The impact of a mid-season FIFA World Cup™ on injury occurrence and patterns in French professional soccer clubs.

This study retrospectively compared the effect of holding the 2022 FIFA World Cup™ (WC) mid-season (season 2022/23) on injury rates and patterns in French Ligue 1 soccer clubs. Epidemiological data in 17 clubs were prospectively recorded by their physicians. Time-loss injuries (injuries leading to a player being unable to fully participate in play over the following 72-hour period) were compared with those reported during a regular season (2021-22). In the WC season, an increase of approximately 23% (training + match-play) was observed for both the total number of injuries and knee, ankle and muscle injuries combined. Incidences for match-play injury overall and for the knee, ankle and muscle regions combined and the hamstrings and calf regions specifically also rose significantly (range: p < 0.05-p < 0.01). These results suggest that injury occurrence and patterns in French L1 soccer clubs were substantially affected during the 2022/23 season when a mid-season WC was held.

[Hamstring muscle injury in football players - Part I : epidemiology, risk factors, injury mechanisms and treatment]. / La lésion musculaire des ischio-jambiers chez le footballeur. Partie 1 : épidémiologie, facteurs de risque, mécanismes lésionnels et traitement.

Due to its high frequency and recurrence rate, hamstring injury represents an important issue in football currently. The mechanisms of injury and the main modifiable and non-modifiable risk factors are now well documented and should allow the implementation of effective preventive strategies. In the treatment of the injured player, the physician will have to rely on a close collaboration with a quality sports physiotherapist and implement a series of key elements allowing an optimal return to the soccer field, which means at the same level of performance compared to the pre-injury period and with a minimal risk of recurrence. This article discusses these different elements in the form of a narrative review of the literature.

La lésion musculaire des ischio-jambiers, de par sa fréquence et son taux de récidive élevés, représente une problématique actuelle importante dans le football. Les mécanismes lésionnels et les principaux facteurs de risque modifiables et non modifiables sont désormais bien documentés et devraient permettre la mise en place de stratégies préventives efficaces. Dans le suivi du joueur blessé, le médecin devra compter sur une collaboration étroite avec un kinésithérapeute du sport de qualité et mettre en place toute une série d'éléments clés permettant un retour optimal sur les terrains de football, c'est-à-dire au même niveau de performance comparativement à la période d'avant blessure et avec un risque minimal de récidive. Cet article aborde ces différents éléments sous l'aspect d'une revue narrative de la littérature.

[Hamstring muscle injury in football players. Part 2 : preventive strategies]. / La lésion musculaire des ischio-jambiers chez le footballeur. Partie 2 : stratégies de prévention lésionnelle.

Prevention of hamstring injuries represents an important issue for football players and clubs. Preventive strategies can be effective if they include multiple dimensions and are well-structured. Five points appear essential in order to obtain a high-quality preventive strategy: progressive muscle strengthening, optimal workload management, lumbopelvic stability exercises, development of physical condition and optimization of sprint technique. While recognizing the limitations of preventive screening and the difficulty of predicting future injury, screening tests appear relevant for the identification of an individual risk profile for each footballer and in defining each player's work priorities. Finally, secondary prevention starts with the implementation of rigorous and high-level rehabilitation, as well as a special attention to players with a history of hamstring injury.

La prévention des lésions musculaires des ischio-jambiers représente une thématique de première importance pour les joueurs et clubs de football. Les stratégies préventives peuvent se révéler efficaces, à condition d'inclure de multiples dimensions à celles-ci et de structurer ces démarches. Cinq points apparaissent incontournables dans l'optique d'obtenir une stratégie préventive de qualité : le renforcement musculaire progressif et raisonné, la gestion équilibrée de la charge de travail, le travail de la stabilité lombo-pelvienne, le développement de la condition physique et l'optimalisation de la gestuelle de course. Tout en reconnaissant les limites du screening préventif et la difficulté de prédire une future blessure, un état des lieux précis peut s'avérer pertinent pour identifier le profil de risque individuel de chaque footballeur et pour définir les priorités de travail de chacun. Enfin, la prévention secondaire démarre par la mise en place d'une rééducation rigoureuse et structurée, ainsi que par une attention particulière aux joueurs avec antécédents de lésions aux ischio-jambiers.

Injury rates and patterns in French male professional soccer clubs: a comparison between a regular season and a season in the Covid-19 pandemic.

This study investigated time-loss injury occurrence and patterns between the first season (2020/21, S2) completed during the Covid-19 pandemic (longer pre-season following cancellation of the 2019/20 season but shorter duration) and a regular season (2018/19, S1) in French Ligue 1 and 2 professional soccer clubs. Epidemiological data were prospectively recorded in a national injury database by each club's physician. In all clubs combined, the mean number of injuries per club was 31.5 and 36.6 in S2 and S1, respectively (-13.9%). Overall match injury incidence (per 1000 hours) in all clubs combined was lower in S2 versus S1 (22.23 vs 25.96, p < 0.01). In Ligue 1 clubs alone, match-play incidences for injury overall (24.92 vs 29.42), muscle strains (10.59 vs 13.24) and strains specifically in the hamstring region (4.52 vs 6.22) were lower in S2 versus S1 (all p < 0.05). No differences in the incidence of match injuries affecting the ankle and knee regions were observed. Changes in the 2020/21 season structure and duration owing to the Covid-19 pandemic seem not to have had a negative effect on injury occurrence and patterns in French professional soccer clubs.

A retrospective analysis of all-cause and cause-specific mortality rates in French male professional footballers.

This study retrospectively compared all-cause and cause-specific mortality in French male professional football players with data from France's national population. Altogether, 6114 individuals born in Metropolitan France or in one of its overseas territories who played at least one competitive match in France's professional football championships between January 1, 1968 and December 31, 2015, were identified and followed up for vital status obtained from a national reference database until December 31, 2015. Data on all-cause and cause-specific mortality were subsequently compared to the expected number of deaths for the national population after standardization for the year, age, and sex. Ratios between observed and expected deaths provided standardized mortality ratios (SMR) along with 95% confidence intervals (95% CI). Linear trends were investigated using the Poisson trend test. Altogether, 662 player deaths were observed. All-cause mortality overall was lower than that of the national population (SMR: 0.69, 95% CI 0.64-0.75). An excess of deaths from dementia was observed in the players (SMR: 3.38, 95% CI: 2.49-4.50) whereas mortality from diseases of the nervous (SMR: 0.64, 95% CI: 0.35-1.08) and cardiovascular systems (SMR: 0.82, 95% CI: 0.70-0.96), and cancer (SMR: 0.67, 95% CI: 0.58-0.76) was lower. Lower overall mortality and that owing to common cardiovascular and cancer-related diseases were reported in French professional football players compared to France's national population. In line with previous studies, however, excess mortality from dementia was observed in the players. Career length was not associated with all-cause or cause-specific mortality. Prospective matched-cohort studies are necessary to identify the neurologic impact of participation in professional football.

Video analysis of concussion mechanisms and immediate management in French men's professional football (soccer) from 2015 to 2019.

In this study, the concussion mechanisms were analyzed in male professional competition football, with the main objective to specify the frequency of head-to-head impact, and immediate management of the concussed players was described in order to check its compliance with the recommendations of football's governing bodies. Based on continuously recorded data from the French Football Federation (FFF), a retrospective database of all reported concussions during matches in the 1st and 2nd French Male leagues was generated comprising seasons 2015/16-2018/19. Injury mechanisms, playing action, immediate medical assessment and management of concussed players, and foul play-referee's decision, were analyzed from video recordings. In total, 41 concussions were reported (incidence rate of 0.44/1000 hours of match exposure [95% CI: 0.40 to 0.49]) of which 36 were identified and analyzed on video sequences. The commonest playing action leading to concussion was aerial challenge (61%), and the main mechanism was head-to-head impact (47%). Following the head impact, 28% of concussed players were not medically assessed on pitch and 53% returned to play the same match. Head-to-head impact was not associated with systematic medical assessment, nor with foul play. In conclusion, the main cause of concussions involved head-to-head impact occurring when two players challenge for heading the ball in the air. The detection of potential concussive head impacts and the immediate management of players possibly concussed during matches remain insufficient according to the international recommendations. Some rules changes, with particular vigilance in case of head-to-head impact, should be discussed.

The effects of a congested fixture period on physical performance, technical activity and injury rate during matches in a professional soccer team.

BACKGROUND: In modern professional soccer, the ability to recover from official match-play and intense training is often considered a determining factor in subsequent performance. OBJECTIVE: To investigate the influence of playing multiple games with a short recovery time between matches on physical activity, technical performance and injury rates. METHODS: The variation of physical (overall distance, light-intensity, low-intensity, moderate-intensity and high-intensity running) and technical performance (successful passes, balls lost, number of touches per possession and duels won) of 16 international players was examined during three different congested periods of matches (six games in 18 days) from the French League and Cup (n=12), and the UEFA Champions' League (n=6) during the 2011-2012 season and compared with that reported in matches outside these periods. Data were collected using a computerised match analysis system (Amisco). Injury rate, time loss injuries, as well as the mechanism, circumstances and severity of the injury were also analysed. RESULTS: No differences were found across the six successive games in the congested period, and between no congested and the three congested periods for all the physical and technical activities. The total incidence of injury (matches and training) across the prolonged congested periods did not differ significantly to that reported in the non-congested periods. However, the injury rate during match-play was significantly higher during the congested period compared with the non-congested period (p<0.001). The injury rate during training time was significantly lower during the congested period compared with the non-congested periods (p<0.001). The mean lay-off duration for injuries was shorter during the congested periods compared with the non-congested periods (9.5±8.8 days vs 17.5±29.6 days, respectively p=0.012, effect sizes=0.5). CONCLUSIONS: Although physical activity, technical performance and injury incidence were unaffected during a prolonged period of fixture congestion, injury rates during training and match-play and the lay-off duration were different to that reported in matches outside this period.

Unmasking mental health symptoms in female professional football players: a 12-month follow-up study.

Objective: To calculate the prevalence rates of mental health symptoms among female professional football players over a 12-month period and to explore the associations of severe injury and related surgery with mental health symptoms among female professional footballers. Methods: An observational prospective cohort study was conducted over a 12-month follow-up period by distributing an electronic questionnaire three times. The questionnaire was based on validated screening tools for assessing mental health symptoms. Results: A total of 74 female professional football players participated in this study. Mental health symptoms ranged from 1% for substance misuse to 65% for sport-psychological distress at baseline, from 6% for anxiety to 53% for sport-psychological distress 6 months postbaseline and from 3% for substance misuse to 55% for sport-psychological distress 12 months postbaseline. The prevalence of disordered eating remained between 15% and 20% over the 12-month period. Only one of the associations was statistically significant. Female professional football players were nearly twice as likely to report sport-related psychological distress following every surgery. Conclusions: The substantial prevalence of mental health symptoms among female professional football players emphasises the need for increased attention, awareness and interventions. Additionally, female professional football players are nearly twice as likely to report sport-related psychological distress after each surgery. Sports medicine physicians and mental health professionals working in female football should provide standard care, which involves identifying, monitoring and implementing tailored interventions for mental health symptoms.

Effect of an injury prevention program on muscle injuries in elite professional soccer.

Due to the continual physical, physiological, and psychological demands of elite level soccer increasing the incidence and risk of injuries, preventative training programs have become a common feature of soccer players training schedule. The aim of the current investigation was to examine the effectiveness of a structured injury prevention program on the number of muscle injuries and the total number of injuries within elite professional soccer. The present study was conducted over 2 consecutive seasons, of which the first (2008-2009) being the intervention season and the second the control season (2009-2010). In total, 26 and 23 elite male professional soccer players competing within the Scottish Premier League and European competition participated. The training program was performed twice weekly for the entirety of the season (58 prevention sessions). The results revealed an increase in the total number of injuries within the intervention season (88 vs. 72); however, this was largely due to the greater number of contusion injuries sustained within the intervention season (n = 44) when compared with control season (n = 23). Significantly less muscle injuries were observed during the intervention season (moderate effect), and this occurred concomitant with a bigger squad size (large effect, p < 0.001). The findings from this study identify that a multicomponent injury prevention-training program may be appropriate for reducing the number of muscle injuries during a season but may not be adequate to reduce all other injuries.

Determination of the Prevalence of Knee and Hip Clinical Osteoarthritis in the Active Professional Male Footballer and Its Association with Pain, Function, Injury and Surgery.

OBJECTIVE: To comment on and explore (1) the prevalence of clinical knee and hip osteoarthritis (OA); (2) the association between pain or function and clinical knee or hip OA; (3) the association between injury or surgery and clinical knee or hip OA. METHODS: Participants were recruited from FIFPRO members. A total of 101 footballers consented to answer (1) a developed questionnaire, (2) patient-reported outcome measures, and (3) be evaluated by their team physician for clinical knee or hip OA. RESULTS: Of the 53% evaluated for clinical knee and hip OA, a prevalence of 9.43% and 7.55% of knee and hip OA, respectively, was found. There was a significant and strong association between knee (p = 0.033; Cramers v Value = 0.523) and hip pain (p = 0.005; Cramers v Value = 0.602) and clinical OA. A significant association existed between Hip dysfunction and Osteoarthritis Outcome short form Scores and clinical OA of the hip (p = 0.036). The odds of clinical knee OA were 1.5 and 4.5 times more after one or more injuries or surgeries, respectively. There was no association between playing position and clinical OA. CONCLUSION: There is a low prevalence of a clinical knee or hip OA in the active professional male footballer. Pain may be a valid symptom to predict or monitor knee or hip OA. Validated assessment tools should be utilised to identify a negative effect on function. The odds of developing clinical OA in the knee with the number of injuries or surgeries. The hip presents with earlier clinical signs of OA compared to the knee.

Incidence and characteristics of COVID-19 in French professional football players during the 2020-2021 season.

OBJECTIVES: To describe the epidemiology of COVID-19 in French professional football players, and to compare the infection incidence with the general population across the first three waves. METHODS: During the 2020-2021 season, all professional football players (n = 1217) in the two primary French leagues underwent weekly testing for SARS-CoV-2 infection by nasopharyngeal PCR, in combination with rigorous infection control measures. RESULTS: Among all players, 572 (47%) tested positive at least once, with no COVID-19-related death or hospital admission. Monthly incidence estimates in players ranged from 1486 to 6731 per 100,000 individuals, i.e. 2-17 times higher than incidence estimates in the general population in France during the study period. CONCLUSION: Almost 50% of professional football players developed SARS-CoV-2 infection during the 2020-2021 season in France, with no severe complication.

A four-season prospective study of muscle strain reoccurrences in a professional football club.

The aim of this investigation was to characterise muscle strain reinjuries and examine their impact on playing resources in a professional football club. Muscle strains and reoccurrences were prospectively diagnosed over four seasons in first-team players (n = 46). Altogether, 188 muscle strains were diagnosed with 44 (23.4%) of these classed as reinjuries, leading to an incidence of 1.32 strain reoccurrences per 1,000 hours exposure (95% Confidence Interval [CI], 0.93-1.71). The incidence of recurrent strains was higher in match-play compared with training (4.51, 95% CI, 2.30-6.72 vs 0.94, 95% CI, 0.59-1.29). Altogether, 50.0% of players sustained at least 1 reoccurrence of a muscle strain, leading to approximately 3 days lost and 0.4 matches missed per player per season. The incidence of recurrent strains was highest in centre-forwards (2.15, 95% CI, 1.06-3.24), peaked in May (3.78, 95% CI, 0.47-7.09), and mostly affected the hamstrings (38.6% of all reoccurrences). Mean layoff for nonreoccurrences and recurrences was similar: approximately 7.5 days. These results provide greater insight into the extent of the problem of recurrent muscle strains in professional football.

The Effects of Menstrual Cycle Phase on Elite Athlete Performance: A Critical and Systematic Review.

Background: In elite athletes, training individualization is widely recommended to optimize competitive performance. Previous studies have evidenced the impact of hormonal fluctuations on different performance parameters among female athletes. While consideration of menstrual cycle (MC) phases as a parameter in training individualization strategies is necessary, systematic evidence identifying such impacts in elite athletes should be evaluated. Objective: Systematically review publications that have investigated the link between MC phases and performance in elite female athletes. The overarching aim is to identify whether a consensus across studies exists enabling evidence-based recommendations for training individualization depending on menstrual cycle phases. Methods: This review followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Three major scientific publication databases were searched from inception until November 3, 2020. Studies included focused on the influence of physiological or psychological parameters throughout at least one phase of the menstrual cycle of elite athletes. Results: A total of 780 search results were yielded and 26 references from a past bibliography were added manually. About 662 papers were reviewed of which 218 studies were assessed for eligibility. Of these, only seven (1%) precisely investigated the influence of a performance or physical parameter during at least one menstrual cycle phase. These seven studies included a total of 314 elite female participants (20.58 ± 1.91 years). Three used interviews, questionnaires or prospective analyses of reports. Four conducted several performance tests or included physical measures although only two performed tests during training or before/during competition. Among the seven studies, five performed hormonal testing through sampling of blood, saliva, or urine. The remaining relied on athletes' menstruation diaries. The current evidence suggests a variable association between menstrual cycle and a few performance-related outcomes, such as endurance or power resistance, ligament stiffness, decision making skills, psychology, or competitiveness. Conclusion: Different sports performance-related parameters are affected during the menstrual cycle among elite athletes, but the parameters themselves and the magnitude and the direction of the effects are inconclusive. Additional longitudinal and prospective studies to systematically monitor on-field performance parameters are urgently required in order to enable recommendations and training individualization in female elite athletes.

Concussion in European professional football: a view of team physicians.

OBJECTIVES: The objective was to explore the view and thoughts of physicians working in professional football about several aspects (eg, education and use of video footages) likely to enhance concussions' recognition and on-field management. METHODS: An observational study based on a cross-sectional design by means of an electronic survey was conducted among physicians working for a professional football club in Belgium, England or France. RESULTS: A total of 96 physicians (95% male; mean age: 44 years) completed the survey. Nearly all participants (95%) were in favour of informational sessions about concussion for players or technical staff. Only 5%-10% of the participants mentioned that they had felt pressured by the technical staff or players not to substitute a player with a (potential) concussion. Most participants were in favour of an additional permanent concussion substitution and a temporary concussion substitution. Four out of five participants reported that the availability of instant video footages (side-line) would ease the recognition of concussion. CONCLUSION: A better recognition and on-field management of concussions in professional football can only be achieved with a holistic approach, including adequate laws of the football game and protocols. Especially, regular education of players and technical staff should be made mandatory while the medical teams should be provided side-line with instant video footages.

Variation in body composition in professional soccer players: interseasonal and intraseasonal changes and the effects of exposure time and player position.

The aim of this study was to examine variations in measures of body composition in elite soccer players. Skinfolds and measures of body mass (BM) recorded on a monthly basis across an entire competitive season in a group of senior professional players (n = 26) were used to estimate percentage body fat (%BF) and provide fat-free body mass (FFBM) values. Mean values in players were compared between 6 specific positional roles (goalkeepers, central and lateral defenders/midfielders and center-forwards). In-season variations in measures were studied by comparing values at 5 separate points across the season. The effects of positional group (goalkeepers, defenders, midfielders, and forwards) and exposure time to play (participation time in training and matches) in relation to in-season variations were also examined. To investigate interseasonal changes, repeated measures were taken in players (n = 9) over 3 consecutive seasons. In relation to positional role, a difference in average %BF and BM values was observed (p < 0.001), with substantial differences observed in goalkeepers, lateral midfielders, and forwards. Across all players, there were significant in-season variations in %BF (between start- and mid-season and mid- and end-season, p < 0.001) and FFBM (between start- and mid-season and start- and end-season, p < 0.001), whereas BM remained unchanged. Further analysis of these fluctuations in %BF and FFBM at different points of the season showed that variations differed across the positional groups (p < 0.01), especially in defenders and midfielders. In contrast, no association was observed between measures and exposure time and no differences were reported across seasons. Practitioners should consider individual positional role when interpreting mean body composition data. They should also take into account positional groups when in-season variations in body composition are identified.

A three-year prospective study of illness in professional soccer players.

The aim of this study was to investigate rates and patterns of illness in professional soccer and their impact on playing resources. Illnesses were prospectively diagnosed over three seasons in first-team professional soccer players (n = 81) by the club's physician. A total of 203 illnesses were diagnosed with a mean of 2.5 +/- 0.7 complaints per player per season. The majority of complaints were classed as upper respiratory (74.5%) and gastrointestinal illnesses (13.7%). Of all illness episodes, 19.7% required a lay-off (mean duration: 2.1 days). Per season, 0.3% +/- 0.01 of all working days were lost due to illness; each player lost an average of 1.1 +/- 0.1 working days and missed 0.1 +/- 0.2 matches. Results from this study suggest that common infectious illnesses have little impact on player availability in professional soccer.

Does an Optimal Relationship Between Injury Risk and Workload Represented by the "Sweet Spot" Really Exist? An Example From Elite French Soccer Players and Pentathletes.

OBJECTIVE: To examine the relationships between the occurrence and severity of injuries using three workload ratios (ACWR, EWMA, REDI) in elite female soccer players and international male and female pentathletes. MATERIALS AND METHODS: Female soccer players in the U16 to U18 national French teams (n = 24) and international athletes (n = 12, 4 women and 8 men) in the French modern pentathlon team were monitored throughout an entire season. The Acute Chronic Workload Ratio (ACWR), the Exponentially Weighted Moving Averages (EWMA), and the Robust Exponential Decreasing Index (REDI) were calculated for internal load by the ROE method in soccer and external load in pentathlon. The occurrence and severity of injuries (determined according to time-loss) were quantified in the sweet spot zone [0.8; 1.3] and compared to the other zones of load variation: [0; 0.8], [1.3; 1.5], [1.5; +8], using the three ratios. RESULTS: Over the study period, a total of sixty-six injuries (2.75 per athlete) were reported in the soccer players and twelve in pentathletes (1 per athlete). The cumulative severity of all injuries was 788 days lost in soccer and 36 in pentathlon: respectively, 11.9 days lost per injury in soccer player and 3.0 per pentathlete. The mean values across the three methods in soccer showed a higher number of injuries detected in the [0; 0.8] workload ratio zone: 22.3 ± 6.4. They were 17.3 ± 3.5 in the sweet spot ([0.8-1.3] zone) and 17.6 ± 5.5 in the [1.5; +8] zone. In comparison to the [1.5; +8] zone, soccer players reported a higher number of days lost to injuries in the presumed sweet spot and in the [0-0.8] zone: 204.7 ± 28.7 and 275.0 ± 120.7 days, respectively. In pentathletes, ten of the twelve injuries (83.3%) occurred in the presumed sweet spot. REDI was the only method capable of tracking workloads over all-time series. CONCLUSION: In the present cohort of elite soccer players and pentathletes, acute chronic workload calculations showed an association with injury occurrence and severity but did not provide evidence supporting existence of a sweet spot diminishing injury risk.

Monitoring the health of transitioning professional footballers: protocol of an observational prospective cohort study.

INTRODUCTION: Transitioning out of professional football is a challenging time in most players' lives. During these preretirement and postretirement years, professional footballers may struggle with their mental, musculoskeletal, neurocognitive and cardiovascular health. Currently, longitudinal data about these health conditions are lacking. This article presents the design of a prospective cohort study with the primary aim of gathering epidemiological evidence about the onset and course of mental, musculoskeletal, neurocognitive and cardiovascular health conditions in professional footballers during their preretirement and postretirement years and evaluating the associations between risk indicators and the health conditions under study in these players. METHODS AND ANALYSIS: An observational prospective cohort study with repeated measurements over a follow-up period of 10 years will be conducted among at least 200 professional footballers (male; 27 (±1) years old). Mental health will be explored by assessing symptoms of distress, anxiety, depression, sleep disturbance, alcohol misuse, drug misuse and disordered eating. Musculoskeletal health will be explored by assessing severe joint injury and related surgery, clinical and radiological osteoarthritis, and joint function (hips, knees and ankles). Neurocognitive health will be explored by assessing the concussion, brain structure and functioning, and neurocognitive functioning. Cardiovascular health will be explored by assessing blood pressure, lipid profile and ECG abnormalities. ETHICS AND DISSEMINATION: Ethical approval for the study was provided by the Medical Ethics Review Committee of the Amsterdam University Medical Centers. The results of the study will be submitted to peer-reviewed journals, will be presented at scientific conferences and will be released in the media (postpublication). TRIAL REGISTRATION NUMBER: The Dutch Trial Registry (Drake Football Study NL7999).

Longitudinal variation of hemoglobin and reticulocytes in elite rowers.

Longitudinal monitoring of athlete's hematologic parameters holds considerable promise as a strategy to detect and thereby deter illicit blood doping. This study documents temporal changes of hemoglobin concentration (Hb) and reticulocyte counts in elite rowers. The 'within subject' variation in rowers was comparable to that of athletes from other sports. Reticulocyte results were dependent on the type of instrument used.