Balance is defined as the process that maintains the center of gravity within the body?s support bases. It requires constant adjustments with muscular activity and articular positioning. Many nervous and musculoskeletal system diseases can alter balance control.¹?³ Maintaining postural balance requires sensorial detection of the body?s movements, integration of sensory-motor information into the central nervous system, and an appropriate motor response. The position of the body in relation to space is determined by the visual, vestibular and somato-sensitive functions. Muscular control and dynamic maintenance of balance involve the activity of coordinated muscular kinetic chains.²,⁴?⁶

The anterior cruciate ligament (ACL) is responsible for 50% of all ligamentous knee injuries. The majority of injuries occur during sports activities, particularly those sports that involve movements of deceleration, rotation and jumping. These injuries occur predominately in males and are more common in the third and fourth decades of life.⁷,⁸ Soccer is the sport with the highest incidence of ACL injuries.⁸?¹⁰ The proprioceptive neurophysiological function of the ACL is important for its biomechanical role in the maintenance of joint stability.⁷,¹¹?¹⁴

Mechanical stability can be successfully recovered after injury to the ACL through reconstructive surgery. However, the restoration of sensory and motor function remains controversial.¹¹

Evaluating postural balance indicates deficiencies (mainly proprioceptive) and determines the efficiency of rehabilitation after injury or after reconstructive surgery.

The Biodex Balance System (BBS) is an instrument that analyzes dislocation of the center of gravity and postural balance by assessing the capacity to maintain dynamic postural stability on an unstable surface.¹⁵?¹⁹

The objective of this study was to compare the dislocation of the center of gravity (CG) and postural balance in sedentary and recreational soccer players with and without reconstruction of the ACL using the BBS.

The study was performed at the Institute of Orthopedics and Traumatology, Hospital das Clinicas da Faculdade de Medicina, Universidade de S?o Paulo and at HCor (Heart Hospital) through approval granted by the Ethics Committee of the University of S?o Paulo (number 013/02).

Sixty-four subjects were divided into three groups: a) soccer players with ACL reconstruction, called the Surgery Group; b) soccer players with uninjured ACLs, called the Non-surgery Group; and c) sedentary individuals, called the Sedentary Group. The common inclusion criteria were: male gender, 20 to 40 years of age and absence of systemic disease.

The Surgery Group consisted of 24 individuals, aged 29?6 years (20?40), height 175 ? 8.5cm (165?188) with an average body mass of 79.6 ? 8.5 kg (65?96). The inclusion criteria were: a) reconstructive surgery of the ACL, with unilateral involvement, and autograft of the patellar tendon, with or without associated injuries for at least 18 months; b) resumption of soccer playing for at least 10 months; c) playing recreational soccer two or more times per week; d) no surgery on the contralateral limb; e) no episodes of instability after surgery; f) no other surgery of the lower limbs; and g) no limitation of joint movement ? 15? of flexion. Thirteen (54.16%) athletes had had an operation on the right knee, and 11 (45.83%) had had an operation on the left knee; 14 (58.33%) had undergone surgery of the dominant limb (defined as the leg for kicking), and 10 (41.66%) had surgery on the support limb; 11 (45.83%) had associated injuries to the meniscus; and 23 individuals were right-handed. The mean time between surgery and evaluation was 36?10 months (minimum: 21 months; maximum: 59). Thirteen (54.16%) athletes played on a soccer field, 12 (50%) played on an indoor soccer field, and seven (29.16%) played on a futsal field; eight (33.33%) played two modalities of soccer. The mean time of rehabilitation was 6?6 months (2?20 months), and the time to return to sport was 10 ? 3 months (6?20 months).

The Non-surgery Group comprised 20 subjects, aged 26?6 years (20?38), mean height 179?0.05cm (170?186), with an average body mass of 74.9?13.0 kg (46?107). The inclusion criteria were: a) no surgery or injury of the lower limbs; b) playing recreational soccer two or more times per week; and c) clinical tests negative for knee instabilities. Seven (35%) athletes played on a soccer field, 12 (60%) played on an indoor field, 11 (55%) played on a futsal field, and 10 (50%) played two modalities of soccer. In 17 persons (85%), the right limb was dominant, and the left limb was dominant in three (15%).

The Sedentary Group consisted of 20 subjects with a mean age of 26?5 years (20?38), a mean height of 171?0.03cm (169?175), and a mean body mass of 71.8?8.7 kg (58?87). The inclusion criteria were: a) no physical activities for at least six months; and b) no surgery on or injury to the lower limbs. In 17 persons (85%), the right limb was dominant, and in three (15%), the left limb was dominant.

The balance test was carried out using the BBS by the same evaluator for all groups. Stability varies according to the resistance level (where level eight is the most stable and level one is the least stable).²,²⁰?²³ The instability protocols used were level eight (more stable) and level two (less stable). Level two allowed an inclination of up to 20? in the horizontal plane in all directions. Three stability indexes were calculated: anteroposterior, mediolateral and a general stability index (sum of the first two).

The groups were similar in age and height, with a significant difference in body mass (as determined by the Tukey test (p=0.027)) between the sedentary group and the uninjured athletes.

There were no significant differences in the dominant limb between the uninjured athletes and the sedentary group, so their data were grouped for the purpose of statistical comparison (Table 1).

The operated side (OS) of the Surgery Group presented less dislocation of the center of gravity and greater stability when compared with the non-operated side (NOS) for all indexes of stability at level eight (Table 1). The OS of the Surgery Group presented less dislocation of the center of gravity and greater stability for all indexes of stability at level two when compared to the Non-surgery Group.

There were no significant differences between the NOS in the Surgery Group and either the dominant or non-dominant side of the Non-surgery Group (Table 2).

The OS in the Surgery Group presented more dislocation and less stability for all indices of stability when compared with the Sedentary Group at level two. The NOS presented more dislocation and less stability for all indices of stability at levels eight and two when compared with the Sedentary Group (Table 3).

The sedentary group presented less dislocation and greater stability in all indices of stability at levels eight and two when compared with the Non-surgery Group. (Table 4).

Evaluating postural balance is important in joint injuries for making treatment decisions such as the type of surgery and the rehabilitation results.²¹ Tests must reproduce the functional activities and measure the postural balance across the vestibular, visual and neuromuscular control systems.²¹ Information on postural balance can be used to plan specific exercises to enhance proprioceptive control and prevent falls.³ This study used the BBS system because it is very simple and easy to use and can provide quick, quantitative measures of postural balance, center of gravity dislocation, and (indirectly) proprioceptive neuromuscular information. This kind of information could be useful during rehabilitation from ACL injuries. The reliability of the balance tests with the BBS was considered satisfactory in previously published studies, which motivated the present study.²⁴,²⁵

In this study, when the OS and NOS in the Surgery Group were compared, the operated side was always more stable for all indices of stability at level eight (more stable). Albeit contradictory, this result suggests that the patients have less compensatory mechanisms to maintain the dynamic stability of the operated limb. Such a finding has not been published in other studies that used the force platform.¹⁶,¹⁷ At level two (greater stability), there were no significant differences between the OS and NOS, perhaps because the compensatory mechanisms are not sufficient to support the balance in this situation. This result is in agreement with Henriksson et al.¹⁶ who used EquiTest Neurocom equipment.

Dominance did not influence the postural balance in the Non-Surgery and Sedentary groups at either level of stability. These results have also been found by Tookuni et al.³ using FScan MAT equipment. These findings are very important because they could be used to evaluate rehabilitation results and postural balance deficit.

In comparing the Surgery Group and the Non-surgery Group at level eight, there was no difference, which indicates a good recovery of the operated site. This result is in agreement with Ochi et al.,⁷ (somatosensory potential and position sense) and Henriksson et al.¹⁶ (balance evaluation using Equitest equipment). However, Bonfim et al.¹⁷ demonstrated a deficit in the operated subjects when compared with a control group using position sense, kinesthesia, latency, and balance (force platform). At level two, for the same comparison (Surgery Group OS and Non-surgery Group), the operated side was more stable. It is important to consider that at level two (greater instability of the equipment), the range of movement of the platform is very extensive and demands more postural adjustments in order to maintain equilibrium.¹⁵,²⁴ Kinetic and kinematic analyses showed that subjects with ACL reconstruction use different strategies to generate joint torque in squat exercises.²⁶ These different motor strategies may influence postural balance. Our data, though contradictory, agree with those authors who consider that surgery continues to affect the functional performance of the joint, even after a period sufficient for the complete re-enervation of the graft.¹⁰,¹⁷,²⁷,²⁸ At level two, the Sedentary Group presented less center of gravity dislocation and better postural balance when compared with the Surgery Group (OS and NOS) and the Non-surgery Group, which is not seen in other studies.

We observed that sensory-motor demand is different in the Sedentary Group. Soccer calls on and stimulates sensory receptors because of the frequent direction, jumping, acceleration and deceleration changes. We expected that soccer players respond better to the balance test, with less dislocation of the center of gravity, but what we observed is a great dislocation of the center of gravity and poor postural balance in the Non-surgery Group. It is possible that the greater dislocation observed is a strategy that helps maintain postural balance even in critical situations. This evaluation does not indicate the speed with which the subjects regain balance, a factor which may be closely related to proprioceptive ability and the maintenance of postural balance. Training these skills with soccer-specific functions can enhance functional limits and joint mobility and may account for the results. BBS evaluation shows that the knees of sedentary individuals and operated athletes are rated as more stable than uninjured athletes? knees. These findings may actually suggest that this equipment does not measure postural balance but shows the dislocation of the center of gravity, which is greater in the uninjured athletes. This greater dislocation would be an adaptation of the athlete to the demand of the sporting activity, at the limit of the maintenance of equilibrium through faster and more complex neuromuscular responses. Athletes develop more strategies for maintaining balance, and they are able to shift their center of load in more unstable situations caused by successive dislocations, changes of direction, and the specific activity of carrying the ball.

The dislocation of the center of gravity and change in postural balance in sedentary individuals and on the operated limb of Surgery Group are less marked than in the soccer players from the Non Surgery Group and on the non-operated limbs (Surgery Group).

At the Surgery Group, the dislocation of the center of gravity and the change in postural balance from the operated limb of the soccer players is less marked than in their non-operated limbs.