# Test-Retest Intra-Session Reliability of Isokinetic Knee Strength Measurements in Obese Children
###### Abstract  
Background: Childhood obesity is one of the most critical global health challenges. Poor knee extensor strength is associated with disability and difficulties with physical function in obese patients. The objective of this study was to evaluate the test-retest reliability of peak torque and total work in concentric flexion and concentric extension in obese children. Methods: 25 obese children aged between 6 and 11 years performed 3F maximal unilateral isokinetic repetitions with the knee extensors/flexors performing concentric actions at 60\({}^{\circ}\)/s. Peak torque (Nm), total work (I), and the ratio antagonist/agonist were recorded and normalized by total and lean mass. The intraclass correlation coefficient (ICC), standard error of measurement (SEM), and smallest real difference (SRD) were computed. Results: Reliability was excellent for almost all variables. The highest ICC values were observed when work or peak torque were not divided by any other variable. However, SEM (%) and SRD (%) were lower when peak torque and work were divided by the total mass or lean mass. The antagonist/agonist ratio showed an ICC value of 0.873. Conclusion: Peak torque, total work, and the antagonist/agonist ratio are reliable in obese children.  
isokinetic strength; children; obesity; reliability +
Footnote †: journal: Medical  
reported that obese children present higher maximum voluntary contraction than non-obese adolescents. However, it is controversial since sedentary obese people may show lower strength levels [10], and their muscle fibers may have more intramyocellular lipids, which could lead to poorer performance [11].  
For a correct interpretation of the results after a physical condition assessment test, it is necessary to know how reliable this test is. The following aspects may affect the reliability of an isokinetic strength evaluation: (1) The accuracy of the isokinetic dynamometer, (2) experience of the assessment, (3) fatigue, (4) motivation, (5) inexperience of the person evaluated in the test (learning effect), and (6) the natural human variability. In this regard, the test-retest reliability of isokinetic strength in non-obese children has been previously described [12; 13; 14]. However, to the best of our knowledge, there is no study evaluating the test-retest reliability of isokinetic strength in obese children. It seems that the functional performance of obese and non-obese children may be different, and this could lead to differences in the reliability of the test. Some of the differences in strength between obese and normal-weight children in the scientific literature are the following: (1) Obese children show a deficit of knee extensors strength linked to their mass [7]; (2) relative strength expressed per unit of body mass may be lower in obese children [15]; (3) Obese children have reduced knee flexion angles in the starting position before starting to walk [16]; (4) \"the overweight/obese children had poorer muscle tissue composition, greater velocity-related impairments in muscle strength, and a smaller range of mean firing rates at the targeted torque that may suggest altered motor unit recruitment strategies\" [17].  
In particular, it is necessary to know test-retest intra-session reliability since several interventions have been proposed, such as whole-body vibration, that could have an acute effect on the knee strength. To verify what the real acute effect of the training is, it is essential to know the test-retest intra-session reliability of the isokinetic knee strength.  
Therefore, the main objective of the present study was to evaluate the test-retest reliability of peak torque and total work in concentric flexion and concentric extension in obese children aged between 6 and 11. Furthermore, given that these measures are sometimes reported as normalized variables (divided by weight or lean mass), as a secondary objective, we intended to verify whether reliability can be affected by this normalization.