₂peak) in elite athletes (Athlete must be involved in regular competition at the national level). This meta-analysis compared the effects of HIIT and conventional training methods (continuous training, repeated-sprint training, high volume low-intensity training, high-intensity continuous running, sprint-interval training, moderate-intensity continuous training)on VO₂max in elite athletes. Nine studies were included, comprising 176 elite athletes (80 female). Compared to that with conventional training, VO₂max was significantly increased after HIIT (overall: 0.58 [0.30, 0.87], I² = 0.49, P = 0.03; males: 0.41 [0.06, 0.76], I² = 0%, P = 0.89). VO₂max had positive training effects when the HIIT recovery period had an interval time ≥2 min (0.44 [0.03, 0.84], I² = 0%, P = 0.99) and recovery phase intensity ≤40% (0.38 [0.05, 0.71], I² = 0%, P = 0.96). Thus, HIIT shows superiority over conventional training methods in improving VO₂max, promoting aerobic capacity, in elite athletes.

Human and African ape myosin heavy chain content and the evolution of hominin skeletal muscle

Humans are unique among terrestrial mammals in our manner of walking and running, reflecting 7 to 8 Ma of musculoskeletal evolution since diverging with the genus Pan. One component of this is a shift in our skeletal muscle biology towards a predominance of myosin heavy chain (MyHC) I isoforms (i.e. slow fibers) across our pelvis and lower limbs, which distinguishes us from chimpanzees. Here, new MyHC data from 35 pelvis and hind limb muscles of a Western gorilla (Gorilla gorilla) are presented. These data are combined with a similar chimpanzee dataset to assess the MyHC I content of humans in comparison to African apes (chimpanzees and gorillas) and other terrestrial mammals. The responsiveness of human skeletal muscle to behavioral interventions is also compared to the human-African ape differential. Humans are distinct from African apes and among a small group of terrestrial mammals whose pelvis and hind/lower limb muscle is slow fiber dominant, on average. Behavioral interventions, including immobilization, bed rest, spaceflight and exercise, can induce modest decreases and increases in human MyHC I content (i.e. -9.3% to 2.3%, n = 2033 subjects), but these shifts are much smaller than the mean human-African ape differential (i.e. 31%). Taken together, these results indicate muscle fiber content is likely an evolvable trait under selection in the hominin lineage. As such, we highlight potential targets of selection in the genome (e.g. regions that regulate MyHC content) that may play an important role in hominin skeletal muscle evolution.

Performance Adaptations to Intensified Training in Top-Level Football

Because physical demands are surging in football (soccer, USA), clubs are more and more seeking players who have a high capacity to perform repeated intense exercise. High-intensity interval training (HIIT), comprising exercise performed at intensities near or exceeding the capacity of aerobic energy systems, effectively enhances the physical conditioning of players. But given that HIIT imposes high loads, it increases the risk of overload-associated match performance decline and injury. This makes some coaches inclined to conduct HIIT in the weeks leading up to the season and during the season. Therefore, the challenge is how to optimize and dose HIIT during these phases, as they can be decisive. Studies have highlighted the utility of conducting periods of intensified training to overcome the risk of overload while at the same time enhancing performance. During intensified training periods of typically a few weeks, intensity is increased by enlarging the amount of HIIT, for example, aerobic high-intensity training or speed endurance training, while volume at low-to-moderate intensity is significantly reduced. The outcome depends on training composition and prescription-most notably, intensity and duration of bouts and recovery. When work intervals are prescribed for a few minutes at intensities > 90% heart rate max (i.e., aerobic high-intensity training), then beneficial adaptations pertaining to aerobic power and capacity are apparent. But when work intervals are conducted at much higher intensities, as all-out efforts or sprinting of typically 10- to 40-s duration with longer recovery periods (i.e., speed endurance training), beneficial adaptations pertaining to anaerobic energy systems, ion handling, and fatigue resilience are commonly observed. In this review, we discuss the utility of conducting intensified training periods to enhance performance in elite football players during the late preparation phase and competitive season.

Cardiopulmonary Exercise Testing in Athletes With Hypertrophic Cardiomyopathy

Patients with hypertrophic cardiomyopathy (HCM) have historically been restricted from athletic participation because of the perceived risk of sudden cardiac death. More contemporary research has highlighted the relative safety of competitive athletics with HCM. However, lack of published data on reference values for cardiopulmonary exercise testing (CPET) complicates clinical management and counseling on sports participation in the individual athlete. We conducted a single-center, retrospective cohort study to investigate CPET in athletes with HCM and clinical characteristics associated with objective measures of aerobic capacity. We identified 58 athletes with HCM (74% male, mean age 18 ± 3 years, mean left ventricular (LV) wall thickness 20 ± 7 mm). LV outflow tract obstruction was present in 22 (38%). A total of 15 (26%) athletes were taking a β blocker (BB), but only 4 (7%) reported exertional symptoms. Overall, exercise capacity was mildly reduced, with a peak myocardial oxygen consumption (peak VO₂) of 37.9 ml/min/kg (83% of predicted peak VO₂). Both LV outflow tract obstruction and BB use were associated with reduced exercise capacity. Limited peak heart rate was more common in athletes taking BB (47% vs 9%, p = 0.002). At a mean 5.6 years follow-up, 5 patients underwent myectomy (9%), and 8 (14%) received an implantable cardioverter defibrillator (ICD) for primary prevention. One individual with massive LV hypertrophy experienced recurrent ICD shocks for ventricular fibrillation and underwent myectomy 7 years after initial evaluation and was no longer participating in sports. There were no deaths over the follow-up period. In conclusion, the prognostic role of CPET remains unclear in athletes with HCM. Mildly reduced exercise capacity was common; however, reduced peak VO₂ did not correlate with symptom status or clinical outcomes. A significant proportion went on to require myectomy and/or ICD, thus highlighting the need for close follow-up. These data provide some initial insight into the clinical evaluation of "real world" athletes with HCM; however, further study is warranted to help guide shared decision-making, return-to-play discussions, and the potential long-term safety of competitive athletic participation.

Changes in Sprint Force-Velocity Profile in International Para Footballers

PURPOSE

Force production is crucial in football, and it is the main limitation of people with cerebral palsy (CP). This study aimed to describe the changes in sprint force-velocity (Fv) profile after a period of 25 weeks of regular training in international football players with CP.

METHODS

The sprint Fv profile and other physical performance variables (ie, linear sprint, vertical jump, change of direction, and intermittent endurance) of 14 international players from the Spanish national team were assessed during 2 consecutive training camps. Pretesting and posttesting sessions were carried out 1 week before and after the 25-week intervention period. The intervention consisted of 2 strength sessions per week added to the usual football training.

RESULTS

The repeated-measures analysis of variance showed changes in players' physical performance (linear sprint: F = 18.05, P < .01; change of direction: F = 16.71, P < .01; and endurance: F = 31.45, P < .01) and in some variables of the sprint Fv profile (maximal horizontal force, maximal power, slope, maximal ratio of force, and decrease of ratio of force; F = 14.28-37.81; P < .01), whereas players' maximal velocity (theoretical and actual) did not change (F = 0.13 and 0.01; P = .72 and .98, respectively).

CONCLUSIONS

This study showed that the implementation of 2 strength-training sessions per week, for 25 weeks, is effective to improve CP football players' physical performance. The main finding of this study is the improvement of force application in the acceleration phase (sprint Fv profile), which is the main attribute in many physical performance tests and is the main limitation of the CP population.

The soccer season: performance variations and evolutionary trends

The physiological demands of soccer challenge the entire spectrum of the response capacity of the biological systems and fitness requirements of the players. In this review we examined variations and evolutionary trends in body composition, neuromuscular and endurance-related parameters, as well as in game-related physical parameters of professional players. Further, we explore aspects relevant for training monitoring and we reference how different training stimulus and situational variables (e.g., competition exposure) affect the physiological and performance parameters of players. Generally, improvements of small magnitude in non- (non-CMJ) and countermovement-based jumps (CMJBased) and in the sprint acceleration (ACCPhase) and maximal velocity phase (MVPhase) are observed from start of preparation phase (PPS) to beginning of competition phase (BCP). A greater magnitude of increases is observed in physiological and endurance performance measures within this period; moderate magnitude in sub-maximal intensity exercise (velocity at fixed blood lactate concentrations; V2-4mmol/l) and large magnitude in VO2max, maximal aerobic speed (MAS) and intense intermittent exercise performance (IE). In the middle of competition phase (MCP), small (CMJBased and ACCPhase), moderate (non-CMJ; MVPhase; VO2max; sub-maximal exercise) and large (MAS and IE) improvements were observed compared to PPS. In the end of competition period (ECP), CMJBased and MVPhase improve to a small extent with non-CMJ, and ACCPhase, VO2max, MAS, sub-maximal intensity exercise and IE revealing moderate increments compared to PPS. Although less investigated, there are generally observed alterations of trivial magnitude in neuromuscular and endurance-related parameters between in-season assessments; only substantial alterations are examined for IE and sub-maximal exercise performance (decrease and increase of small magnitude, respectively) from BCP to MCP and in VO2max and IE (decrements of small magnitude) from MCP to ECP. Match performance may vary during the season. Although, the variability between studies is clear for TD, VHSR and sprint, all the studies observed substantial increments in HSR between MCP and ECP. Finally, studies examining evolutionary trends by means of exercise and competition performance measures suggests of a heightened importance of neuromuscular factors. In conclusion, during the preseason players "recover" body composition profile and neuromuscular and endurance competitive capacity. Within in-season, and more robustly towards ECP, alterations in neuromuscular performance seem to be force-velocity dependent, and in some cases, physiological determinants and endurance performance may be compromised when considering other in-season moments. Importantly, there is a substantial variability in team responses that can be observed during in-season. Consequently, this informs on the need to both provide a regular training stimulus and adequate monitorization throughout the season.

Effects of short-term repeated sprint training in hypoxia or with blood flow restriction on response to exercise

This study compared the effects of a brief repeated sprint training (RST) intervention performed with bilateral blood flow restriction (BFR) conditions in normoxia or conducted at high levels of hypoxia on response to exercise. Thirty-nine endurance-trained athletes completed six repeated sprints cycling sessions spread over 2 weeks consisting of four sets of five sprints (10-s maximal sprints with 20-s active recovery). Athletes were assigned to one of the four groups and subjected to a bilateral partial blood flow restriction (45% of arterial occlusion pressure) of the lower limbs during exercise (BFRG), during the recovery (BFRrG), exercised in a hypoxic room simulating hypoxia at FiO2 ≈ 13% (HG) or were not subjected to additional stress (CG). Peak aerobic power during an incremental test, exercise duration, maximal accumulated oxygen deficit and accumulated oxygen uptake (VO2) during a supramaximal constant-intensity test were improved thanks to RST (p < 0.05). No significant differences were observed between the groups (p > 0.05). No further effect was found on other variables including time-trial performance and parameters of the force-velocity relationship (p > 0.05). Thus, peak aerobic power, exercise duration, maximal accumulated oxygen deficit, and VO2 were improved during a supramaximal constant-intensity exercise after six RST sessions. However, combined hypoxic stress or partial BFR did not further increase peak aerobic power.

Effects of a shock microcycle after COVID-19 lockdown period in elite soccer players

Objectives

Decreases in physical fitness are inevitable after two to six week period of detraining in athletes. Lockdown period changed the characteristics of soccer players’ training.

Aim of the study

The aim of our study was to apply a HIIT shock-microcycle (SM) after return to training and assess its effect on players’ performance.

Equipment and methods

Nineteen elite professional soccer players during the lockdown period (LP) from March to May 2020 (8 weeks) performed 3-4 individual training sessions per week. The training sessions included running boots of anaerobic short and aerobic prolonged duration intervals. Intensity was determined according to lab ergospirometry test 2 weeks before LP. All the players followed an indoor program, 3–6 sessions per week consisted of core, balance and flexibility exercises (∼45 minutes). SM training content was same for both groups and took place the first two weeks after LP, consisted by eight high intensity interval training sessions (HIIT), two technique, two tactical sessions and two days off.

Results

Repeated sprint ability mean time (RSA_MeanTime) and Repeated sprint ability performance–sprint no 4,5,6 (RSA_4,5,6S) improved after SM (P = 0.025, Effect size: r² = 0.331, P = 0.010, Effect size: r² = 0.411, p = 0.009, Effect size: r² = 0.418, P = 0.037, Effect size: r² = 0.293, respectively. Yo-Yo intermitted recovery test level 2 (YYIR2_{TotalDistance}) that covered by players during the 2nd measurement was 10.8% longer (P = 0.004, Effect size: r² = 0.483). Also, the YYIR2_{HeartRateRecovery} percentage was lower during the 2nd measurement (P = 0.014, Effect size: r² = 0.107).

Conclusions

These results indicate that SM can improve YYIR2 and RSA performance thus it is a useful tool to regain physical attributes in a short period.

The Salzburg 10/7 HIIT shock cycle study: the effects of a 7-day high-intensity interval training shock microcycle with or without additional low-intensity training on endurance performance, well-being, stress and recovery in endurance trained athletes-study protocol of a randomized controlled trial

BACKGROUND

Performing multiple high-intensity interval training (HIIT) sessions in a compressed period of time (approximately 7-14 days) is called a HIIT shock microcycle (SM) and promises a rapid increase in endurance performance. However, the efficacy of HIIT-SM, as well as knowledge about optimal training volumes during a SM in the endurance-trained population have not been adequately investigated. This study aims to examine the effects of two different types of HIIT-SM (with or without additional low-intensity training (LIT)) compared to a control group (CG) on key endurance performance variables. Moreover, participants are closely monitored for stress, fatigue, recovery, and sleep before, during and after the intervention using innovative biomarkers, questionnaires, and wearable devices.

METHODS

This is a study protocol of a randomized controlled trial that includes the results of a pilot participant. Thirty-six endurance trained athletes will be recruited and randomly assigned to either a HIIT-SM (HSM) group, HIIT-SM with additional LIT (HSM + LIT) group or a CG. All participants will be monitored before (9 days), during (7 days), and after (14 days) a 7-day intervention, for a total of 30 days. Participants in both intervention groups will complete 10 HIIT sessions over 7 consecutive days, with an additional 30 min of LIT in the HSM + LIT group. HIIT sessions consist of aerobic HIIT, i.e., 5 × 4 min at 90-95% of maximal heart rate interspersed by recovery periods of 2.5 min. To determine the effects of the intervention, physiological exercise testing, and a 5 km time trial will be conducted before and after the intervention.

RESULTS

The feasibility study indicates good adherence and performance improvement of the pilot participant. Load monitoring tools, i.e., biomarkers and questionnaires showed increased values during the intervention period, indicating sensitive variables.

CONCLUSION

This study will be the first to examine the effects of different total training volumes of HIIT-SM, especially the combination of LIT and HIIT in the HSM + LIT group. In addition, different assessments to monitor the athletes' load during such an exhaustive training period will allow the identification of load monitoring tools such as innovative biomarkers, questionnaires, and wearable technology.

TRIAL REGISTRATION

clinicaltrials.gov, NCT05067426. Registered 05 October 2021-Retrospectively registered, https://clinicaltrials.gov/ct2/show/NCT05067426 . Protocol Version Issue date: 1 Dec 2021. Original protocol. Authors: TLS, NH.

International football players with cerebral palsy maintained their physical fitness after a self-training program during the COVID-19 lockdown

Background

The COVID-19 global pandemic caused a complete stop in sport participation which meant a detraining period for athletes. High-level athletes had to train at home guided by their coaches and conditioning trainers in an effort to maintain their physical fitness. The aim of maintaining the training adaptations and physical fitness during the COVID-19 mandatory lockdown was especially important for CP athletes, in which the detraining period was expected to cause early declines in motor function, poor coordination and muscle weakness due to their disability.

Methods

The present study assessed the effect of a guided self-training program on international CP football players' physical fitness during the COVID-19 mandatory lockdown. Fifteen CP football players from the Spanish National Team participated in the study. An experimental design with a pre- (T1) and a post-intervention (T2) assessment was carried out, with a 12-week period of players' self-training (divided in two periods of 6 weeks) which combined strength and endurance training. Physical performance assessment consisted in the free countermovement jump (CMJ), 5, 10 and 20-m sprint, the modified agility T-test (MAT) and a dribbling test. The Kruskal-Wallis test was used for between-group comparisons, while the Student's paired t test or the Fisher Pitman permutation test, based on the normality of the data, were used for within-group comparisons.

Results

The results showed no differences between sport classes (FT1, FT2 and FT3) in physical fitness change after the training program (Chi² = 0.16 to 1.73; p = 0.42 to 0.92). Within-group comparisons showed an increase of jump height in the CMJ (4.19 cm [2.46, 5.93]; p < 0.001) and a maintenance of the 5, 10 and 20-m sprint, MAT and dribbling ability (<0.01 to 0.09 s; p = 0.19 to 0.97).

Discussion

To the authors' knowledge, this is the first study that examined the physical fitness adaptations to a training program with CP football players. The results show that a 12-week guided self-training program without football-specific stimulus may be effective to maintain or even improve the specific physical performance of international CP football players during a non-competitive period (as the COVID-19 lockdown). This study reveals that CP football players are able to show adaptations to the strength and endurance training and this could be the basis for future research regarding training adaptations in CP football players.

Effects of Resistance Training Cessation on Cycling Performance in Well-Trained Cyclists: An Exploratory Study

ABSTRACT

Bláfoss, R, Rikardo, J, Andersen, AØ, Hvid, LG, Andersen, LL, Jensen, K, Christensen, PM, Kvorning, T, and Aagaard, P. Effects of resistance training cessation on cycling performance in well-trained cyclists: an exploratory study. J Strength Cond Res 36(3): 796-804, 2022-Supplementary (i.e., concurrent) resistance training can enhance cycling performance among competitive cyclists. However, a lack of knowledge exists about the retention (decay profile) in mechanical muscle function and cycling performance after concurrent resistance and endurance training. The present exploratory intervention study investigated the effect of 6 weeks of resistance training cessation when preceded by 8 weeks of concurrent resistance and endurance training on mechanical muscle function and cycling performance in 9 male well-trained competitive cyclists (V̇o2max = 66 ± 7 ml·min-1·kg-1). Cyclists performed periodized resistance training targeting leg and core muscles for 8 weeks as a supplement to their normal endurance (cycling) training. This was followed by 6 weeks of endurance training only (retention period) leading up to the start of the competitive season. Maximal leg extensor power, isometric leg extensor strength (maximal voluntary contraction [MVC]), rate of force development (RFD), and long-term cycling performance (2-hour submaximal cycling at 55% of Wmax), followed by 5-minute max cycling were evaluated. After 8 weeks of concurrent resistance and endurance training, leg extensor power, MVC, and RFD increased by 12, 15, and 17%, respectively while mean power output (W) during 5-minute max cycling increased by 7% (p < 0.05). Training-induced gains in MVC and 5-minute max cycling power were retained after 6-week cessation of resistance training (p < 0.05). These findings indicate that competitive cyclists can focus on cycling training alone for at least 6 weeks leading up to competition without losing attained gains in maximal muscle strength and cycling performance achieved by preceding periods of concurrent resistance training.

The Relationship between VO2 and Muscle Deoxygenation Kinetics and Upper Body Repeated Sprint Performance in Trained Judokas and Healthy Individuals

The present study sought to investigate if faster upper body oxygen uptake (VO₂) and hemoglobin/myoglobin deoxygenation ([HHb]) kinetics during heavy intensity exercise were associated with a greater upper body repeated-sprint ability (RSA) performance in a group of judokas and in a group of individuals of heterogenous fitness level. Eight judokas (JT) and seven untrained healthy participants (UT) completed an incremental step test, two heavy intensity square-wave transitions and an upper body RSA test consisting of four 15 s sprints, with 45 s rest, from which the experimental data were obtained. In the JT group, VO₂ kinetics, [HHb] kinetics and the parameters determined in the incremental test were not associated with RSA. However, when the two groups were combined, the amplitude of the primary phase VO₂ and [HHb] were positively associated with the accumulated work in the four sprints (ΣWork). Additionally, maximal aerobic power (MAP), peak VO₂ and the first ventilatory threshold (VT₁) showed a positive correlation with ΣWork and an inverse correlation with the decrease in peak power output (Dec-PPO) between the first and fourth sprints. Faster VO₂ and [HHb] kinetics do not seem to be associated with an increased upper body RSA in JT. However, other variables of aerobic fitness seem to be associated with an increased upper body RSA performance in a group of individuals with heterogeneous fitness level.

Applicability of Field Aerobic Fitness Tests in Soccer: Which One to Choose?

A desire to make fitness testing cheaper and easier to conduct in a team-sport setting has led to the development of numerous field aerobic fitness tests. This has contributed to a growing confusion among strength and conditioning coaches about which one to use. The main aim of this narrative review was to examine the reliability, validity, sensitivity and usefulness of the commonly used field aerobic fitness tests and to provide practical guidelines for their use in soccer. The University of Montreal track test (UMTT) and Vam Eval test seem the best options for estimation of maximal oxygen uptake (VO_2max) while the highest signal-to-noise ratio of the 30-15 intermittent fitness test (30-15IFT) suggests its superior sensitivity to track changes in fitness. The UMTT and 30-15IFT are the best solutions for prescription of long and short high-intensity interval training sessions, respectively. All field tests mostly present with marginal usefulness, but the smallest worthwhile change for UMTT or Vam Eval test, Yo-YoIRT2 and 30-15IFT are smaller than their stage increment making the improvement of only one stage in the test performance already worthwhile. Strength and conditioning coaches are advised to choose the test based on their specific purpose of testing.

Six high-intensity interval training sessions over 5 days increases maximal oxygen uptake, endurance capacity, and sub-maximal exercise fat oxidation as much as 6 high-intensity interval training sessions over 2 weeks

BACKGROUND

High-intensity interval training (HIIT) induces similar or even superior adaptations compared to continuous endurance training. Indeed, just 6 HIIT sessions over 2 weeks significantly improves maximal oxygen uptake (VO_2max), submaximal exercise fat oxidation, and endurance performance. Whether even faster adaptations can be achieved with HIIT is not known. Thus, we aimed to determine whether 2 sessions of HIIT per day, separated by 3 h, every other day for 5 days (double HIIT (HIIT-D), n = 15) could increase VO_2max, submaximal exercise fat oxidation, and endurance capacity as effectively as 6 sessions of HIIT over 2 weeks (single HIIT (HIIT-S), n = 13).

METHODS

Each training session consisted of 10 × 60 s of cycling at 100% of VO_2max interspersed with 75 s of low-intensity cycling at 60 watt (W). Pre- and post-training assessments included VO_2max, time to exhaustion at ∼80% of VO_2max, and 60-min cycling trials at ∼67% of VO_2max.

RESULTS

Similar increases (p < 0.05) in VO_2max (HIIT-D: 7.7% vs. HIIT-S: 6.0%, p > 0.05) and endurance capacity (HIIT-D: 80.1% vs. HIIT-S: 79.2%, p > 0.05) were observed. Submaximal exercise carbohydrate oxidation was reduced in the 2 groups after exercise training (HIIT-D: 9.2%, p = 0.014 vs. HIIT-S: 18.8%, p = 0.012) while submaximal exercise fat oxidation was significantly increased in HIIT-D (15.5%, p = 0.048) but not in HIIT-S (9.3%, p = 0.290).

CONCLUSION

Six HIIT sessions over 5 days was as effective in increasing VO_2max and endurance capacity and was more effective in improving submaximal exercise fat oxidation than 6 HIIT sessions over 2 weeks.

Muscle Ionic Shifts During Exercise: Implications for Fatigue and Exercise Performance

Exercise causes major shifts in multiple ions (e.g., K⁺ , Na⁺ , H⁺ , lactate^- , Ca²⁺ , and Cl^- ) during muscle activity that contributes to development of muscle fatigue. Sarcolemmal processes can be impaired by the trans-sarcolemmal rundown of ion gradients for K⁺ , Na⁺ , and Ca²⁺ during fatiguing exercise, while changes in gradients for Cl^- and Cl^- conductance may exert either protective or detrimental effects on fatigue. Myocellular H⁺ accumulation may also contribute to fatigue development by lowering glycolytic rate and has been shown to act synergistically with inorganic phosphate (Pi) to compromise cross-bridge function. In addition, sarcoplasmic reticulum Ca²⁺ release function is severely affected by fatiguing exercise. Skeletal muscle has a multitude of ion transport systems that counter exercise-related ionic shifts of which the Na⁺ /K⁺ -ATPase is of major importance. Metabolic perturbations occurring during exercise can exacerbate trans-sarcolemmal ionic shifts, in particular for K⁺ and Cl^- , respectively via metabolic regulation of the ATP-sensitive K⁺ channel (K_ATP ) and the chloride channel isoform 1 (ClC-1). Ion transport systems are highly adaptable to exercise training resulting in an enhanced ability to counter ionic disturbances to delay fatigue and improve exercise performance. In this article, we discuss (i) the ionic shifts occurring during exercise, (ii) the role of ion transport systems in skeletal muscle for ionic regulation, (iii) how ionic disturbances affect sarcolemmal processes and muscle fatigue, (iv) how metabolic perturbations exacerbate ionic shifts during exercise, and (v) how pharmacological manipulation and exercise training regulate ion transport systems to influence exercise performance in humans. © 2021 American Physiological Society. Compr Physiol 11:1895-1959, 2021.

Effects of COVID-19 Lockdown and a Subsequent Retraining Period on Elite Athletes' Workload, Performance, and Autonomic Responses: A Case Series

PURPOSE

To describe the effects of COVID-19 lockdown and a subsequent retraining on the training workloads, autonomic responses, and performance of a group of elite athletes.

METHODS

The training workloads and heart rate variability (assessed through the log-transformed root mean square of successive R-R intervals) of 7 elite badminton players were registered daily during 4 weeks of normal training (baseline), 7 to 10 weeks of lockdown, and 6 to 8 weeks of retraining. Physical performance was assessed at baseline and after each phase by means of a countermovement jump and the estimated squat 1-repetition maximum.

RESULTS

A reduction in training workloads was observed in all participants during the lockdown (-63.7%), which was accompanied by a reduced heart rate variability in all but one participant (-2.0%). A significant reduction was also observed for countermovement jump (-6.5%) and 1-repetition maximum performance (-11.5%), which decreased in all but one participant after the lockdown. However, after the retraining phase, all measures returned to similar values to those found at baseline. At the individual level, there were divergent responses, as exemplified by one athlete who attenuated the reduction in training workloads and increased her performance during the lockdown and another one who markedly reduced his workload and performance, and got injured during the retraining phase.

CONCLUSIONS

Although there seems to be a large interindividual variability, COVID-19 lockdown is likely to impose negative consequences on elite athletes, but these detrimental effects might be avoided by attenuating reductions in training workloads and seem to be overall recovered after 6 to 8 weeks of retraining.

The effects of residential environment on the condition and fitness of soccer players in the summer

Exercise performance is reduced in hot environments due to physiological responses caused by increased body temperature. A proper residential environment is important for improving the performance and maintaining physical condition of soccer players in the summer. The purpose of this study was to determine the effect of indoor temperature of the resting space during the summer on the fitness and condition of soccer players. A total of 12 K-3 League semiprofessional players without serious injuries in the last 3 months voluntarily participated in the study. Participants performed speed (10 m, 20 m, and 30 m), soccer-specific coordination skill (dribbling), agility, repeated sprints, Yo-Yo intermittent level 2, vertical jump, and questionnaire (fatigue, sleep quality, muscle soreness, stress, and mood) after staying indoor temperature at 20°C, 26°C, and 30°C for one night, respectively. There was no difference among groups in physical fitness (speed, agility, jump, coordination, Yo-Yo intermittent level 2, and repeated sprints). The differences in fatigue and sleep quality were not statistically significant among groups, but they tended to be different. Muscle soreness was similar among all groups. Significant differences were observed between the 20°C and 30°C groups in stress and mood levels. The present study concluded that, while the physical fitness did not differ among groups, the 30°C residential environment was shown to have a negative psychological effect. Considering that many diseases associated with hot weather occur in low residential temperatures, a room temperature of 26°C is recommended for elite soccer players in hot summer weather.

Detrimental Effects of the Off-Season in Soccer Players: A Systematic Review and Meta-analysis

BACKGROUND

The off-season period in soccer leads necessarily to changes in fitness status. However, there is a lack of systematization that allows identifying the magnitude of these changes in groups participating in off-season training programs compared with those subjected to training cessation.

OBJECTIVE

This systematic review with meta-analysis was conducted to assess the effects of training cessation in off-season training programs on men soccer players' body fat, maximal oxygen uptake (VO_2max), yo-yo intermittent recovery test (YYIRT), vertical jump, sprinting time, and repeated-sprint ability.

METHODS

To qualify for inclusion in the systematic review, studies must have included: (1) a detraining period of ≥ 2 weeks; (2) controlled trials or cohorts of healthy men soccer players with no restriction on age; and (3) a pre-post training cessation or off-season training programs measure of body fat (%), VO_2max (mL kg^-1 min^-1), YYIRT performance (meters), vertical jump (height), sprinting (time), and repeated-sprint ability (total time).

RESULTS

The electronic search yielded 563 articles, and 12 were subsequently included. Significant (all p < 0.05) detrimental training cessation effects were noted for body fat (ES = 0.26), VO_2max (ES = - 1.48), YYIRT (ES = - 0.46), vertical jump (ES = - 0.81), and repeated-sprint ability (ES = 0.68). Similarly, significant (all p < 0.05) detrimental off-season training programs effects were noted for body fat (ES = 0.26), VO_2max (ES = - 0.48), vertical jump (ES = - 0.51), and sprinting time (ES = 0.86). When training cessation and off-season training programs effects were compared, greater detrimental effects were noted after training cessation for VO_2max (p = 0.002) and repeated-sprint ability (p < 0.001).

CONCLUSIONS

Detrimental effects on body composition and physical fitness were observed after both training cessation and off-season training programs. However, off-season training programs seem to ameliorate such detrimental effects on VO_2max and repeated-sprint ability to some extent. The results presented here call for the implementation of more effective off-season training programs among male soccer players.

COVID-19-Related Restrictions and Quarantine COVID-19: Effects on Cardiovascular and Yo-Yo Test Performance in Professional Soccer Players

The present study aimed to verify the quarantine's effects during a serious viral outbreak on the cardiovascular and performance associated with the Yo-Yo test in a sample of professional soccer players. 20 high-level soccer players (n = 20; age: 26 ± 4 years-old; weight: 76.85 ± 6.7 kg; height: 179 ± 6 cm) participated in this study. The intermittent Yo-Yo test was performed pre- and post- COVID-19 quarantine in a random order. During each test, the soccer players' running performance outcomes were monitored using a portable 5-Hz GPS with a 100 Hz accelerometer and a paired t-test was conducted at a p-value of ≤ 0.05. The main results demonstrated significant differences between pre- versus post-COVID-19 quarantine in the following variables: relative distance (161.7 ± 5.9 > 141.1 ± 33.8 m/min), maximal speed (18.7 ± 0.9 > 18.2 ± 0.6 km/h), acceleration (60 ± 20 frequency > 52 ± 16 frequency), deceleration (34 ± 13 frequency > 27 ± 6 frequency), sprints > 19 km/h [0.8 (0.2;3)% >0.5 (0;0.5)%], and in high intensity running distance [16.48 (2.68;41.64)m > 0.827 (0.164;3.0)m]. We concluded that COVID-19-related restrictions and quarantine COVID-19 demonstrated adverse effects on professional soccer players' Yo-Yo tests performance.

High-intensity Interval Training Shock Microcycle Improves Running Performance but not Economy in Female Soccer Players

This study aimed to evaluate the effect of high-intensity interval training shock microcycles (HIIT_SM) on endurance, running economy and change of direction economy in female soccer players. Nineteen sub-elite female soccer players were randomised to two groups: HIIT_SM (10 HIIT sessions over 13 days) or HIIT_TRAD (4 HIIT sessions over 13 days) interventions. Endurance performance was evaluated through the 30-15 intermittent fitness test (30-15_IFT); running economy over a 5-min treadmill run; and change of direction economy over two conditions: (1) 5-min 20m shuttle run, and (2) 5-min 10m shuttle run. HIIT_SM significantly improved 30-15_IFT scores compared to baseline (+4.4%, p=0.009; d=0.96) and 30-15_IFT scores relative to HIIT_TRAD (p=0.002; d=2.01). There was no significant interaction (group×time) for running economy and change of direction economy. Pre- to post- intervention there was a significant main time effect for blood lactate over 20m and 10m shuttle runs (p<0.001 and p=0.037, respectively), with large (d=0.93) and moderate (d=0.53) changes observed for the HIIT_SM over the two distances, respectively. HIIT_SM may be more effective than HIIT_TRAD to improve 30-15_IFT over shorter training periods but may not affect running economy and change of direction economy.

Repeated Wingate sprints is a feasible high-quality training strategy in moderate hypoxia

Sprint-interval training (SIT) is efficient at improving maximal aerobic capacity and anaerobic fitness at sea-level and may be a feasible training strategy at altitude. Here, it was evaluated if SIT intensity can be maintained in mild to moderate hypoxia. It was hypothesized that 6 x 30 s Wingate sprint performance with 2 min active rest between sprints can be performed in hypoxic conditions corresponding to ~3,000 m of altitude without reducing mean power output (MPO). In a single-blinded, randomized crossover design, ten highly-trained male endurance athletes with a maximal oxygen uptake ([Formula: see text]O2max) of 68 ± 5 mL O2 × min-1 × kg-1 completed 6 x 30 s all-out Wingate cycling sprints separated by two-minute active recovery on four separate days in a hypobaric chamber. The ambient pressure within the chamber on each experimental day was 772 mmHg (~0 m), 679 mmHg (~915 m), 585 mmHg (~ 2,150 m), and 522 mmHg (~3,050 m), respectively. MPO was not different at sea-level and up to ~2,150 m (~1% and ~3% non-significant decrements at ~915 and ~2,150 m, respectively), whereas MPO was ~5% lower (P<0.05) at ~3,050 m. Temporal differences between altitudes was not different for peak power output (PPO), despite a main effect of altitude. In conclusion, repeated Wingate exercise can be completed by highly-trained athletes at altitudes up to ~2,150 m without compromising MPO or PPO. In contrast, MPO was compromised in hypobaric hypoxia corresponding to ~3,050 m. Thus, SIT may be an efficient strategy for athletes sojourning to moderate altitude and aiming to maintain training quality.

Metabolic and Cardiorespiratory Responses of Semiprofessional Football Players in Repeated Ajax Shuttle Tests and Curved Sprint Tests, and Their Relationship with Football Match Play

In this study, the Ajax Shuttle Test (AST) and the Curved Sprint Test (CST) were conducted on semiprofessional football players to evaluate (1) their test performance, (2) the extent of anaerobic glycolysis by measuring blood lactate, (3) performance decrement and onset of fatigue, and (4) the correlation between selected physiological variables and test performance. Thirty-two semiprofessional Polish football players participated in this study. Both AST and CST were conducted on an outdoor football ground and were conducted in two sets; each set had six repetitions. In the case of AST, the total duration for 6 repetitions of the exercise in Sets 1 and 2 were 90.63 ± 3.71 and 91.65 ± 4.24 s, respectively, whereas, in the case of CST, the respective values were 46.8 ± 0.56 and 47.2 ± 0.66 s. Peak blood lactate concentration [La] after Sets 1 and 2 of AST were 14.47 ± 3.77 and 15.00 ± 1.85 mmol/L, and in the case of CST, the values were 8.17 ± 1.32 and 9.78 ± 1.35 mmol/L, respectively. Performance decrement in AST was more than in CST, both after Set 1 (4.32 ± 1.43 and 3.31 ± 0.96 in AST and CST, respectively) and Set 2 (7.95 ± 3.24 and 3.71 ± 1.02 in AST and CST, respectively). Only in a few of the repetitions, pulmonary ventilation (V_E) and oxygen uptake (VO₂) were found to be significantly correlated with the performance of the volunteers in both AST and CST. Respiratory exchange ratio (RER) was significantly correlated with most of the repetitions of AST, but not with CST. The study concludes that (1) AST shows more dependence on the anaerobic glycolytic system than shorter repetitive sprints (as in CST), (2) there is more performance decrement and fatigue in AST than in CST, and (3) early decrease in performance and fatigue in the semiprofessional football players in AST and CST may be due to the insufficiency of their aerobic energy system.

High-Intensity Interval Training Shock Microcycle for Enhancing Sport Performance: A Brief Review

Dolci, F, Kilding, AE, Chivers, P, Piggott, B, and Hart, NH. High-intensity interval training shock microcycle for enhancing sport performance: A brief review. J Strength Cond Res 34(4): 1188-1196, 2020-High-intensity interval training (HIIT) is a powerful strategy to develop athletes' fitness and enhance endurance performance. Traditional HIIT interventions involve multiple microcycles (7-10 days long) of 2-3 HIIT sessions each, which have been commonly supported to improve athletic performance after a minimum period of 6 weeks training. Regardless of the efficacy of such an approach, in recent years, a higher frequency of HIIT sessions within a unique microcycle, commonly referred to as an HIIT shock microcycle, has been proposed as an alternative HIIT periodization strategy to induce greater and more efficient endurance adaptation in athletes. This review article provides an insight into this new HIIT periodization strategy by discussing (1) HIIT shock microcycle format and design; (2) the sustainability of this training strategy; (3) effects on performance and physiological parameters of endurance; and (4) potential mechanisms for improvements. Evidence advocates the sustainability and effectiveness of HIIT shock microcycle in different athletes to improve intermittent and continuous running/cycling performance and suggests mitochondria biogenesis as the main acute physiological adaptation following this intervention.

How has COVID-19 modified training and mood in professional and non-professional football players?

Background

Coronavirus disease 2019 (COVID-19) has restricted freedom of movement with several countries ‘locked down’ worldwide. During this isolation period or quarantine, habits have been modified. This might have had negative effects on physiological variables but also influenced numerous emotional aspects, especially in elite athletes, which can have a negative impact on training and sleep quality, affecting their performance.

Methods

175 Spanish professional and non-professional association football players answered an online survey about demographic and training habits, as well as two validated questionnaires to assess psychological variables (POMS and WLEIS-S).

Results

The results showed that the confinement period reduced the load of training (p < 0.01), and modified the sleeping behaviour (both, sleep time (p < 0.05) and quality (p < 0.001)) across soccer players. Higher emotional intelligence (EI) values were positively related to training variables and strongly correlated with the mood. Interestingly, athletes’ mood was affected differently depending on gender.

Conclusion

We found that confinement period affects both, training load and recovery process and that mood states and EI could predict the training variables and performance of top-level football players.

Effectiveness of a Generic vs. Specific Program Training to Prevent the Short-Term Detraining on Repeated-Sprint Ability of Youth Soccer Players

Rodríguez-Fernández, A, Villa, JG, Sánchez-Sánchez, J, and Rodríguez-Marroyo, JA. Effectiveness of a generic vs. specific program training to prevent the short-term detraining on repeated-sprint ability of youth soccer players. J Strength Cond Res 34(8): 2128-2135, 2020-The aim of this study was to analyze the effects of 2 short-term training programs to prevent the negative effect of detraining on repeated-sprint ability (RSA) performance. The study was performed during a 2-week midseason break without official matches. Forty-five youth soccer players (17.7 ± 0.8 years, 175.4 ± 5.5 cm, and 67.2 ± 5.1 kg) were split into 3 groups during the intervention period: inactivity group (IN; N = 16), generic high-intensity training group (GG; N = 15), and specific training group (SG; N = 14). IN was instructed to avoid performing physical activity during the 2-week training intervention. However, GG and SG performed 8 training sessions. GG performed a generic aerobic interval training consisting of 4 repetitions of 4 minutes of exercise at 90-95% of maximal heart rate. SG performed a specific conditioning through small-sided games (4 vs. 4, 4 × 4-minute) and repeated sprints (6 × 30-m). Testing sessions included an RSA test and a Yo-Yo intermittent recovery test level 1 (YYIR1). Repeated-sprint ability performance only improved after the training intervention in SG (∼2%, p < 0.01, (Equation is included in full-text article.)= 0.23-0.25). Both GG and IN declined their performance in post-test (∼2%, p < 0.01, (Equation is included in full-text article.)= 0.19-0.22). No significant effect, group × time, was analyzed for YYIR1 performance. This study suggests that only specific training, based on small-sided games and repeated sprints, leads to short-term improvements on RSA performance in youth soccer players.

Relationship between maximal incremental and high-intensity interval exercise performance in elite athletes

This descriptive study aimed to explore the physiological factors that determine tolerance to exertion during high-intensity interval effort. Forty-seven young women (15–28 years old) were enrolled: 23 athletes from Taiwan national or national reserve teams and 24 moderately active females. Each participant underwent a maximal incremental INC (modified Bruce protocol) cardiopulmonary exercise test on the first day and high-intensity interval testing (HIIT) on the second day, both performed on a treadmill. The HIIT protocol involved alternation between 1-min effort at 120% of the maximal speed, at the same slope reached at the end of the INC, and 1-min rest until volitional exhaustion. Gas exchange, heart rate (HR), and muscle oxygenation at the right vastus lateralis, measured by near-infrared spectroscopy, were continuously recorded. The number of repetitions completed (R_lim) by each participant was considered the HIIT tolerance index. The results showed a large difference in the R_lim (range, 2.6–12.0 repetitions) among the participants. Stepwise linear regression revealed that the variance in the R_lim within the cohort was related to the recovery rates of oxygen consumption (V˙O2), HR at the second minute after INC, and muscle tissue saturation index at exhaustion (R = 0.644). In addition, age was linearly correlated with R_lim (adjusted R = −0.518, p < 0.0001). In conclusion, the recovery rates for V˙O2 and HR after the incremental test, and muscle saturation index at exhaustion, were the major physiological factors related to HIIT performance. These findings provide insights into the role of the recovery phase after maximal INC exercise testing. Future research investigating a combination of INC and HIIT testing to determine training-induced performance improvement is warranted.

Human skeletal muscle fiber type percentage and area after reduced muscle use: A systematic review and meta-analysis

The main objective of this systematic review was to examine the effect of reduced muscle activity on the relative number of type 1 muscle fibers (%) in the human vastus lateralis muscle. Other objectives were changes in type 2A and 2X percentages and muscle fiber cross-sectional area. We conducted systematic literature searches in eight databases and included studies assessing type 1 fiber percentage visualized by ATPase or immunohistochemical staining before and after a period (≥14 days) of reduced muscle activity. The reduced muscle activity models were detraining, leg unloading, and bed rest. Forty-two studies comprising 451 participants were included. Effect sizes were calculated as the mean difference between baseline and follow-up and Generic Inverse Variance tests with random-effects models were used for the weighted summary effect size. Overall, the mean type 1 muscle fiber percentage was significantly reduced after interventions (-1.94%-points, 95% CI [-3.37, -0.51], P = .008), with no significant differences between intervention models (P = .86). Meta-regression showed no effect of study duration on type 1 fiber percentage (P = .98). Conversely, the overall type 2X fiber percentage increased after reduced muscle activity (P < .001). The CSA of the muscle fiber types decreased after the study period (all P-values < 0.001) with greater reductions in type 2 than type 1 fibers (P < .001). The result of this meta-analysis display that the type 1 muscle fiber percentage decrease as a result of reduced muscle activity, although the effect size is relatively small.

Wales Anaerobic Test: Reliability and Fitness Profiles of International Rugby Union Players

Beard, A, Ashby, J, Chambers, R, Millet, GP, and Brocherie, F. Wales Anaerobic Test (WAT): Reliability and fitness profiles of international rugby union players. J Strength Cond Res XX(X): 000-000, 2019-To provide strength and conditioning coaches a practical and evidence-based test for repeated-sprint ability (RSA) in rugby union players, this study assessed the relative and absolute test-retest reliability of the Wales Anaerobic Test (WAT) and its position-specific association with other fitness performance indices. Thirty-four players (forwards: n = 19; backs: n = 15) of the Welsh rugby union male senior national team performed the WAT (10 × 50-m distance, 25-30 seconds of passive recovery) twice within 4 days. Time for each repetition was recorded, with the best (WATBest) and total time (WATTT) retained for analysis. Relative (intraclass correlation coefficient [ICC]) and absolute (SEM) reliability of the WAT indices were quantified. Furthermore, association (Pearson's product-moment correlations and stepwise backward elimination procedure) with other fitness performance indices (10- and 40-m sprinting times, 30-15 intermittent fitness test [30-15IFT] and the Yo-Yo intermittent recovery test level 2 [YYIR2]) was investigated. Pooled values revealed "moderate" to "high" ICCs for WATBest (ICC = 0.89, p = 0.626) and WATTT (ICC = 0.95, p = 0.342). Good test sensitivity was reported for forwards and backs' WATTT (p > 0.101). Both WATBest and WATTT correlated with 10-m and 40-m sprinting times (r > 0.69, p < 0.001) as well as with 30-15IFT (r < -0.77, p < 0.001) and YYIR2 (r < -0.68, p < 0.001) for pooled values. The WAT proved to be a reliable and sensitive test to assess the rugby union specific RSA-related fitness of international players.

Repeated-Sprint Training in Hypoxia in International Rugby Union Players

Purpose: To investigate the effects of repeated-sprint training in hypoxia vs in normoxia on world-level male rugby union players' repeated-sprint ability (RSA) during an international competition period. Methods: A total of 19 players belonging to an international rugby union senior male national team performed 4 sessions of cycling repeated sprints (consisting of 3 × eight 10-s sprints with 20 s passive recovery) either in normobaric hypoxia (RSH, 3000 m; n = 10) or in normoxia (RSN, 300 m; n = 9) over a 2-wk period. Before and after the training intervention, RSA was evaluated using a cycling repeated-sprint test (6 × 10-s maximal sprint and 20-s passive recovery) performed in normoxia. Results: Significant interaction effects (all P < .05, ηp2>.37 ) between condition and time were found for RSA-related parameters. Compared with Pre, maximal power significantly improved at Post in RSH (12.84 [0.83] vs 13.63 [1.03] W·kg^-1, P < .01, ηp2=.15 ) but not in RSN (13.17 [0.89] vs 13.00 [1.01] W·kg^-1, P = .45, ηp2=.01 ). Mean power was also significantly enhanced from Pre to Post in RSH (11.15 [0.58] vs 11.86 [0.63] W·kg^-1, P < .001, ηp2=.26 ), whereas it remained unchanged in RSN (11.54 [0.61] vs 11.75 [0.65] W·kg^-1, P = .23, ηp2=.03 ). Conclusion: As few as 4 dedicated specific RSH sessions were beneficial to enhance repeated power production in world-level rugby union players. Although the improvement from RSA to game behavior remains unclear, this finding appears to be of practical relevance as only a short preparation window is available prior to international rugby union games.

Progressive high-intensity interval training (HIIT) is not superior to unmodified non-progressive HIIT in an uncontrolled setting

BACKGROUND

High-intensity interval training (HIIT) is an important training component to improve aerobic and anaerobic exercise capacity. Higher HIIT workloads in general may generate additional effects on the improvement of exercise capacity, while missing adherence to more strenuous training regimes may affect training success. This study investigated if higher training workload generated by progressive HIIT (proHIIT) is superior to HIIT when used in an uncontrolled setting.

METHODS

Thirty-four moderately trained females and males performed a 4-week training intervention with three exercise sessions per week. Participants were randomized into two HIIT groups using the individual lactate threshold at baseline: Group 1 (N.=17), HIIT, four runs at maximal speed (all-out) with 30 s active recovery (total: 48 runs), Group 2 (N.=17), proHIIT, 4 runs at maximal speed (all-out) with 30-second active recovery with one extra repetition every week (up to seven runs, for a total of 66 runs). An incremental field test protocol with standard blood lactate (LA) diagnostic and heart rate monitoring was used to access changes in exercise capacity.

RESULTS

Overall, power output (running speed) at LA threshold (baseline LA+1.5 mmol/L) increased by +3.6% (P=0.004, effect size [ES] 0.38) after 4 weeks of HIIT. However, no significant between-group differences pre- vs post-intervention were detected.

CONCLUSIONS

Our data suggest that proHIIT does not provide additional improvement of running speed at individual lactate threshold over HIIT in an uncontrolled setting.

In-season adaptations to intense intermittent training and sprint interval training in sub-elite football players

This study investigated the in-season effect of intensified training comparing the efficacy of duration-matched intense intermittent exercise training with sprint interval training in increasing intermittent running performance, sprint ability, and muscle content of proteins related to ion handling and metabolism in football players. After the first two weeks in the season, 22 sub-elite football players completed either 10 weeks of intense intermittent training using the 10-20-30 training concept (10-20-30, n = 12) or sprint interval training (SIT, n = 10; work/rest ratio: 6-s/54-s) three times weekly, with a ~20% reduction in weekly training time. Before and after the intervention, players performed a Yo-Yo intermittent recovery test level 1 (Yo-Yo IR1) and a 30-m sprint test. Furthermore, players had a muscle biopsy taken from the vastus lateralis. Yo-Yo IR1 performance increased by 330 m (95%CI: 178-482, P ≤ 0.01) in 10-20-30, whereas no change was observed in SIT. Sprint time did not change in 10-20-30 but decreased by 0.04 second (95%CI: 0.00-0.09, P ≤ 0.05) in SIT. Muscle content of HADHA (24%, P ≤ 0.01), PDH-E1α (40%, P ≤ 0.01), complex I-V of the electron transport chain (ETC) (51%, P ≤ 0.01) and Na⁺ , K⁺ -ATPase subunits α₂ (33%, P ≤ 0.05) and β₁ (27%, P ≤ 0.05) increased in 10-20-30, whereas content of DHPR (27%, P ≤ 0.01) and complex I-V of the ETC (31%, P ≤ 0.05) increased in SIT. Intense intermittent training, combining short sprints and a high aerobic load, is superior to regular sprint interval training in increasing intense intermittent running performance during a Yo-Yo IR1 test and muscle content of PDH-E1α and HADHA in sub-elite football players.

5-Hydroxymethylfurfural and Alpha-Ketoglutaric Acid as an Ergogenic Aid During Intensified Soccer Training: A Placebo Controlled Randomized Study

Intensified training may lead to fatigue or even a state of overreaching with temporary reductions in performance. Any aid helping to prevent these consequences and to better tolerate such a training regime would be of great importance. 5-hydroxymethylfurfural (5-HMF) and α-ketoglutaric acid (α-KG) supplementation has been suggested to support favorable training outcomes but its effectiveness to facilitate adaptations during an intensified training period has never been investigated. During an in-season competition break (2 weeks), seventeen young outfield soccer players (age:14.7 ± 0.4 yr) performed a 9-day lasting shock microcyle including 5-7 repeated sprint exercise sessions in addition to the regular training (∼6 sessions/wk) and match (1-2 matches/wk) schedule. Before the training period a treadmill test to exhaustion, a YOYO intermittent recovery level 2 (YYIR2) and a repeated sprint ability (RSA) test were performed. The treadmill test was repeated 3 days after the shock microcycle whereas the YYIR2 and the RSA test on day 10 after the training. Magnitude based inference analysis showed likely positive effects of the 5-HMF/α-KG compared to the control group for changes in the maximal running velocity (+0.3 ± 0.7 vs. -0.3 ± 0.8 km/h) and running velocity at lactate turn-point 1 (+0.2 ± 0.4 vs. -0.2 ± 0.6) and lactate turn-point 2 (+0.4 ± 0.4 vs. -0.2 ± 0.6 km/h, for the 5-HMF/α-KG and placebo group, respectively). Training improved YYIR2 performance (+180 ± 67 vs. +200 ± 168m) and RSA (mean time: -0.1 ± 0.1 vs. -0.1 ± 0.1s, for the 5-HMF/α-KG and placebo group, respectively) in both groups and to the same extent. In conclusion, an in-season shock microcyle including repeated sprint training improves YYIR2 performance and RSA in youth soccer players. Supplementation with 5-HMF/α-KG did not modify training adaptations but led to likely positive exercise performance responses shortly after the intensified training regime.

Movement Economy in Soccer: Current Data and Limitations

Soccer is an intermittent team-sport, where performance is determined by a myriad of psychological, technical, tactical, and physical factors. Among the physical factors, endurance appears to play a key role into counteracting the fatigue-related reduction in running performance observed during soccer matches. One physiological determinant of endurance is movement economy, which represents the aerobic energy cost to exercise at a given submaximal velocity. While the role of movement economy has been extensively examined in endurance athletes, it has received little attention in soccer players, but may be an important factor, given the prolonged demands of match play. For this reason, the current review discusses the nature, impact, and trainability of movement economy specific to soccer players. A summary of current knowledge and limitations of movement economy in soccer is provided, with an insight into future research directions, to make this important parameter more valuable when assessing and training soccer players’ running performance.

Effects of short-term in-season break detraining on repeated-sprint ability and intermittent endurance according to initial performance of soccer player

To better understand the detraining effects in soccer, the purpose of the study was to analyse if performance level of soccer players modulate repeated-sprint ability (RSA) and intermittent endurance changes during 2-weeks of detraining (i.e., in-season break). Seventeen professional and sixteen young elite soccer players of two different teams performed, before and after 2-weeks of detraining, the RSA test and the Yo-Yo Intermittent Recovery Test, level 1 (YYIR1). Before detraining, professional players perform better (p < 0.05) RSA best time (RSA_best) than young players. A decrease (p < 0.05) in RSA_best, RSA total time (RSA_total) and mean time (RSA_mean) performance was observed in both teams, without changes in RSA fatigue index (Sdec). No significant changes in distance covered during YYIR1 was observed in any team. Before detraining, faster players from both teams (FG) (following the median split technique, soccer players with RSA_best ≤ 3.95 s) performed better (p < 0.01) in RSA_total, RSA_mean and RSA_best, but worse (p < 0.01) in Sdec. Although FG and the slower players (SG, RSA_best > 3.95 s) showed a worse (p < 0.05) RSA_total, RSA_best and RSA_mean performance after detraining (ES = 1.5, 1.4 and 2.9; ES = 0.6, 1.2 and 0.6; for FG and SG, respectively), the deterioration was greater in the FG for RSA_best (p < 0.05) and RSA_total (ES = 1.46). After detraining, FG improved (p < 0.05) Sdec performance. In conclusion, a 2-week in-season break (detraining) period induced a worse RSA, with no effect on intermittent endurance in professional and elite young soccer players, with greater detrimental effects on RSA_total and RSA_best in FG. In addition, Sdec does not seem to be sensitive to changes in RSA after a 2-week in-season break.

The Yo-Yo Intermittent Tests: A Systematic Review and Structured Compendium of Test Results

Background: Although Yo-Yo intermittent tests are frequently used in a variety of sports and research studies to determine physical fitness, no structured reference exists for comparison and rating of test results. This systematic review of the most common Yo-Yo tests aimed to provide reference values for test results by statistical aggregation of published data.

Methods: A systematic literature search for articles published until August 2017 was performed in MEDLINE, Web of Science, SPORTDiscus and Google Scholar. Original reports on healthy females and males ≥16 years were eligible for the analysis. Sub-maximal test versions and the Yo-Yo Intermittent Recovery Level 1 Children's test (YYIR1C) were not included.

Results: 248 studies with 9,440 participants were included in the structured analysis. The Yo-Yo test types most frequently used were the Yo-Yo Intermittent Recovery Level 1 (YYIR1, 57.7%), the Yo-Yo Intermittent Recovery Level 2 (YYIR2, 28.0%), the Yo-Yo Intermittent Endurance Level 2 (YYIE2, 11.4%), and the Yo-Yo Intermittent Endurance Level 1 (YYIE1, 2.9%) test. For each separate test, reference values (global means and percentiles) for sports at different levels and both genders were calculated.

Conclusions: Our analysis provides evidence that Yo-Yo intermittent tests reference values differ with respect to the type and level of sport performed.The presented results may be used by practitioners, trainers and athletes to rate Yo-Yo intermittent test performance levels and monitor training effects.

The effect of a concentrated period of soccer-specific fitness training with small-sided games on physical fitness in youth players

BACKGROUND

It is unknown whether a concentrated period of small-sided games and high intensity training is an effective training approach in youth soccer players. The aim of the study was to examine the effect of a concentrated period of soccer specific training on physical fitness in youth players.

METHODS

Nineteen male soccer players, from two teams, participated (mean age: 16.2±0.8 years; body mass: 58.2±7.6 kg; height: 170.8±7.7 cm). One team performed 5 days of small-sided games and high intensity training (SSG & HIT; N.=12) and the other team regular soccer training with 1 day of SSG and HIT (REG; N.=7) weekly for 4 weeks. The 30-15 intermittent fitness test (30-15 IFT), countermovement jump (CMJ) and change of direction (COD) performance were measured pre and post intervention. Heart rate (HR) and rating of perceived exertion (RPE) were recorded and session load calculated (RPE x minutes).

RESULTS

Average percentage of maximum HR and session load were 83% and 344 AU for the SSG & HIT versus 73% and 253 AU for the REG (P<0.05). 30-15 IFT improved for the SSG & HIT (from 17.0±1.1 to 18.4±0.8 km/h; P<0.05; ES=0.57) with no difference for the REG group (Pre: 17.9±1.3, Post: 18.2±1.6 km/h, ES=0.10). CMJ and COD were unchanged in both groups and no injuries were reported.

CONCLUSIONS

A 4-week concentrated period of daily SSG & HIT is effective for improving endurance performance in youth soccer players. This was without injuries and without negating performance in power and change of direction.