Proprioceptive balance training is used in rehabilitation following sports-related injuries and is becoming recognized as an important element in injury prevention in sports.^{12,13,14,15,16,17,18,19} Running, jumping or pivoting on one leg relies on a sense of joint position and muscular control for joint stability. There is evidence that static balance improves following proprioceptive balance training using a wobble board.^20,21,22,23 However, most of these studies did not examine the effect of dynamic proprioceptive balance training, which may improve postural control in athletic situations and prevent some injuries.

There is evidence from randomized trials that multifaceted prevention programs, including proprioceptive balance training using a wobble board, are effective in reducing injuries to the lower extremities in specific sports.^{12,13,14,15,16,17,18,19} However, the programs in these trials were multifaceted (i.e., included warm-up, flexibility, jump training, strength training, rehabilitation and sport-specific technical components), and balance was not measured. The effectiveness of balance training alone on balance ability and prevention of injury remains unclear. Moreover, the use of these techniques in adolescents and non-elite athletes has not been studied.

We included subjects if they were between the ages of 14 and 19 years, regularly attended classes and participated in physical education classes. We excluded subjects if they had a history of a musculoskeletal injury in the 6 weeks before recruitment, a previous history of a serious musculoskeletal disorder (e.g., fracture, rheumatologic disease, systemic disease or surgery) or an important medical condition (e.g., hypertension, or recurrent fainting or dizzy spells).

Each subject was asked to complete a baseline questionnaire, which included questions about previous history of injuries and participation in sports. At the initial assessment, the physiotherapist measured each participant's height and weight. Each subject completed, with their eyes closed, a timed static unipedal balance test on the gym floor and a timed dynamic unipedal balance test on an Airex Balance Pad (Fitter International Inc., Calgary). We have previously shown adequate test–retest reliability for these 2 measurements (intraclass correlation 0.7 and 0.5 respectively).²⁴ During these tests, time was recorded when the subject's balance was lost or eyes opened, or when the maximum time allowed for each trial (180 seconds) was reached. The baseline assessment also included a vertical jump test²⁵ to examine functional strength and the Canadian version of the 20-m shuttle run to test endurance.²⁶

A physiotherapist taught each participant in the intervention group a progressive, home-based, proprioceptive balance-training program to be used daily for 6 weeks and then weekly for maintenance for the remainder of the 6-month study period. A 16-inch (40-cm) wobble board (Fitter International Inc.) was provided. At the 2- and 4-week follow-up assessments the program was reviewed and progressed. Progression at 2 weeks included bipedal to unipedal exercise progression and increased duration of eye-closed elements of the program. At 4 weeks progression involved wobble board adjustment to level 2, which increased the amount of wobble board instability. Core stabilization, including isometric contraction of abdominal and gluteal muscles was incorporated into the program. Each daily session was expected to last about 20 minutes, and self-reported compliance with the training program was assessed by a daily record sheet and weekly telephone calls over the 6-week training period. Each subject was retested (i.e., balance, vertical jump and shuttle run tests) biweekly over 6 weeks by the physiotherapist. For the 6-month follow-up period, each subject was asked to complete a sport participation record sheet and an injury report form as required. An athletic injury was defined as any injury occurring during a sporting activity that required medical attention (i.e., visit to an emergency department or physician's office, chiropractic, physiotherapy or athletic therapy) or resulted in the loss of at least 1 day of sporting activity, or both. Injury report forms included a section to be completed by any attending medical professional. The physiotherapist made biweekly telephone calls to all study participants during the 6-month follow-up period to ensure that all eligible injuries were reported.

The primary outcome measures included the change from baseline to 6-week follow-up in the maximum time that balance was maintained during the static and dynamic tests over 6 trials, 3 for each leg. Measurements for both legs were pooled because there is no evidence that balance differs by side.²⁴ The primary injury outcome measure included all self-reported sports-related injuries and ankle sprain injuries.

Because of the cluster randomization design of the study, to calculate the sample size we had to take into account the possible similarity in the response of individuals within each cluster.²⁴ We assumed an intracluster correlation of ρ = 0.01 based on a comparison with the mean ρ found by Murray and associates²⁷ of ρ = 0.006 in examining adolescent smoking behaviour. We also adjusted for a potential drop-out and noncompliance rate in the intervention group and a contamination rate in the control group (R_o = 0.10). On the basis of the primary outcome variable static balance, this trial was powered to detect an effect size of d = δ/σ = 0.8 (where δ = μ₁ – μ₂ = mean [intervention group] – mean [control group] = 9 seconds; and σ = 11 seconds = the estimated common standard deviation of the timed balance test measurement in the control and intervention group), assuming a type I error (α = 0.05) and type II error (β = 0.10).

We report descriptive statistics for baseline characteristics. Baseline variables were compared between the 2 groups. We calculated the mean difference in static and dynamic balance test results from baseline to the 6-week follow-up for the intervention and control groups and compared them using both independent and cluster-adjusted t tests.²⁸ Where the assumptions of normality and equal variance were not met, the data were logarithmically transformed.²⁹ In this case the measure of central tendency used was a geometric mean, which was estimated by back-transformation from the mean of the log-transformed data. Our analyses were based on the intention-to-treat principal. We used multivariable mixed-effects regression analysis (i.e., allowing random effects for cluster) to examine further the effectiveness of the training program in improving both static and dynamic balance test results, controlling for other baseline covariates.²⁸ To determine our final model, we eliminated covariates through a stepwise process, with α at 0.05.

Improvements in static and dynamic balance during the follow-up period were greater in the intervention group than in the control group (Fig. 2 and Fig. 3). The results of the individual level and cluster-adjusted analyses examining static and dynamic balance favoured the intervention group (Table 2). After adjustment for covariates, mixed-effects linear regression analyses reproduced the main findings for static and dynamic balance (Table 3). At 6 weeks, improvements in static and dynamic balance were observed in the intervention group but not in the control group (mean difference in static balance from baseline 20.7 seconds, 95% confidence interval [CI] 10.8 to 30.5 seconds; mean difference in dynamic balance from baseline 2.3 seconds, 95% CI 0.7 to 4.0 seconds).

Compliance with balance training sessions had an effect on the change in static balance: the observed change among students in the intervention group who reported fewer than 18 sessions over 6 weeks was 6.1 seconds (95% CI –8.4 to 20.7), as compared with 25.8 seconds (95% CI 16.4 to 35.1) among those who reported 18 or more sessions. Compliance did not have a significant effect on change in dynamic balance.

Twelve (5 female and 7 male) subjects reported athletic injuries over the 6-month observation period: 2 were in the intervention group, and 10 were in the control group (Table 4). The median time lost from a sporting activity because of an injury was 13 (range 7–28) days. The median time to injury occurrence from the start of the study was 13 (range 2–24) weeks. The injuries reported occurred while the students were playing basketball (4/12), soccer (3/12), football (2/12), hockey (2/12) and volleyball (1/12). The relative risk of all injury was 0.20 (95% CI 0.05 to 0.88), and of ankle sprain 0.14 (95% CI 0.18 to 1.13). Compliance in collecting prospective sports participation data was low (43.3%), which resulted in insufficient data to estimate incidence density (i.e., number of injuries per 1000 participation hours).

There was an important difference in the incidence rate of self-reported injury (number of injuries per 100 adolescents) between the intervention and control groups: 3 (95% CI 0 to 12) versus 17 (95% CI 8 to 29) respectively, for a difference of 14 (95% CI 3 to 24). The number needed to treat to avoid 1 injury over 6 months was 8 (95% CI 5 to 35). The training program was more effective among subjects who reported an injury in the previous year (relative risk [RR] 0.13, 95% CI 0.02 to 1.0) than among those who reported no previous injury (RR 0.28, 95% CI 0.03 to 2.43).

The improvement in static balance following balance training with a wobble board is consistent with findings of other studies.^17,21,22,23 In addition, we found that the improvement was greater with increased reported compliance. Despite the small number of clusters and wide confidence intervals around the estimates for injury incidence rates, there was a significant and clinically important difference in injury rate between the intervention and control groups. The relative risk of injury found in our study (RR 0.20, 95% CI 0.05 to 0.88) is consistent with the finding in the only other randomized controlled trial examining a similar prevention program for adolescents (RR 0.17, 95% CI 0.09 to 0.32) and in studies involving adults (RR 0.06–0.51).^{12,13,14,15,16,17,18,19} In our study, we also found evidence that previous injury may be associated with future injury, independent of study group (OR 3.51, 95% CI 0.98 to 12.49), which is consistent with previously reported findings.¹²

Our study has limitations. Compliance in collecting prospective sports participation data was poor, probably because of the time intensity to report daily sports participation. Although some minor injuries may have been missed through self-reporting, the likelihood of this would not differ between the 2 groups, since biweekly follow-up by a physiotherapist was identical for both groups. Second, the moderate reliability and small inter-subject variability associated with the dynamic balance test could lead to an increased similarity between the study groups for this study variable. The resultant nondifferential measurement bias may have diluted the association found between the study groups on change in dynamic balance. Third, the ceiling effect of the static balance test led to the inability to examine changes in balance among subjects who reached the maximum time allowed (180 seconds) for this test. Notwithstanding these limitations, our study has many strengths. The effectiveness of the training program is valid and cannot be accounted for by differences in baseline characteristics between the 2 groups. A cluster randomized controlled trial with random recruitment of schools and subjects, and comprehensive primary and secondary end points, reduces the biases associated with the results and increases the generalizability of the study results. The high rate of consent to participate and the low dropout rate limited potential selection bias.