Convergent Validity between Field Tests of Isometric Core Strength, Functional Core Strength, and Sport Performance Variables in Female Soccer Players

Introduction: Previous research has failed to definitively explain the role that core fitness plays during sport performance. Movements of sport performance require the core musculature to simultaneously provide spinal stability while producing external forces that aid limb movement. The core is central to most kinetic chains; therefore, a better understanding of core function during sport should help to benefit performance. Purpose: The purpose of this study was to compare tests of isometric core strength that evaluate the ability of the core to provide a stable base of support, and tests of concentric functional core strength that evaluate the ability of the core to produce and transfer forces to the limbs, with the soccer kick and throw-in, to see which plays a greater role in soccer sport performance. It was hypothesized that the concentric functional core strength tests would correlate more strongly with the soccer performance tests than the isometric strength tests due to their ability to be performed in an explosive manner that better mimics sport. Methods: To test this hypothesis, 11 female participants (age: 19.73 ± 0.9 y, height: 1.63 ± 0.04 m, weight: 64.41 ± 11.73 kg) from the College of Idaho soccer team volunteered for this study. Isometric core strength was measured using a dynamometer during movements of trunk flexion and bi-lateral rotation. Concentric functional core strength was measured by performing the front abdominal power test (FAPT) and side abdominal power test (SAPT). Soccer performance was evaluated with a standing soccer-style kick and throw-in for maximal speed. Isometric trunk flexion and the FAPT were correlated with the soccer throw-in, while bi-lateral trunk rotation and bi-lateral SAPT were compared with the contralateral soccer kick. By correlating the tests in this manner, the muscular contributions during similar movement patterns (flexion and bi-lateral rotation) could be analyzed in different manners (isometrically, concentric/functionally) to see which correlates more strongly with tests of soccer sport performance. Results: A Pearson’s product correlation found that the isometric core strength correlated more strongly with tests of soccer sport performance than concentric functional core strength. Discussion: It was found that the core plays a greater role in providing a stable base of support rather than producing/transferring force during tests of soccer sport performance. Consistent with previous studies, the external load and direction of the load placed on the core affects the muscular activation that is produced. The isometric tests had a much larger load placed on them, which elicited a greater muscular activation and could explain why there was a greater correlation with tests of soccer sport performance. The validity of the isometric and concentric functional strength tests to accurately measure force of the intended musculature remains in question. More future research is warranted to better explain the relationship between core fitness and sport performance