# Validation and Application of Two New Core Stability Tests in Professional Football
## 4 Discussion
### Study II: (longitudinal study)  
The purpose of the study was to determine whether these preliminary validated tests (PPT and CCT) would be able to detect changes in the force application capacities of professional football players after the implementation of a motor control-based core stability training program. We found a significant improvement in the measured core strength values (PPT and CCT tests) after the core stability training program was implemented. The original result obtained from this longitudinal study was that participants were able to produce higher strength values in spite of the fact that the progression criteria were core neuromuscular control challenge-based, instead of being based on traditional resistance progression criteria. In fact, neuromuscular control training for athletes has traditionally been focused on resistance exercises for trunk musculature. However, these types of core training programs could not only lead to excessive applications of mechanical loads to the spine, but they can also fail to satisfy the biomechanical stabilization requirements necessary to counteract the acting internal and external forces during athletic maneuvers [30; 31]. Even so, previous studiesusing non-elite or sub-elite instead of elite subjects have clearly demonstrated the effectiveness of core stabilization programs in improving core strength when compared with no or regular training only [32; 33]. Regarding the literature available, it is difficult to find longitudinal studies recruiting elite professional football players due to the inability to have a control group and the high level competitive demand that often challenges this type of scientific effort. A weak core body region has been linked with alterations in the transfer of energy throughout the whole kinetic chain, potentially resulting in reduced sport performance and increased injury risk [2; 15; 34]. As a result, the implementation of a core strength training program has become popular in sport and clinical settings as a means to improve performance and reduce injury risk. Several studies have examined the effects of improving core-related strength following a core training program, finding a wide variety of results. Similarly, few studies have analyzed the possible relationship between core strength and sport-performance, concluding that if founded, the correlations were weak to moderate [31; 34; 35; 36]. One of the main sources that could explain the inconsistency of these results could involve the methodology used to measure the core strength. Most of the studies followed McGill testing protocol-derived methods. These studies, despite demonstrating excellent test-retest reliability, seem to be more related to trunk muscle endurance capacity, rather than being focused on the more functional and faster movements demands that most multi-directional sports seem to require [35]. Furthermore, in the present study, we found that those players displaying a limb symmetry index greater than 15% (described in the literature as the maximum physiological difference) displayed a trend toward the equalization of the observed deficit at the end of the season. In contrast, players reporting more symmetric limb-to-limb values at baseline significatively improved their performance in the tests at the end of the season. Whether this fact could be linked to both injury risk reduction and/or athletic performance enhancement should be appropriately addressed in future studies. The present study had several limitations. The first is associated with the use of the HHD, which has been demonstrated to reproduce some tester bias as the evaluator has to produce sufficient force to ensure the actual isometric contraction of the participant. As such, inadequate force application by the evaluator may lead to decreased intertester reliability [23; 24]. To avoid or diminish this bias, both testing sessions (test 1 and test 2) were conducted by the same tester in the longitudinal study [25].