Due to its acyclical nature and intensity, soccer is classified as a high intensity intermittent team sport.¹ During competitive soccer match play, elite players cover a distance of about 10–12 km^2–,⁵ at an average intensity close to the anaerobic threshold, being 80–90% of maximal heart frequency (Hf_max) or 70–80% of maximal oxygen uptake (Vo_2max).^3,6,7 It is estimated that aerobic metabolism provides 90% of the energy cost of soccer match play.¹ Therefore, it is a prerequisite in the modern game for the elite soccer player to have high aerobic endurance fitness. Pate and Kriska⁸ have described a model that incorporates the three major factors that account for inter-individual variance in aerobic endurance performance, namely Vo_2max, lactate threshold (LT), and work economy (C), with numerous studies supporting this model.^7,9–,¹³ The physiological elements of this model can be used when studying the physiological profiles of soccer players and the physiological adaptations to specific training interventions.

Vo_2max is considered to be the most important component of endurance performance.^14,15 Small alterations in LT occur in response to running interventions when LT is expressed as a percentage of Vo_2max.⁷ Minor changes have been shown for running economy (C_R) after running interventions, although C_R is positively influenced by maximal strength training based on maximal mobilisation of force and rate of force development (RFD).¹⁶

The mean Vo_2max of elite soccer players is normally reported to be between 55 and 68 ml kg⁻¹ min⁻¹.^14,17–,²⁴ These moderate to high values are similar to those found in other team sports,²⁵ but are substantially lower than elite endurance performers where values close to 90 ml kg⁻¹ min⁻¹ have been found.¹⁵ Individual values higher than 70 ml kg⁻¹ min⁻¹ for modern soccer players have been recently reported.^7,15,24

The importance of Vo_2max in soccer has been reflected by rank correlation of the most successful teams in the Hungarian 1st Division Championship.²⁶ Wisløff et al²⁴ supported this aerobic power-success relationship by demonstrating a clear difference in Vo_2max between the top team Rosenborg (67.6 ml kg⁻¹ min⁻¹) and a lower placed team Strindheim (59.9 ml kg⁻¹ min⁻¹) in the Norwegian elite division. The observation of a high correlation between Vo_2max and distance covered during match play supports the adoption of training regimes that raise the Vo_2max of soccer players to high levels.^7,27,28 Smaros²⁸ reported that in addition to the strong correlation with the total distance covered in the game (r = 0.89), Vo_2max also influenced the number of sprints attempted during a match. It has been recently shown that by improving the mean Vo_2max of youth soccer players by 11% over an 8 week period, a 20% increase in total distance covered during competitive match play was manifested, along with a 23% increase in involvements with the ball and a 100% increase in the number of sprints performed by each player.⁷

In activities that involve dynamic work with large muscle mass utilisation, as in playing soccer, it is generally assumed that Vo_2max is primarily limited by maximal cardiac output.²⁹ Whereas untrained subjects are limited peripherally, trained individuals are primarily limited centrally, with the maximal stroke volume of the heart being accepted as the major limiting factor of Vo_2max.^30,31 Increased stroke volume has recently been shown up to the level of Vo_2max.³²

Hoff and Helgerud¹⁵ thus argue that continuous interval training for 3–8 min with a working intensity of 90–95% of Hf_max should increase Vo_2max by increasing the maximal cardiac output by improving stroke volume. Recently, Helgerud et al^7,33 showed the effectiveness of such a training regime, significantly increasing mean Vo_2max and C_R of soccer players using running and treadmill running as training modes, respectively. Similar physiological load could be obtained by using a specifically designed soccer track where the subjects dribble a soccer ball instead of regular running.³⁴ This might however raise different problems in terms of the subjects and coaches’ control of intensity, control of the ball, etc, that might influence training execution and response.

The main aim of this study was to intervene in a professional youth soccer team using high intensity interval training with the players dribbling a soccer ball around a specifically designed track. An improvement in Vo_2max of similar magnitude as that attained by Helgerud et al⁷ for youth players and by Helgerud et al³³ for Champions League players was hypothesised, with no adverse effects on sprinting and jumping ability. A secondary aim of this study was to present normative physiological data on professional youth soccer players.

METHODS

Training intervention

The aerobic training intervention consisted of interval training, comprising four bouts of 4 min work periods dribbling a soccer ball (Mitre ISO League, Mitre, Wilmslow, Cheshire, UK) around a specially designed track³⁴ (fig 1⇓) on an artificial field-turf pitch. Training cones used in the dribbling circuit were 0.3 m high and 0.15 m wide. Hurdle height was set at 0.5 m. Working intensity was at 90–95% of each player’s Hf_max, with work periods separated by 3 min of jogging at 70% of Hf_max. All players wore a Polar Team System heart rate belt and monitor (Polar Electro) throughout the interval training. The interval training was performed two times a week at the end of the soccer training session, on the same days and time of day throughout the intervention period. No emphasis was placed on improving strength, sprinting, or jumping performance throughout the intervention period. The 10 week intervention period was carried out directly after the off-season intermission period, encompassing the 6 week pre-season preparation period and the first 4 weeks of the competitive season.

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Figure 1

 Soccer specific dribbling track used for high intensity interval training sessions. Players dribble a soccer ball around the track, lift the soccer ball over the hurdles, and jump over the hurdles. Players dribble backwards with the soccer ball between points A and B.

RESULTS

During the 10 week, twice a week intervention, the players in this study completed 16 (80%) of the scheduled 20 interval training sessions. Four sessions were uncompleted due to the players competing in a 1 week tournament and mid-week competitive matches. Five players of the 16 assessed pre-intervention were withdrawn from the study due to injuries not connected with the training intervention (three players) and unavailability at the post-intervention session (two players).

The 10 week aerobic training intervention manifested significant improvements in Vo_2max (table 2⇓) of 9% (p<0.001). The mean Vo_2max increased from 4.45 (0.37) to 4.87 (0.45) l min⁻¹, which equates to a change from 63.4 (5.6) to 69.8 (6.6) ml kg⁻¹ min⁻¹, or 183.3 (13.2) to 201.5 (16.2) ml kg^−0.75 min⁻¹. Mean body mass was unchanged after the intervention period.

When treadmill running at 9 km h⁻¹ at an inclination of 5.5%, C_R was unchanged by the intervention. However, mean Hf at this workload decreased significantly from 162 (14) to 154 (14) beats min⁻¹ (p<0.05).

No significant changes in RFD and 10 m sprint performance were evident. SJ and CMJ height increased from 37.7 (6.2) to 40.3 (6.1) cm and 52.0 (4) to 53.4 (4.2) cm, respectively (p<0.05).

DISCUSSION

The training intervention used in this study elevated mean Vo_2max by 6.4 ml kg⁻¹ min⁻¹ or 9.0%. Mean Vo_2max increased from a typical soccer player value of 63.4 (5.6) to 69.8 (6.6) ml kg⁻¹ min⁻¹, or 183.3 (13.2) to 201.5 (16.2) ml kg^−0.75 min⁻¹. Removing the goalkeeper’s Vo_2max value from the pre- and post-intervention results shows an improvement in mean Vo_2max from 64.7 (3.9) to 70.9 (5.7) ml kg⁻¹ min⁻¹, or 186.3 (9.2) to 204.0 (14.8) ml kg^−0.75 min⁻¹ for the outfield players (n = 10).

Helgerud et al⁷ used 4 min intervals at 90–95% Hf_max with youth soccer players over an 8 week training period and promoted a Vo_2max increase of 11%, an increase from 58.1 (4.5) to 64.3 (3.9) ml kg⁻¹ min⁻¹. Although the players in the present study had a mean Vo_2max score greater than the post-intervention values found by Helgerud et al,⁷ large improvements in Vo_2max were still attained. The training intervention of Helgerud et al⁷ manifested improvements in Vo_2max of 0.67% per training session, compared to 0.56% in the present study. The somewhat greater efficiency of Helgerud’s intervention may be due to the lower initial aerobic fitness levels of the Norwegian junior players in that experiment.

The initial and post-intervention mean Vo_2max values of the Scottish youth players assessed in the present study are greater than those of Norwegian youth players of similar age⁷ and also of Spanish Premier Liga soccer players.⁴⁰ Indeed, the Vo_2max of the Scottish players in this study are comparable to elite Norwegian Champions League soccer players, who possessed a mean (SD) Vo_2max of 200.2 (8.4) ml kg^−0.75 min⁻¹, which are the highest oxygen uptake values reported for a soccer team.²⁴ It is suggested that the modern 75 kg soccer player should have a Vo_2max above 70 ml kg⁻¹ min⁻¹, or 205 ml kg^−0.75 min⁻¹.¹⁵ Five players in this study attained a Vo_2max greater than 205 ml kg^−0.75 min⁻¹ after the intervention period, with none of the players possessing this level of aerobic fitness pre-intervention.

The first 6 weeks of the intervention period in this study encompassed the pre-season training regime, and it is plausible that some of the improvements in Vo_2max manifested were attributable to the players returning to pre-season in a de-trained state, due to the 6 week summer intermission period. It is also plausible that improvements in Vo_2max in this study may have resulted from the conventional soccer training the players participated in (for example, small-sided game play, technical/tactical training), although it should be noted that the only endurance training that the players performed over the pre-season period in this training study were the high intensity aerobic interval sessions. Some studies have not reported large changes in Vo_2max of soccer players after pre-season training. For example, Bangsbo¹ reported only small increases in Vo_2max after a 5 week pre-season training period in professional soccer players.

The authors of this study are aware that this study would have benefited from data from a control group. The use of controls is difficult, if not impossible, when dealing with professional soccer clubs and players. However, control group data from a previous study by Helgerud et al⁷ can be referred to. The control group performed additional technical training whilst the training group performed high intensity aerobic interval training (4×4 min intervals at 90–95% Hf_max) twice a week for 8 weeks. The Vo_2max of the training group improved by 11% over the 8 week intervention, but the Vo_2max of the control group was unchanged (58.4 (4.3) to 59.5 (4.4) ml kg⁻¹ min⁻¹).

The C_R of the players in this study was not significantly changed after the training intervention. This is in contrast to the findings of Helgerud et al⁷ who reported an improved C_R of 6.7% in junior players, but of only 3.7% when using treadmill training with elite players,³³ with the latter improvement explained by gains arising from concurrent maximal strength training. It should be noted that the training intervention of Helgerud et al⁷ involved straight line running only, therefore an improved C_R may be expected for junior players with relatively low endurance values on the basis of more running practice. The aerobic endurance intervention in the present study was carried out dribbling a soccer ball around a specially designed track.³⁴ Dribbling a soccer ball around the track raises the oxygen demand when compared to running,⁴¹ as does the frequent changes in direction and pace involved. Hoff et al³⁴ reported that a mean exercise intensity of 93.5% Hf_max when dribbling a soccer ball around the track corresponded to 91.7% Vo_2max. C_R was unchanged in the present study, but it can be postulated that soccer specific work economy may have improved somewhat, with relevant gains not detectable by conventional treadmill testing.⁷

Although C_R was unchanged after the intervention period, mean Hf for treadmill running at 9 km h⁻¹ at 5.5% inclination decreased significantly from 162 (14) to 154 (14) beats min⁻¹ (p<0.001). This decrease in Hf could be attributed to an increased Vo_2max. A reduction in sub-maximal Hf of approximately 10 beats min⁻¹ is indicative of an increase in Vo_2max of approximately 3–5 ml kg⁻¹ min⁻¹.^7,13,33

Several authors have concluded that endurance training inhibits or interferes with strength^42–,⁴⁵ and therefore power performance.⁴⁶ 1RM half squat strength was measured pre- and post-intervention in the present study, but due to the players being limited by technique and apprehension as they were not familiar with this exercise movement, the recorded values for the pre-intervention assessment results were artificially low. During the final 4 weeks of the endurance training intervention period technical training for carrying out a safe half squat was performed twice a week using an unloaded Olympic bar. Therefore, only post-intervention squat strength scores for the players in this study are reported. RFD and 10 m sprint times in the present study were unaffected by the aerobic intervention program and the lifting technique program, although jumping height improved significantly over the 10 week period (p<0.05). Therefore, it appears that power related performance was not hindered by the aerobic interval training regime used in the present study. Similarly, Helgerud et al⁷ showed substantial gains in Vo_2max from an 8 week endurance training intervention with no reduction in sprinting and jumping abilities.

High intensity aerobic 4 min intervals are an effective tool for increasing Vo_2max in soccer players, with the training effect of each session being approximately 0.5%. The results from this study and that of Helgerud et al^7,33 demonstrate that it is possible to elevate the Vo_2max of youth soccer players from typical values (for example, 58–64 ml kg⁻¹ min⁻¹) to levels approaching or exceeding 70 ml kg⁻¹ min⁻¹ without hindering power related performance. For example, soccer players with a moderate initial Vo_2max of approximately 60 ml kg⁻¹ min⁻¹ can raise their Vo_2max to a level approaching 70 ml kg⁻¹ min⁻¹ by performing four sets of 4 min high intensity intervals at 90–95% Hf_max twice a week over a 10 week training period, in addition to regular soccer training.

In conclusion, high intensity aerobic interval training can be effectively performed by dribbling a soccer ball around a specially designed track. The addition of high intensity interval training at 90–95% Hf_max to the training regimes of professional youth soccer players can elevate Vo_2max levels from low to moderate values to that of elite Champions League soccer players, with no negative interference effects on strength, jumping ability, and sprint performance.