Efforts to improve soccer performance often focus on technique and tactics at the expense of fitness and applied physiology. During a 90-min game, elite level players run about 10 km (4,6,37) at an average intensity close to the anaerobic threshold(7,32,40). Within this endurance context, numerous explosive bursts of activity are required, including jumping, kicking, tackling, turning, sprinting, changing pace, and sustaining forceful contractions to maintain balance and control of the ball against defensive pressure.
Previous studies demonstrate a significant relationship between maximal oxygen uptake (˙VO2max) and both distance covered during a game(7,37) and number of sprints attempted by a player(37). Rank-order correlation between average˙VO2max and placing for the first four teams in the Hungarian First Division Championship was shown by Apor (2). Mean˙VO2max of elite soccer players is normally reported between 55 and 65 mL·kg-1·min-1(14,27,33,39,42,45-47), with few individual values over 70 mL·kg-1·min-1. There is some evidence that differences in physiological demands exist among offensive, midfield, and defensive players, based on a presumption of higher endurance demands on the more active midfield position. Several studies have concluded that midfield players have higher ˙VO2max values when expressed per kilogram body weight (14,30).
Comparisons of ˙VO2max using the traditional expression mL·kg-1·min-1 are both very routine and functionally imprecise. The oxygen cost of running at a standard velocity does not increase in direct proportion to body mass. Similarly, ˙VO2max does not increase in direct proportion to body mass(10,23). Dimensional scaling of geometrically similar individuals suggests that the cross-section area of the aorta will increase in proportion to the square of height (L2) while body mass is dependent on body volume, which varies according to L3(3). Consequently, ˙VO2max, which is primarily limited by maximal cardiac output, should be proportional to body mass(mb) raised to the power of 0.67 (mb0.67). This dimensional scaling approach was supported by Bergh et al.(10) who found that ˙VO2max relative to body mass raised to the power of 0.75 was most indicative of performance capacity in running. Since defense players might be consistently heavier compared with midfield and forward players, as found by Davis et al.(14), they will be underestimated using the traditional expression mL·kg-1·min-1.
Strength and power share importance with endurance in soccer. Maximal strength refers to the highest force that can be performed by the neuromuscular system during one maximum voluntary contraction (1RM), whereas power is the product of strength and speed and refers to the ability of the neuromuscular system to produce the greatest possible impulse in a given time period. Maximal strength is one basic quality that influences power performance; an increase in maximal strength is usually connected with an improvement in relative strength and therefore with improvement of power abilities. A significant relationship has been observed between 1RM and acceleration and movement velocity (11,24). This maximal strength/power performance relationship is supported by jump test results as well as in 30-m sprint results (36). By increasing the available force of muscular contraction in appropriate muscles or muscle groups, acceleration and speed in skills critical to soccer such as turning, sprinting, and changing pace may improve (7). High levels of maximal strength in upper and lower limbs may prevent injuries in soccer by increasing the cross-section area of muscles and strength and mobility of tendon and ligaments (31,32).
Different tests have been used for evaluation of strength parameters for elite soccer players. Most studies(14,15,25,33) have used isokinetic equipment with different speeds and joint angles, making direct comparisons difficult. Muscular power has traditionally been measured by means of vertical jumps, and reported values are between 50 and 60 cm for elite soccer players(19,20). Raven et al. (28) used one repetition maximum bench press to test muscle strength of professional soccer players and reported a mean value of 73 kg (SD = 4.0).
Dimensional scaling must also be considered when evaluating strength measures (3). In two geometrically similar and quantitatively identical individuals, one may expect all linear dimensions(L) to be proportional. The length of the arms, the legs, and the individual muscles will have a ratio L:1, the cross-section areaL2:1, and the volume ratio L3:1. Since muscular strength is directly proportional to the muscle cross-section area, and body mass (mb) varies directly with body volume, whole body muscular strength measures will vary in proportion to mb0.67.
In the present study, cardiovascular endurance capacity as well as muscular strength and power were evaluated in two teams of Norwegian elite soccer players. Data were collected to test the following hypotheses. First, a difference exists between the two teams regarding maximal strength and aerobic endurance. Second, a relationship exists between results from the physiological tests and placing in the elite soccer league. Third,˙VO2max and maximal strength do not increase in proportion to body mass. Fourth, no position-specific differences for ˙VO2max exist for players due to mass differences. Finally, a secondary aim was to establish normative data of Norwegian elite soccer players.
MATERIALS AND METHODS
Two teams from the Norwegian elite division participated in the study. One of the teams in the study, Rosenborg, is the most successful team in the elite soccer league in Norway in the last 8 yr and is also presently successful in the Champions League. The other team, Strindheim, was elevated to the elite league for the first year at the time of the study. Physiological assessments were made of 29 players (13 defense players, 7 midfield players, and 9 forward players) in their preparatory training phase (March). Eight of the players tested were members of either the Norwegian national team (non-age restricted) or Olympic team (under 23). Each subject reviewed and signed consent forms approved by the Human Research Review Committee before participating in the study. Physical and physiological characteristics of the subjects are presented in Table 1. All of the players within a given team were assessed on the same day, and the tests were performed in the same order. Upon entering the laboratory, hemoglobin (Hb), hematocrit (Hct), and lung function were measured for normative data comparisons. For Hb and Hct determination, blood was drawn from a fingertip and analyzed immediately using the Refletron (Boehringer Manheim, Germany) and Ames microspin (Bayer Diagnostic, Germany) devices, respectively. Vital capacity (VC) and forced expiratory volume in one second (FEV1) were determined using a flow screen (Jaeger, Germany). After these preliminary tests, subjects completed a 20-min warm up at approximately 50-60% of ˙VO2max. Vertical jump height was determined using a force platform (Scan Sense AS, Norway) in combination with software developed specifically for the platform. Jump height was determined as center of mass displacement calculated from force development and measured body mass. Strength testing consisted of one repetition maximum of bench press and of squats (90° angle of the knee joints) performed with a competition standard Olympic style bar and weights(T-100G, Eleiko, Sweden). The athletes were familiar with both movements as part of their regular strength training programs.
After the strength tests, each athlete ran for 10 min on a motorized treadmill (Challenger LE5000) at 50-60% of ˙VO2max before measuring˙VO2max and maximal heart rate (fcmax). The specific procedure for ˙VO2max and fcmax determination is routinely used and has been previously described(22). The speed of the treadmill was increased every minute to a level that brought the subject close to exhaustion after approximately 5 min. Inclination was constant at 3°. Immediately after˙VO2max determination, each subject ran for 2 min at an exercise intensity of 50-60% of ˙VO2max directly followed by a supramaximal intensity run, resulting in exhaustion after ≈3 min. Heart rate(fc) was determined using short range radio telemetry (Polar Sporttester, Polar Electro, Finland). The highest heart rate frequency during the last minute of the supraintensity run was recorded asfcmax. Oxygen uptake (˙VO2), minute ventilation(˙VE), and breathing frequency (fb) were measured during work using an Ergo Oxyscreen (Jaeger EOS sprint, Germany). Allometric equations was used to determine the relationship between maximal oxygen uptake/maximal strength and body mass; ˙VO2 = a · mbb and 1RM = a · mbb, where a is the mass coefficient, mb is the body mass in kilograms, and b is the reduced exponent, the numerical value of which can be obtained from the log-log plot of the experimental data, as the logarithmic expression is a straight line (log ˙VO2 or log 1RM = log a + b · logmbb) (3).
Statistical analyses. The results are reported as means([horizontal bar over]X) and standard deviation (SD) calculated by conventional procedures. One-way ANOVA was used to determine differences of parameters between different playing positions. A P value ≤ 0.05 was considered statistically significant.
Rosenborg became champions while Strindheim ended last in the Norwegian elite soccer league. Rosenborg also did win the Norwegian elite soccer cup and participated in the Champions league. Rosenborg players had higher values of˙VO2max and squat 1RM as a team compared with players from Strindheim (Table 3). There was a significant correlation(r = 0.61, P < 0.01) between squat 1RM and vertical jump height. There were no significant differences in squat or bench press 1RM between the three different playing positions. Vertical jump height, however, was significantly higher in defense and forward players compared with midfield players. Defense players were significantly heavier than midfield and forward players.
Neither ˙VO2max nor maximal strength does increase proportionally to body mass in elite soccer players. The exponent b was found to be significantly less than unity for the entire group, and the mean value was 0.66 (SD = 0.04), 0.55 (SD = 0.06), and 0.74 (SD = 0.06) for˙VO2max, bench press, and squats, respectively. Thus, both the oxygen uptake and maximal strength per kilogram of body mass were inversely proportional to body mass. Midfield players had significantly higher˙VO2max compared with defense players using the expression mL·kg-1·min-1, whereas no significant differences were found expressing ˙VO2max either absolutely(L·min-1) or in relation to body mass(mL·kg-0.75·min-1) among players grouped by position (Table 2). Average results of squats were 150 kg(SD = 17.2) or 8.0 kg·mb-0.67 (SD = 0.9), for bench press 79.9 kg (SD = 13.6) or 4.4 kg·mb-0.67 (SD = 0.8), and for vertical jump height 54.9 cm (SD = 5.3).
In the 1995 season, Rosenborg became champions in the Norwegian elite soccer league, while Strindheim ended last. In addition, Rosenborg won the Norwegian elite soccer cup and participated in the Champions league. Rosenborg also participated in the Champions league in the 1996 season and had qualified for the quarter-final round. The results of the present study support previous investigations indicating a positive relationship between endurance capacity, physical strength, and performance results in elite soccer. As only two teams participated in the study, more work has to be done before conclusions can be made, but it seems natural regarding all the advantages the more endurance- and strength-trained individuals will have compared with less well-trained counterparts. Higher level of endurance capacity and 1RM squats compared with Strindheim (Table 3) will give Rosenborg a better base for on-field performance regarding playing intensity and power abilities such as acceleration and movement velocity among others(12,24) as discussed below.
Mean ˙VO2max in the present study (188.6 mL·kg-0.75·min-1 or 63.7 mL·kg-1·min-1) was in the upper range of values normally reported, and to the authors' knowledge, mean ˙VO2max for Rosenborg (Table 3) is the highest ever reported for a professional soccer team. This, hopefully, reflects that the volume and/or the methods of training in soccer have been improved. Compared with other sports,˙VO2max reported in the present study is not very high. It is the authors' view that professional soccer trainers should try to elevate the aerobic power of their team players. Considering ˙VO2max, it would be reasonable to expect about 70 mL·kg-1·min-1 for a 75-kg male, or about 205 mL·kg-0.75·min-1 independent of body weight. In activities that involve dynamic work with large muscle mass, as in soccer, it is generally assumed that ˙VO2max primarily is limited by maximal cardiac output(3,16). This should be taken into consideration when choosing training regimen for endurance training. Interval training, with a working intensity above 90% of maximal heart rate, primarily increases the maximal cardiac output. Rosenborg has, in previous seasons, organized the endurance training purely as playing sessions and reached satisfactory results this way. Whether endurance training should be organized as a playing session or as pure running must be considered by each team. Monitoring the training intensity during a playing session, with the assistance of a heart rate monitor, will be helpful in this regard. Experience tell us that there are problems in getting high enough intensity during a playing session, especially for teams in the lower divisions.
Several studies (7,30) have reported the average intensity of a soccer game to be around anaerobic threshold (80-90% offcmax). In two consecutive 45-min continuous work bouts, it will be physiologically impossible to work at an average intensity markedly higher than the anaerobic threshold. Expressing the intensity as an average value hides important factors incorporated during a game and provides less information. Soccer has periods with high intensities that result in accumulation of blood lactate and must necessarily lead to periods with low intensity for elimination of lactate. Most time spent in a soccer game is doing aerobic exercise, but during the most decisive and interesting situations, intense anaerobic exercise is performed.
Players should ideally be able to maintain the same level of effort throughout a game. The commonly found decline in distance covered, ratios of high intensities to low intensity work, fc, blood glucose levels, and blood lactate levels all indicate a reduction in activity levels as a game progresses (17,18). Players with a high˙VO2max have a faster recovery and greater stores of muscle glycogen (3,5,18). A correlation between˙VO2max and number of sprints attempted by a player as found by Smaros (37) is not surprising in this context. As the stores of muscle glycogen are reduced, an increasing part of substrate utilization must be taken from metabolism of fat. Athletes with better endurance capacity would be expected to “spare” glycogen during moderate intensity exercise, providing greater reserves for fueling intense sprints in the later, often decisive stages of a game(29). This glycogen sparing effect would be a distinct advantage because players could run longer and at a higher intensity before reduced glycogen contents, and accumulation of blood lactate forces them to reduce their work rates and the quality of technical and tactical elements(17,18).
High blood lactate level and decreased muscle glycogen are usually connected with impaired neuromuscular performances (3). The negative impact of high levels of blood lactate on coordinative function was demonstrated in a study by Ekblom (18); players were able to juggle the ball on average 64 times consecutively before a hard training bout, compared with 3 times immediately after the training bout(blood lactate level approximately 15 mmol·L-1). These factors suggest that players with the highest values of ˙VO2max and overall endurance capacity will have the potential to participate in more decisive and interesting situations. In addition, they will be able to perform technical and tactical elements at a higher intensity. Full glycogen stores in the muscles may last for about 90 min (13), and glycogen sparing may have little effect. If this is the case, focus should be attended to maximizing the stores of muscle glycogen before games and training. Saltin(35) and Ekblom (18) found that players with low glycogen content in their thigh muscles at the start of the game covered 25% less distance than others. An even more marked difference was noted for running speed; players with initially low glycogen content covered 50% of the total distance walking and 15% at top speed compared with 27% walking and 24% sprinting for the players with initially high muscle glycogen levels (35).
˙VO2max was proportional to mb0.66; i.e., the oxygen uptake per kilogram of body mass displayed an inverse relationship to body mass. This is in agreement with previous studies(10,23) and supports the argument that dimensional scaling should be used when comparing individuals with different body mass. Thus, it is reasonable to expect light individuals to have a higher oxygen uptake per kilogram of body mass than their heavier counterparts.˙VO2max proportional to mb0.66 is in line with mb0.75 found by Bergh et al. (10) and mb0.67 as suggested from the theory of similarity(3,21,44). Whether expressing˙VO2max in relation to mb0.67 or mb0.75, it may not be critical as long as the unit approximates the theoretical value and not the traditional mb. Considering the number of subjects in the present study and to avoid introducing yet another exponent, it seems reasonable to concur with the conclusions of Bergh et al.(10) and express ˙VO2max in relation to mb0.75, which has become quite common for evaluating running economy or to indicate the performance capacity of runners.
Previous reports of higher ˙VO2max for midfield players were not supported by the results of the present study. This may be a result of both higher movement demands of attack and defensive positions in modern soccer and the failure of previous studies to apply appropriate scaling for body mass differences. If defense players are consistently heavier compared with midfield and forward players, as found in the present study and by Davis et al. (14), the argument for using the expression mL·kg-0.75·min-1 becomes even stronger.
As no standardized 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 employing free barbells will reflect the functional strength of the soccer player more accurately. Furthermore, free barbells are readily available to most teams and provide more teams the potential to develop a meaningful functional testing program in conjunction with strength training. In strength training studies, it has been observed that measured increases in strength are dependent on the similarities between training and testing exercise. This specificity in movement patterns in strength training probably reflects the role of learning and coordination(1,34). The neuromuscular system also reacts sensitively in terms of adaptation to slow or fast contraction stimuli(36,38). Increased peak torque has been observed at or near the velocity of training (8,9) and at speeds below training velocity (26). Nevertheless, in sports-specific training for high velocity movements, a combination of maximum strength training in a basic nonspecific movement with emphasis on high velocity and high mobilization of power, and training the fast movement in the same period of time, gave a substantially higher increase in movement velocity(24,43) than training the fast movement itself, even with supramaximal velocities (41). These findings question some of the fundamentals trying to establish both movement and velocity specificity as basics for strength development. Higher values for vertical jump height of defense and forward players compared with midfield players (Table 2) may be explained by the tendency for defense and forward players to be involved in more jumping and tackling compared with midfield players.
The values of the strength parameters in the present study are not high compared with other team sports such as football or handball(3). Considering maximal strength, from testing of other explosive events it would be reasonable to expect, for a 75-kg male, squat values higher than 200 kg (90° in the knee joint) or about 11.0 kg·mb-0.67. The expected values for bench press would be 100 kg or about 5.5 kg·mb-0.67. For vertical jump height, it would be reasonable to expect that the elite soccer player have values higher than 60 cm. A higher level of all strength parameters would be preferable and would reduce the risk for injuries and allow for more powerful jumps, kicks, tackles, and sprints among other factors. Both maximal strength and rate of force development are important factors in successful soccer players because of the demand on the organism that the situations in a game give, which should be considered while choosing regimens for maximal strength training. Such training regimens for maximal strength training involve few repetitions with high loads and high velocity of movement and are described thoroughly elsewhere (34,36).
In the present study, maximal strength was proportional to mb0.55 and mb0.74 for squats and bench press, respectively; i.e., maximal strength per kilogram of body mass displayed an inverse relationship to body mass. Considering the small number of subjects in the present study, we suggest that maximal strength in soccer players should be expressed in relation to mb0.67 as suggested from the theory of scaling (3). Absolute strength is important when attempting to move an external object such as the ball or an opponent. Strength relative to body mass is the important factor when carrying body weight, especially for acceleration and deceleration in the soccer play. Relative strength comparisons are not functionally representative when values are divided by body mass. If maximum strength is divided by body mass for comparative purposes, the heavier individual's capacity will be underestimated and not representative of on-field work capacity. This information is important for coaches and especially for evaluating physical fitness or work capacity in younger soccer players in different periods of growth where body weight and size differ significantly at the same age.
Values for Hb, Hct, VC, FEV1/VC, and VE in the present study were within the normal range of the male population(3) and not markedly different from values reported from studies of athletes by others(14,28,33).
The results of the present study support previous investigations indicating a relationship between endurance capacity, physical strength, and performance results in elite soccer.
Studying the results of ˙VO2max, Rosenborg, theoretically, does have one player more on the field with a ˙VO2max of 77 mL·kg-1·min-1 (67.6-59.9 · 10 outfield players) compared with Strindheim. In addition, again theoretically, Rosenborg as a team manages to lift 296 kg more in squats compared with Strindheim(164.6-135.0 · 10 outfield players). Considering all the advantages of a high level of endurance and strength, as discussed above, these factors have high impact upon achievement of success in elite soccer. Neither˙VO2max nor maximal strength comparison is functionally representative when values are divided by body mass. Therefore, it is concluded that, for soccer players, ˙VO2max should be expressed in relation to mb0.75 and maximal strength in relation to mb0.67. In the authors' opinion, further improvements in the level of play in soccer require greater emphasis on optimizing the functional strength and endurance capacity of the athletes. Superior technical and tactical ability in soccer can only be consistently demonstrated throughout the course of a 90-min competition by athletes with high endurance capacity and strength.
Address for correspondence: Ulrik Wisløff, Department of Physiology and Biomedical Engineering, Faculty of Medicine, Norwegian University of Science and Technology, N-7005 Trondheim, Norway. E-mail:[email protected].
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