A number of fitness components have been identified as being important for the soccer player, including aerobic fitness, anaerobic fitness components (such as jumping ability and acceleration), strength, and flexibility.¹ The importance of aerobic fitness in professional soccer is highlighted by the facts that elite players cover 10–12 km during a competitive match at an average intensity of around 75% of their maximal oxygen uptake (Vo_2max) and that the aerobic system contributes approximately 90% of the total energy cost of match play.^2,3 High aerobic endurance performance has the potential to optimise soccer performance by enhancing recovery from high intensity intermittent bouts during match play, thereby contributing to the ability to sustain a high work rate throughout a full competitive match.^3,4 Thus, it is of importance to monitor the aerobic endurance performance of professional soccer players periodically throughout the soccer season.

Sub-maximal blood lactate assessment is a useful tool for detecting changes in endurance fitness,⁵ with lactate threshold determination possibly being a more sensitive indicator of aerobic endurance performance than Vo_2max.^6–8 A higher lactate threshold theoretically means that a player could maintain a higher average intensity during a soccer match without accumulation of lactate.^3,9 Sub-maximal blood lactate assessment has been previously utilised by researchers when assessing the aerobic fitness of soccer players. Blood lactate assessment has been used to monitor aerobic fitness changes across a pre-season and in-season intermission period with professional Danish players,² and throughout a soccer season with professional English players,^10,11 across two soccer seasons with an elite Scottish soccer team,¹² and over a 3 year period with international female Danish players.¹³

The aims of this study were: (a) to examine the changes in sub-maximal blood lactate measurement of professional Scottish youth soccer players throughout the soccer season; (b) to test whether two different established methods for determining lactate thresholds correlate; and (c) to discuss the possible limitations of lactate threshold measurements in terms of discrimination between the factors contributing to aerobic endurance performance.

METHODS

Subjects

Thirty seven male youth professional soccer players with a mean (standard deviation, SD) age of 18.3 (0.3) years participated in this study. The players were required to sign a consent form and medical questionnaire after being informed of the procedures and risks involved before each assessment. The study was approved by the local ethics committee.

The study involved the players being assessed by sub-maximal blood lactate measurement on a treadmill at six time points throughout a soccer season: July (beginning of pre-season training); October; December; January; April; and June (end of soccer season). Data collected from players who missed one or more testing time points were excluded from the analysis. Nine players were available at all six testing time points and their physical characteristics are presented in table 1.

Testing

Each player performed a 5 min warm up on a Woodway ERGO ES2 treadmill (Cranlea, Birmingham, UK) at a velocity that elicited a heart frequency (Hf) of approximately 60% of estimated maximum (Hf_max). Thereafter, the subjects carried out stretching exercises of their own choice for 5 min. The test protocol consisted of individual 4 min exercise stages, with a 0.5 km h⁻¹ increase in treadmill velocity until test termination. Each assessment started at a treadmill velocity that elicited a Hf of approximately 60% Hf_max. The test was terminated when the blood lactate concentration of the exercising subject exceeded 4 mmol l⁻¹ or by the subject’s own request. Hf was recorded at 3 min 45 s (Polar Accurex, Kempele, Finland). Blood samples were withdrawn from the subject’s thumb at the end of each exercise stage by pin prick and were analysed for whole blood lactate concentration using an Analox GM7 analyser (Analox Instruments, London, UK). Treadmill inclination was set at 0% throughout the assessment, and velocity was validated with an odometer (Trumeter, Radcliffe, UK).

Lactate threshold determination

Lactate threshold running velocity (v-T_lac) was identified as the first significant elevation of blood lactate above resting levels.¹⁴ In order to achieve objectivity, a two phase linear regression model¹⁵ was used in order to estimate the inflection point of the lactate profile “curve” (corresponding to the v-T_lac). A log transformation to both the running velocity and blood lactate concentration was applied in an attempt to gain a better estimate of the lactate threshold.¹⁶ Running velocity at a lactate concentration of 4 mmol l⁻¹ (v-4mM) was calculated by linear extrapolation, using the lactate concentration points at the running velocities directly before and after attainment of 4 mmol l⁻¹.

Maximum heart frequency determination

Hf_max for the players were determined from the highest Hf recorded (Polar Accurex) from a 20 m multi-stage shuttle test¹⁷ completed before and after the pre-season training period.

Training

A diary of squad training throughout the full season was obtained. A typical training week consisted of four to seven training sessions and one/two soccer matches per week (table 2).

Statistical analysis

There were always some players unavailable at each testing time point, sometimes because of illness or international youth team commitments, but mostly because of injury. Therefore, considerable imbalance in the data in terms of missing values over the six testing time points occurred. Nine players had complete data and all results refer to this sub-set. A repeated measures ANOVA (with Greenhouse-Geisser corrections as necessary) was used to determine any significant changes in mean v-T_lac and v-4mM across the soccer season. Pearson’s product-moment correlation was used to determine the relationships between variables. Data are expressed as means (SD).

RESULTS

Mean running velocity at v-T_lac and v-4mM increased from the start of the pre-season training to October (fig 1), resulting in significantly lower lactate levels at each measured running velocity (p<0.001) (fig 2). Mean running velocity at both lactate threshold markers increased from the start of pre-season training to October: 11.67 (0.29) to 12.96 (0.28) km h⁻¹ for v-T_lac and 13.62 (0.25) to 14.67 (0.24) km h⁻¹ for v-4mM (p<0.001). There are no significant differences in lactate responses to increased running velocity at any of the other testing time points.

When lactate values are plotted against Hf at the running velocities tested from pre-season to October, that is, the period showing significant changes in lactate response, no significant differences in the lactate concentration to Hf relationship were found (fig 3). Hf_max was unchanged across this time period, and subsequently v-T_lac and v-4mM expressed as %Hf_max were unchanged.

The two different methods for determining lactate threshold were significantly correlated. Correlation between v-T_lac and v-4mM was r = 0.87 (p<0.001). A significant correlation was also observed between Hf at v-T_lac and v-4mM (r = 0.82, p<0.001).

DISCUSSION

The main conclusion of this study is that aerobic endurance performance increased significantly from the start of the pre-season training period to the early weeks of the competitive playing season (October), as evidenced by significant increases in mean running velocity at both v-T_lac and v-4mM. This change is probably mainly attributable to the fact that the players returned to pre-season training in a detrained state, due to a summer intermission break of approximately 5 weeks duration. During the summer break the players were not involved in any structured aerobic fitness programme. Amigo et al¹⁸ investigated the effects of the summer intermission on amateur soccer players who had been training for 11 months prior to the summer break and found significant decrements in the cross-sectional area of slow twitch and fast twitch fibres, and a significant decrement in the activities of aerobic enzymes.

The finding of an improved aerobic endurance performance over the pre-season preparation period is supported by the studies of Bangsbo² and Brady et al,¹² although Dunbar¹¹ reported no change in fixed blood lactate concentrations (FBLC) of 2 and 3 mmol l⁻¹ over the pre-season period in a squad of English professional players. Bangsbo² tested elite soccer players using sub-maximal blood lactate and Vo_2max assessment before and after a 5 week pre-season training period. At the end of the training period, Vo_2max was only slightly higher than before the pre-season, but blood lactate concentrations during sub-maximal running were significantly lower, resulting in a 25% higher mean running velocity at an FBLC of 3 mmol l⁻¹, similar to the findings in the present study. Another study by Bangsbo² involved quantifying aerobic fitness changes in a Danish soccer team over a 7 week in-season intermission period. Mean Vo_2max increased from 58.6 to 60.3 ml kg⁻¹ min⁻¹ across the preparation period and the mean blood lactate concentration during sub-maximal treadmill running (9 to 13 km h⁻¹) was found to be significantly lower at all the treadmill velocities. The changes in blood lactate concentrations of approximately 1 mmol l⁻¹ at the same running velocity were identical to the improvements in this study.

A change in lactate threshold can be expressed relative to Vo_2max or to Hf_max. In this study, we have presented lactate values relative to Hf (fig 3), indicating that there is no significant change in relative lactate threshold from the start of pre-season training to the early days of the competitive season (October), although there are clearly higher running velocities at the same lactate values in October when compared to pre-season (fig 2). These findings are in line with those of Helgerud et al³ who showed no change in lactate threshold (relative to Hf_max and Vo_2max ) when significantly elevating Vo_2max after an endurance intervention period in soccer players. Although running velocity increased by 16% from 11 to 13.5 km h⁻¹ (p<0.05) in the study of Helgerud et al,³ lactate threshold expressed relative to Hf_max (87.4 (2.3) to 87.6 (2.4) %Hf_max) and Vo_2max (82.4 (3.1) to 86.3 (2.1) %Vo_2max) was not significantly changed. It seems that lactate threshold changes in tandem with Vo_2max in soccer players, and in terms of %Hf_max and %Vo_2max the adaptability seems to be minor (<5%).^3,10,19

Reproducibility of the v-T_lac and v-4mM using the same incremental treadmill protocol as in this study has been investigated recently²⁰ with no differences in reproducibility found between v-T_lac and v-4mM. As can also be observed in fig 1, any fixed lactate value between 2 and 4 mmol l⁻¹ would reveal similar changes, and may be used for lactate assessment of soccer players.

Pate and Kriska²¹ have described a model that incorporates the three major factors accounting for inter-individual variance in aerobic endurance performance, namely Vo_2max, lactate threshold, and work economy, with numerous published studies supporting this model.^9,22,23 Thus, the model serves as a useful framework for comprehensive examination of the effects of aerobic training on endurance performance. Since lactate threshold in relative terms does not change in this study, the explanation for improved aerobic performance may be attributable to changes in Vo_2max and/or running economy (C_R).

Vo_2max has been suggested to be a very important factor for determining success in an aerobic endurance sport.^24,25 Previous studies have demonstrated a significant relationship between Vo_2max and distance covered during soccer match play^3,26,27 and number of sprints attempted.²⁶ No correlation, however, has been documented between %Vo_2max at lactate threshold and performance during soccer match play, although a significant correlation has been found between v-4mM and distance covered by soccer referees (r = 0.73, p<0.05).²⁸ An improved Vo_2max will lower the lactate concentration and Hf at a standardised running velocity, similar to what has been shown in this study. The improvements in running velocity at a standard lactate level and reduced Hf of approximately 10 beats min⁻¹ at a fixed speed after the pre-season preparation period may indicate an improved Vo_2max in the range of 3–5 ml kg⁻¹ min⁻¹.⁹ Even though Vo_2max was not measured in this study, the most plausible explanation for the improvements may be related to an improved Vo_2max.

Improvement in endurance performance in this study may also be attributed to improvements in C_R. Helgerud et al³ observed improvement in C_R of soccer players when using a two-sessions-a-week specific running intervention. Hoff and Helgerud^3,29 have also demonstrated a 5% improvement in C_R in soccer players when using a maximal strength training intervention. In the present study, however, no strength training in terms of heavy neuromuscular weight training was employed. C_R was not measured in the present study, but it is plausible that C_R may have changed across the season.

This study has shown that aerobic endurance performance of professional youth soccer players increased significantly over the pre-season training period, but did not change significantly during the competitive season. The findings of the present study echo the findings of the historical study of Thomas and Reilly³⁰ who reported that endurance fitness increased during the first third of the season and remained relatively stable throughout the course of the competitive season. The findings of the present study are in contrast to those of Brady et al¹² who demonstrated deterioration of the player’s endurance levels over the course of the competitive season following the achievement of peak fitness at the end of the pre-season training period. The authors suggested that the decrement of aerobic fitness levels towards the end of the season may have been a consequence of coaches “scaling down” training in order to save the players’ efforts for competition. Time normally allocated for soccer training may be reduced as the soccer season progresses, due to an accumulation of competitive matches. A backlog of previously postponed matches and more matches in the latter stages of cup competitions for successful teams may decrease weekly time available for endurance training, especially in the last few months of the season. The endurance fitness gains manifested from the pre-season preparation period were successfully maintained for the remainder of the season in the present study. This may indicate that the physiological load of competing in two competitive soccer matches per week in conjunction with the training regime documented in the present study may be sufficient for maintaining endurance fitness levels during the in-season period. This means that it is important that soccer players who are not participating regularly in competitive matches perform additional individual endurance training to maintain, or even increase, their endurance levels.³¹ Soccer coaches are challenged to employ training strategies that increase the aerobic endurance performance of soccer players during the competitive in-season period, whilst not increasing the risk of over-reaching and over-training or promoting fatigue on match days that would adversely affect performance.

Although 37 players were available for testing throughout the season, only nine players were available for testing at all six testing time points. Three players left the squad during the study, and seven players were unavailable at one or more of the testing time points due to international commitments or illness. Therefore, most data collected in this study were excluded due to 18 players (49% of the squad) being injured at the time of one or more of the testing time points. Although this may seem a high occurrence of injury, studies by Hawkins et al³² and Woods et al^33–35 have highlighted the high incidence of injury in professional soccer, especially in younger age groups.³³

Conclusions

This study has shown that the endurance fitness of the soccer players increased over the pre-season to the early weeks of the competitive season, and was unchanged for the remainder of the season, in line with earlier findings. The results of this study also indicate that sub-maximal blood lactate assessment of soccer players can be used as an indicator of change in soccer players’ endurance performance levels over a specified time period. This study has shown that any fixed blood lactate level between 2 and 4 mmol l⁻¹ is appropriate during lactate assessment to determine sub-maximal responses to change in aerobic endurance levels. Since lactate threshold alterations appear to change in tandem with Vo_2max for soccer players, and also because C_R of soccer players may be altered by specific endurance running and strength training interventions, it is suggested that Vo_2max and C_R of professional soccer players should be primarily assessed.