# Test-Retest Intra-Session Reliability of Isokinetic Knee Strength Measurements in Obese Children
## 4 Discussion  
The main finding of this study was that the reliability of strength values was very high. The values of the ICC from concentric extension were always higher than 0.95. Furthermore, the reliability of the concentric flexion oscillated between 0.887 and 0.956. This difference between flexors and extensors was common in previous studies on the reliability of concentric flexion and extension [12,13] and may indicate a poorer ability to exert controlled concentric force with knee flexors. Therefore, after one single familiarization repetition, the second and third repetitions were not different (\(p>0.05\)), and the reliability was good or excellent (ICC \(>0.7\) and 0.9, respectively). These results were similar to those observed in other studies, which were summarized in the study by [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [19]. The PT values of knee flexors reported in Table 2 were similar to those reported by a previous study with 3587 children [22]. However, the PT of knee extensors was higher in the obese children in the present study compared with the 50 th percentile of the previous study. Absolute strength was greater in obese subjects. However, when the data were normalized (e.g., by muscle volume), strength decreased in obese subjects. Therefore, it is commonly accepted that obesity increases absolute strength,  
Figure 2: Bland–Altman plots work. probably as the greater cross-section mass is lower. This may suggest a deficit in contractile muscle function that is compensated for by increased muscle mass [26; 27; 28]. The present study included not only reliability indices for non-normalized PT and TW, but also normalized by total body mass and lean mass. To our knowledge, this is the first article reporting on the reliability of strength variables during an isokinetic test in a sample comprised of obese children. The incidence of knee injuries in obese children is higher than in non-obese children [29]. The balance of knee flexor and extensor muscles is suggested to be a relevant marker of the capacity for stabilization of the knee and the reduction of risk of injury [30; 31]. The present study assesses the hamstrings/quadriceps balance ratio using the PT of a maximum voluntary contraction. Results showed that the mean hamstring/quadriceps ratio was 51.56 and 53.22 in the test and retest, respectively. This ratio showed good reliability, with an ICC close to 0.90 (0.874), a SEM of 8.62%, and a SRD of 23.91%. However, those mean values are lower than those found for non-obese children in a previous study [32], which may indicate that, to some extent, the higher incidence of knee injury in obese children may be partially explained by a greater imbalance between knee flexor and extensor muscles. However, further studies are needed to test that hypothesis. Visual inspection of the Bland-Altman plot for peak torque and work did not show signs of any systematic bias in the differences between test and retest. Our data showed that the work may have higher variability than peak torque in obese children. It appears that the percentage differences in the normalized torque values tend to decrease with higher levels of normalized torque. This may suggest poorer reliability in those with lower levels of muscle strength. In this sense, it is important to evaluate the intra-session test-retest reliability of isokinetic knee strength in this population because they could have knee extensor strength deficits [7] and the strength per unit of body mass may be lower, which could have implications for weight-bearing activities [15]. In this way, our study could help to adequately interpret measurements on muscle strength, which is related to their physical autonomy, the ability to perform daily activities, and their health-related quality of life [16]. Additionally, the results obtained in our study may be used to interpret the acute effects of physical training on the peak torque and work of knee extensors and flexors.