Functional asymmetries in the lower-limbs have been the subject of numerous recent investigations concerning many different contact, limited-contact and non-contact sports aimed at understanding the role of conditioning in performance and in injury prevention (Fousekis et al., 2010; Impellizzeri et al., 2007; Lawson et al., 2006; Newton et al., 2006; Sannicandro et al., 2011a; 2011b; Stephens et al., 2005). Functional asymmetries in the lower-limbs are determined by strength deficits between the two limbs (Fousekis et al., 2010) and differ from muscular imbalances, which represent an alteration in the strength relationship between agonist and antagonistic muscle pairs (Jones and Bampouras, 2010; Knapik et al., 1991; Schlumberger et al., 2006).
In children and adolescents that practice sport at either the recreational or competitive level, the presence of strength asymmetries between the two lower limbs is correlated with a high risk of injury (Hickey et al., 2009). In the presence of functional asymmetries in young athletes, compensation training programmes should be undertaken aimed at eliminating, or at least limiting, the asymmetries in order to avoid negatively affecting the health of young athletes on the long-term. For instance, Emery et al. (2006) showed that 50% of subjects who had suffered knee injuries 20-25 years earlier presented signs of knee osteoarthritis compared to just 5% of the sample population who had not suffered any prior knee injury.
Tennis is characterised by the execution of a series of high intensity and explosive actions, very brief sprints, changes of direction and abrupt deceleration (Fernandez-Fernandez et al., 2010; Fernandez et al., 2005; Girard and Millet, 2004); these specific movements put the tennis player under physical stress. In young and adult tennis players presenting lower-limb functional asymmetries in strength capacity resulting from the practice of sport-specific movements (Ellenbecker et al., 2009), personalised and specific training should be provided to minimise this risk factor for injury.
Over recent years, athletic conditioning programmes have started to introduce balance training exercises, which may involve the use of resistances, with the aim of reducing risk of injury (Anderson and Behm, 2005; Caraffa et al., 1996; Gioftsidou et al., 2008; Granacher et al., 2011; Malliou et al., 2004; 2010; Olsen et al., 2005; Wedderkopp et al., 2003; Yaggie and Campbell, 2006). The introduction, and now widespread use, of training techniques involving the use of unstable surfaces (i.e. balance training) represents an important methodological innovation in sports training and therapy (Behm, Anderson, 2006). Exercises performed on unstable surfaces may be included into a training programme as part of an injury prevention/management strategy or with the primary aim of improving athlete performances, especially in relation to team and open skill sports. An unstable surface places an increased demand on the neuromuscular system to stabilise the joints involved in the execution of a movement. All of the advantageous effects of the movements performed on unstable surfaces have beneficial outcomes in sport-specific performances; for instance, in tennis, the correct activation of core muscles achieved when per-forming balance training exercises is a prime example of the positive transfers that can be obtained in the execution of sport-specific movements, from weight lifting to hitting a tennis ball (Behm and Colado, 2012; Behm et al., 2010). However, the relationships between training on unstable surfaces, injury prevention and the reduction of strength asymmetries are not yet completely clear (Behm et al., 2010).
The aims of this study were (i) to examine the presence of functional asymmetries in the lower-limbs of young tennis players in strength and speed drills; (ii) to verify whether balance training provides an effective programme able to reduce functional asymmetries.
The study was performed using a sample of young tennis players (n = 23); the players were randomly divided into an Experimental Group (EG) (n = 11: 4 females, 7 males; mean age, 13.2 [+ or -] 0.9 years; mean weight, 50.8 [+ or -] 8.9 Kg; mean height, 1.63 [+ or -] 0.08 m) and a Control Group (CG) (n = 12: 4 females, 8 males; mean age, 13.0 [+ or -] 0.9 years; mean weight, 51.1 [+ or -] 9.2 Kg; mean height, 1.61 [+ or -] 0.09 m).
This study conforms to the policy statement relating to the Declaration of Helsinki. Before data collection, all subjects and their parents provided their informed consent in accordance with the Institutional Ethics Committee for the Department of Clinical and Experimental Medicine of the University of Foggia, Italy.
Specific tests were used to evaluate functional asymmetries between the lower-limbs in strength and speed drill performance:
one-leg hop test (OLH) (Augustsson et al., 2006; Gustavsson et al., 2006), for the evaluation of explosive strength and stability in the sagittal plane;
side hop test (SH) (Docherty et al., 2005; Gustavsson et al., 2006), for the evaluation of strength and stability in the frontal plane;
10 and 20m sprint tests, from a standing start position, for the evaluation of acceleration in a straight line; the two sprint tests were carried out separately and test two different elements of speed capacity, as previously employed in a similar study (Girard and Millet, 2009);
Foran test (Foran, 2001), for the evaluation of acceleration with a change of direction;
side steps and forward 4.115-m test (4m-SSF) (Salonikidis and Zafeiridis, 2008), for the evaluation of lateral speed by means of lateral sprints.
The tests of acceleration and speed were performed with the assistance of photocells (TTsport, San Marino). All tests were performed in the tennis court where the athletes regularly played tennis. Each subject's leg preference was determined by asking the athlete to perform a one-legged jump, as previously defined in the literature (Brophy et al., 2010). In the tests of leg strength, the subject's dominant leg was assessed first, followed by the non-dominant leg. In order for the participants to familiarise themselves with the tests, two practice trials were performed with an interval rest time of 2 minutes. For both groups (EG and CG), testing was conducted before (T0) and after (T1) the completion of the 6-week training period.
The order of presentation of the tests was as follows:
On the first (T0) and penultimate day of training (T1): one-leg hop test, side hop test and Foran test.
On the second (T0) and last day of training (T1): 10-20m sprint test, side steps and forward 4.115-m test.
The EG completed two 30-minute sessions per week dedicated to balance training. The two weekly training sessions proposed to the EG were differentially structured (see Tables 1 and 2 for details); balance training was performed after a 15 minute warm-up consisting of slow...