METHODS

Subjects

Six male soccer players from a Norwegian first division team volunteered to participate in the three different training modes. Before the study, each subject reviewed and signed consent forms in accordance with the Declaration of Helsinki and the human research review committee of the Norwegian University of Science and Technology. Subjects were informed about the test protocols, without being informed about the aim of the study. The mean (SD) age of the soccer players was 22.2 (3.3) years, weight was 77.5 (12.4) kg, height was 180.2 (5.5) cm, and maximal heart rate was 198.3 (5.5) beats/min. The laboratory tests were carried out first, and the dribbling course and small group play were carried out in randomised order.

Testing

Laboratory tests

All laboratory tests were performed on the same day. Room temperature was 21–22°C and relative humidity was 50%. Subjects carried out a 20 minute warm up at 50–60% of Vo₂max, running on a treadmill. Vo₂max was determined with the treadmill inclined at 3° (Jaeger LE 5000; Erich Jaeger GmbH, Germany), as described previously.¹⁰ Briefly, running speed was increased by 1 km/h every minute to a level that brought the subjects to Vo₂max after about five minutes. Vo₂max was defined as levelling off of oxygen uptake despite increased exercise intensity, and unhaemolysed blood lactate concentration above 6 mmol/l. Immediately after Vo₂max determination, each subject ran for two minutes at an exercise intensity of 50–60% of Vo₂max directly followed by a supramaximal intensity run, which resulted in exhaustion within three minutes. The highest heart rate (f_c), measured by short range radiotelemetry (Polar Sporttester; Polar Electro, Oy, Finland) during the last minute of running, was recorded as f_cmax. Oxygen uptake, ventilation (Ve), respiratory exchange ratio (R), and breathing frequency (f_b) were measured using an Ergooxyscreen Sprint (EOS; Erich Jaeger). Unhaemolysed blood lactate was determined using a lactate analyser (YSI Model 1500 Sport Lactate Analyzer; Yellow Springs Instruments Co, Yellow Springs, Ohio, USA).

Dribbling track

The field running test measurements were performed a minimum of three days and a maximum of nine days after the laboratory test on an indoor high quality soccer field consisting of artificial curled nylon grass filled with sand. All field tests were performed after a 30 minute soccer training warm up period. Figure 1 shows the dribbling track for the endurance training.

The soccer players dribbled the ball through the cones and lifted the ball over the 30 cm high hurdles. Between point A and B the players moved backwards while controlling the ball, before turning and starting on a new round. Players were instructed to increase running intensity gradually to a level that brought them to 90–95% of maximal heart rate after about 60 seconds in the four minute training bout. The players carried out two four minute intervals, separated by a three minute exercise at 70% of maximal heart rate. Heart rate was monitored using Polar heart rate monitors which were continuously observable by the player during the run. The player was also assisted in assessing heart rate by the test leader observing the heart rate transmitted by telemetry (Polar Sport Tester). One person replaced cones and hurdles that fell down. Vo₂, Ve, R, and f_b were measured using the portable metabolic test system Metamax II (MMX II) (Cortex Metamax, Leipzig, Germany). The MMX II has been shown to be valid, reliable, and comparable to the EOS used in the laboratory test.¹⁴

Small group play

Small group play was organised as five a side, including goalkeepers as shown in fig 2. Pilot studies indicated that it would be necessary to use four minute periods of play to reach at least three minutes in the high intensity zone. Intensity was sought to be as high as possible through the four minute playing periods. Two four minute playing periods were carried out, separated by three minutes of active rest. Pilot studies showed that active coaching by encouragement and constructive messages was necessary to achieve a high enough intensity for some of the players. The measuring equipment was similar to that used for the dribbling track.

Statistical analysis

Data are expressed as means (SD). The Kruskall-Wallis test was used to evaluate differences between tests. Spearman correlation coefficients were used when appropriate. Cautious calculations from previous studies^3,9 and pilot experiments showed that six players were enough to test the hypothesis with a statistical power of 0.90 (p<0.05).

RESULTS

During small group play, exercise intensity was 91.3% of f_cmax or 84.5% of Vo₂max. Corresponding intensity in the dribbling track was 93.5% of f_cmax or 91.7% of Vo₂max. f_c corresponding to 90–95% of f_cmax was reached after 61.5 (10.8) seconds on the dribbling track, and after 62.5 (12.5) seconds in small group play (table 1).

Figure 3 shows the relation between Vo₂ and f_c measured at several submaximal intensities on the treadmill (correlation coefficient r = 0.844, p<0.01). The Vo₂-f_c relation in either of the soccer specific training situations was not significantly different from running on the treadmill (fig 3). f_c was higher on the dribbling track than during small group play (p<0.05).

Average Vo₂ and the corresponding f_c were similar in both playing sessions, whereas, during the dribbling course, Vo₂ was significantly (p<0.05) reduced by 3.9%, with unchanged f_c in the second interval. The players with the highest Vo₂max showed the lowest percentage of Vo₂max during the small group play.

DISCUSSION

The major finding of this study is that specifically designed soccer training fulfils the criteria for aerobic interval training. Furthermore, heart rate monitoring is a valid measure of actual exercise intensity in these types of training modes.

The results therefore show that it is possible to perform soccer specific endurance training in the form of specially designed small group play or on a dribbling track, within the intensity zone for effectively developing Vo₂max and corresponding soccer performance.³ It should be emphasised that this requires good organisation, as satisfactory exercise intensity was not reached during small group play without active coaching—that is, constructive instructions and encouragement to the players when necessary. Several factors must be considered when designing soccer specific aerobic endurance training. Firstly, intensity has to be higher than in normal soccer matches. This can be achieved by altering the number of players and field size and reducing the time the ball is out of play.¹ The fact that players with the highest Vo₂max had the lowest percentage of Vo₂max during small group play indicates that the playing situation designed for this experiment may have a ceiling effect for developing aerobic endurance. Therefore, players with a high Vo₂max may have to train on the dribbling track because higher exercise intensity is achievable in this way than during small group play, or they may have to run uphill to have the same training response as players with lower Vo₂max.

A problem to be considered when designing a dribbling track is that soccer is played on a flat surface, and research has shown that subjects running on a flat surface may not be able to reach exercise intensities close to Vo₂max.¹⁴ Therefore, Vo₂max is usually measured with the treadmill at a 3° inclination. In the design of the dribbling track, the inclination should be compensated for by changes in pace and direction, both of which increase workload.

Intermittent work—that is, part of the time is spent standing still, as seen in soccer play—may overestimate Vo₂ based on f_c measurements compared with a continuous workload. The dribbling track should therefore be designed to be continuous, but allow for some variation in exercise intensity. Previously, the measuring equipment has been too complicated for field testing of soccer players, both in terms of weight and size. Smaller and lighter (800 g) equipment—for example, Metamax Cortex II—has been tested in our laboratory and found to be valid, reliable, and comparable to the EOS Sprint used for the laboratory measurements.¹⁵

On the dribbling track, the soccer players could have exercised at an intensity considerably higher than that shown in this experiment. In fact, in line with observations from running training (without a ball), the players had to be told to reduce their intensity in the first interval, so as not to exceed the planned intensity of 90–95% of f_cmax. Higher exercise intensities lead to increased lactate levels and a lower aerobic training response, and often result in fatigue and failure to complete the planned training session. A work intensity of 90–95% of f_cmax is higher than the anaerobic threshold. Activity during pauses is thus important to reduce lactate build up in the muscles and subsequently in the blood.¹⁶ The high intensity—close to Vo₂max—also limits the length of the working periods and the number of intervals that it is possible to carry out. Helgerud et al³ used 4 × 4 minute intervals, which seemed to be very effective for soccer players with a Vo₂max in the range 55–65 ml. Better trained subjects would probably have optimised training responses through somewhat longer interval periods or a higher number of intervals, and the opposite is probably true for less trained subjects.

The soccer players in this experiment showed a higher Vo₂max than that reported previously,⁴ but in line with results reported by Wisløff et al.⁹ However, the players in our study do not represent a whole team, and, as volunteers for an endurance experiment, they probably have a genetic predisposition for endurance performance. Therefore, the Vo₂max results cannot be generalised to all groups of soccer players. Monitoring heart rate during play would give the coach valuable information about training effects.

Conclusions

High intensity, aerobic, endurance, interval training can be carried out in a more soccer specific way than plain running. A specially designed dribbling track and small group play can produce the intended work intensity. Heart rate is shown to be a valid and reliable indicator of oxygen uptake in small group play with no, or only short, stops, as well as on the dribbling track, as shown in this experiment.