Fatigue-Induced Balance Impairment in Young Soccer Players

Context:

Although balance is generally recognized to be an important feature in ensuring good performance in soccer, its link with functional performance remains mostly unexplored, especially in young athletes.

Objective:

To investigate changes in balance induced by fatigue for unipedal and bipedal static stances in young soccer players.

Design:

Crossover study.

Setting:

Biomechanics laboratory and outdoor soccer field.

Patients or Other Participants:

Twenty-one male soccer players (age = 14.5 ± 0.2 years, height = 164.5 ± 5.6 cm, mass = 56.8 ± 6.8 kg).

Intervention(s):

Static balance was assessed with postural-sway analysis in unipedal and bipedal upright stance before and after a fatigue protocol consisting of a repeated sprint ability (RSA) test (2 × 15-m shuttle sprint interspersed with 20 seconds of passive recovery, repeated 6 times).

Main Outcome Measure(s):

On the basis of the center-of-pressure (COP) time series acquired during the experimental tests, we measured sway area, COP path length, and COP maximum displacement and velocity in the anteroposterior and mediolateral directions.

Results:

Fatigue increased all sway values in bipedal stance and all values except COP velocity in the mediolateral direction in unipedal stance. Fatigue index (calculated on the basis of RSA performance) was positively correlated with fatigue/rest sway ratio for COP path length and COP velocity in the anteroposterior and mediolateral directions for nondominant single-legged stance.

Conclusions:

Fatigued players exhibited reduced performance of the postural-control system. Participants with better performance in the RSA test appeared less affected by balance impairment, especially in single-legged stance.Key Words: postural sway, repeated sprint ability test, athletes

Key Points

• In young soccer players, balance was adversely affected by fatigue.
• A moderate correlation existed between balance impairment and repeated sprint ability performance.

In soccer, as in many other disciplines, researchers continuously attempt to define which anthropometric, physiologic, psychological, and cognitive characteristics are most relevant to identifying talent at an early age. Such features, together with the development of new assessment methods in the laboratory or on the field, are essential to improving the chances of recognizing future top players.In particular, somatotype, aerobic and anaerobic power, agility, joint flexibility, and muscular development are considered essential contributors to achieving high-level performance.¹ Nevertheless, it remains unclear how these factors are to be objectively selected, measured, weighed, and combined with coaches'' subjective perceptions, which are mainly based on personal experience.^1,2Somewhat surprisingly, balance is not included among the most important features in athletic success; this quality is considered important mostly as a cofactor that helps reduce the risk of injuries.³ Also, soccer is a discipline that relies a great deal on single-legged support under unstable conditions. In fact, players use 1 limb (the dominant limb) to control the force and direction of the ball while dribbling, maintaining ball possession, and kicking; the nondominant limb basically ensures the necessary stability to optimally perform the required technical maneuver.⁴ Thus, it appears very important for players to have (and possibly improve during their maturation) excellent balance skills, especially for unipedal stance.Although previous authors^5–7 have shown that soccer players are generally characterized by superior balance performance compared with athletes in other sports (except gymnasts) or nonathletes, few data are available on the relationship between balance variables and functional performance in soccer players.⁸ Similarly, the possibility of improving postural-control performance in healthy athletes using specific balance-training protocols remains partly unexplored. However, recent investigators^9,10 have demonstrated that young athletes may benefit from proper stimulation of the proprioceptive system.

Balance and Fatigue

Both localized and whole-body fatigue cause degradation of postural-control performance, which is evident in the form of increased postural sway (ie, constant, slight corrective deviations from vertical when standing upright).¹¹ This phenomenon, which has been extensively reported in the literature, is mainly due to changes in cardiac and respiratory contractions, fluid movement in the body, and release of metabolic products by muscle fibers and is caused by altered sensory information from the proprioceptive system.¹² The magnitude of sway increase is partly influenced by exercise intensity and duration, but it also depends on the muscular groups involved.¹³ From this point of view, localized fatigue and whole-body fatigue are thought to trigger different disturbances of the postural-control system,¹⁴ although the net effect in terms of balance impairment may appear similar.Typical experiments aimed at assessing the effect of fatigue on static balance involve exercise such as the treadmill,^15–17 cycle or rowing ergometer,^14,15,18 isokinetic concentric actions, squat jumps, and heel raises.^14,19 A few authors^20,21 have evaluated the fatiguing effects of actual sport performance in triathletes²² and soccer players. In particular, Zemkova and Hamar²⁰ and Brito et al²¹ analyzed center-of-pressure (COP) velocity of 19- to 21-year-old players (unipedal and bipedal stances with eyes open or closed) before a game, in the break period between the first and second halves, and after a game. When visual input was present, single-legged balance was reduced after the match. Less evident was the effect of fatigue on bipedal standing: changes were observed only when the support surface was unstable and visual input suppressed.The lack of experimental data regarding the effects of fatigue on balance in soccer players is somewhat surprising, considering that fatigue is implicated in injury occurrence²³ and that a deficit in postural control may increase the risk of ankle injuries.²⁴ Thus, it is reasonable to hypothesize that not only may fatigue associated with performance in a match impair balance (and thereby increase the risk of injuries) but also that superior physical fitness in better-trained athletes may limit this effect.Our goal was to assess postural-sway changes induced by fatigue subsequent to a controlled field test (repeated sprint ability RSA]) representative of actual soccer activity in a cohort of young elite soccer players. We had 2 questions: (1) Is unipedal and bipedal static balance impaired by fatigue? (2) Is there a functional relationship between performance level in RSA and balance alterations?