The results of this study confirm that a strong correlation exists between maximal strength, sprinting, and jumping performance in elite soccer players, which supports the findings from earlier work.12,13,20 The strongest correlations were found, as expected and echoing Newton’s 2nd law of motion, where acceleration was greatest. There were also strong correlations between maximal strength and the 30 m sprint test, including the recorded times between 10 and 30 m where the acceleration is substantially smaller than between 0 and 10 m, and the 10 m shuttle run test, where breaking velocity is part of the performance. Vertical jump height was in line with previous reports on elite soccer players,16,17 and the relatively strong correlation between vertical jumping performance and sprint times was expected as both are derivatives of maximal strength.
It should be noted that at Rosenborg FC, strength training is performed on an individual basis without any supervised regimen from the coach. However, all players do perform half squats as part of their normal strength training programme. Nine of the players used in this study received additional advice from our research group and consequently integrated a strength training programme twice a week into their normal schedule. This involved using few repetitions with high loads and high velocity of movement as described below. These nine players had considerably higher 1RM values than the other eight players. We have recently shown the effectiveness of such a training programme; increasing 1RM in half squats by about 35% (from 160 kg to 215 kg). The programme consists of five repetitions performed twice a week over a period of eight weeks, with the load being increased by 5 kg each time the athlete successfully completes a weekly work load.14 A higher level of maximal strength and power in the “high strength” group is supposed to result in more powerful jumps, kicks, tackles, and sprints and reduce the risk of injury.13,14,16 As the high strength group had carried out a training regimen with emphasis on maximal mobilisation of force, which normally results in large training effects on rate of force development, it may mean that the correlation between maximal strength and all sprint and jump variables is not a global finding. Furthermore, it should be emphasised that the timing for the sprint test was from a moving position, starting 30 cm behind the photocell beam, which may have influenced the correlations observed.
If other training regimens for improving maximal strength are used as background for enhanced strength, the correlation with sprinting and jumping performance may be different. As no standardised protocol for testing strength of soccer players exists, it is difficult to compare results among different studies. In our view, commonly used isokinetic tests do not reflect the movement of the limbs involved during soccer. Tests using free barbells will reflect the functional strength of the soccer player more accurately. Furthermore, free barbells are readily available to most teams and provide the potential to develop a meaningful functional testing programme in conjunction with strength training. The result from 1RM half squats in this study is in line with previous studies using free barbells.7,16,17 Both maximal strength and rate of force development are important factors in successful soccer performance because of the demands apparent from game play.2 Both should therefore be systematically worked on within a weekly schedule using few repetitions with high loads and high velocity of contraction as described elsewhere.13,18–20
A recent study reported that 96% of sprint bouts during a soccer game are shorter than 30 m,21 with 49% being less than 10 m. The 30 m sprint times reported here were in line with earlier studies on elite soccer players.7,17 However, the data also show that there were substantial time differences within the 30 m test. In particular, two of the players had similar timed performances on the overall test, but significant differences between how they ran the first and the last part of the test. The implication is that it is possible to differentiate the focus of sprint training individually on the basis of split time recordings.
The present shuttle run/sprint test involved a sharp change in direction which tested the balance, strength, flexibility, breaking, and acceleration capacity of the players. In line with previous work,22 the results from the study indicated that there was no relation between the 10 m shuttle run and the 30 m sprint test. However, both sprint capacities are of importance in soccer, and our data show that both capacities should be included and evaluated in a sprint test battery of soccer players.
As reported in previous studies,16,23 elite soccer players have high VO2MAX. A VO2MAX of 65.7 ml/kg/min or 194 ml/kg0.75/min as found in this study is in line with earlier work from our laboratory16 and others.24 In activities that involve dynamic work with large muscle mass, as in soccer, it is generally assumed that VO2MAX is primarily limited by maximal cardiac output.7 Interval training with a working intensity of 90–95% of maximal heart rate increases the maximal cardiac output. We have recently shown the effectiveness of such a training programme, increasing VO2MAX about 5 ml/kg/min by interval training (4 x 4 minutes intervals, twice a week) for eight weeks in well trained elite junior soccer players.7 This led to several positive on field adaptations such as increased distance covered, intensity of play, number of sprints performed, and ball involvements, highlighting some of the advantages of increasing VO2MAX in soccer players. The results in this group of players showed that a high level of maximal strength did not compromise a high VO2MAX.
It has been concluded from several studies that endurance training inhibits or interferes with strength development. Nelson et al25 claimed that simultaneous training of strength and endurance inhibits the normal adaptation to either training regimen when performed alone. Thus up to a reasonably high level of both strength and endurance capacity, there seems to be little interference between training strength and endurance capacity simultaneously, which obviously have to be carried out in soccer as both capacities are needed.
Maximal strength in half squats determined the sprint performance in these elite soccer players. Strong correlations are evident in all aspects of 0–30 m sprints as well as the 10 m shuttle run test and jumping height. The players with a high level of strength in this team had used a training regimen with few repetitions, high loads, and emphasis on maximal mobilisation of force in the concentric part of the half squat. High strength half squat performance does not imply a lower level of oxygen consumption in elite soccer players. Future research should involve the effect of maximal strength training on performance during soccer games.