# Criterion-Related Validity and Reliability of the Front Plank Test in Adults: The ADULT-FIT Project
## 4 Discussion
### Criterion-Related Validity  
The EMG activity of the trunk muscles during the FPT showed significantly greater differences for the URA, LRA, and EO muscles compared to the ES and MF muscles. These results highlight the predominance of flexor and lateral muscle activation in maintaining the front plank position. The particularly high activation of the EO muscle suggests its crucial role in providing lateral stability and counteracting rotational forces during exercise. In accordance with our findings, [PERSON] et al. (2018), [PERSON] et al. (2010), and [PERSON] et al. (2018) also found a greater activation of the EO concerning the RA and of these two muscles for the back muscles [21, 47, 48]. In contrast, the relatively lower activation of the ES and MF muscles indicates that these muscles contribute less to the stabilization demands of the FPT. Moreover, previous studies realized by [PERSON] et al. (2017) and [PERSON] and [PERSON] (2019) obtained similar values of ES activation to our study (24.81 and 24.32% MVIC, respectively) and slightly lower in the RA (34.93 and 51.83% MVIC, respectively) [49, 50]. On the other hand, two studies observed higher values of RA than EO during a front plank exercise [51, 52]. This may be because they analyzed muscle activation in a front plank for a certain time and not to failure. As demonstrated in previous research and corroborated by the present study, the EO usually has a greater activation for the MVIC than the rest of the muscles assessed [21, 48, 53]. However, we found similar levels of activation of URA and EO in the adult group. Also, it has been shown that the front plank exercise activates the flexor muscles of the trunk more than the extensor muscles [54]. To increase the activation of the ES muscles, a recent review showed that free-weight exercises are those that produce the greatest activation, with back extension exercises showing high activation. In addition, if an increase in MF activation is desired, it was recommended to work on the front plank exercise on a Swiss ball or free weight, such as squat or deadlift, and other forms, such as core stability training with abdominal drawing-in maneuver technique or Pilates [54, 55, 56]. Regarding correlations, no correlations were found between FPT time with physical activity or RPE at the end. However, low and moderate correlation were found with body fat percentage and trunk fat mass percentage regardless of sex and age groups. [PERSON] et al. (2018) [17] found a very low or low correlation between FPT and BMI and waist circumference as well as the end of the RPE in young adults. This may be due to the fact that the sample in the [PERSON] et al. (2018) [17] had overweight (BMI = 25.4) participants, whereas our sample had normal weight (BMI < 25.00) participants. Furthermore, [PERSON] et al. (2018) [17] did not carry out correlations by sex and age group, nor was body composition measured, making it difficult to compare the results of our study by groups. In the study by [PERSON] et al. (2021), they found no agreement between the fatigue presented by the electromyographic data and RPE at the end, as the mean obtained was3.5 for the sedentary group and 3.8 for the resistance group. This may occur because the FPT is an isometric test in which there is no cardiovascular involvement, so the perceived effort may be lower, and the participants finish the test due to local fatigue. However, [PERSON] et al. (2019) [32] measured RPE every 5 s in a FPT to failure, and they found a high association between RPE and neuromuscular fatigue during the performance of this exercise. A major finding in our study was that, both in the total sample and the sample divided by sex and age groups, there was a significant and negative correlation between the FPT time and the percentage of body fat and the percentage of trunk fat mass, indicating that the time in FPT is lower when the percentage of body fat and trunk fat mass is higher.