Impellizzeri Franco M, Rampinini Ermanno, Sassi Aldo, Mognoni Piero, and Marcora Samuele M
Several studies investigated different lower limb strength imbalance indices as a risk factor for developing knee or hamstring injuries, and to monitor rehabilitation programs (e.g. Dvorak and Junge, 2000). Isokinetic measurements are frequently used to assess lower limbs muscular strength. However, the nature of isokinetic tests (open chain and isokinetic movements) is different from movements performed by athletes during most sport activities (closed chain, isotonic) (Abernethy et al, 1995). The aims of this study were 1) to develop and validate a test which address the limitation of isokinetic measures by assessing the difference in strength between the two lower limbs (strength asymmetry index, SAI) using vertical jump on a force platform, 2) to determine the reliability of this test, and 3) to provide some preliminary reference values for soccer players. Study 1: we measure the peak force of the right and left lower limb in 35 subjects during a) maximal isometric voluntary contraction on a leg-press equipped with a load cell, and b) during vertical jump on force platform. SAI was expressed as percent difference. Study 2: 60 subjects repeated the jump test in two occasions. Five measures for each leg were collected. Study 3: 190 soccer players were tested to determine the distribution of SAI values before the beginning of the competitive season. Results: there was a strong and significant correlation between isometric SAI and vertical jumps SAI (r = 0.84; p<0.001). Intraclass correlation coefficient from the two trials was 0.92 (CI 95%: 0.86-0.95), and the bias ± total error was 0.48 ± 6.94 %. The SAI values collected on soccer players showed a normal distribution (1.1 ± 8.6 %). Discussion: the results of this study demonstrate high concurrent validity of SAI determined during vertical jumps when compared to isometric SAI. The high intraclass correlation and the relative small confidence limits of the total error suggest that this test is also reliable. Based on the distribution data, we calculated abnormal SAI as values 2SD above (18%) and below (-16%) the mean. In conclusion, the results of the present study suggest that this test can be used to identify athletes with abnormal SAI. The significance of abnormal SAI as risk factor for sport related injuries need further investigations. Additional studies should also assess the sensibility of this test in monitoring rehabilitation programmes aimed to reduce strength difference between lower limbs.