Evaluation of the Field Tests of Flexibility of the Lower Extremity: Reliability and the Concurrent and Factorial Validity

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The purpose of this study was to evaluate the standard field tests of flexibility of the lower extremity through their (a) intratrial and test-retest reliability, (b) concurrent validity with respect to a high-precision 3D system for kinematic recording, and (c) factorial validity. Physically active men (n = 84) were tested over 2 separate sessions by means of standard simple devices (kinanthropometry, ruler, and protractor) on the following flexibility tests: leg raise in a supine position, hip abduction test, single-legged knee bend (SLKB), sideward leg splits (SdLS), sit and reach (SR), sideways leg splits, and lengthwise leg splits. Additionally, 17 participants were simultaneously tested by means of a laboratory 3D kinematic analysis system. Most of the tests demonstrated high intratrial and test-retest reliability and concurrent validity. The exceptions were SLKB and SR tests that revealed not only a somewhat lower reliability but also a lower concurrent validity. For estimation of factorial validity, we applied a principal component analysis of the intercorrelations among the evaluated flexibility tests that revealed a single extracted principal component. Strong mutual relationships among them also suggest that very few of them, if not a single one, should be applied within the standard batteries of physical performance tests. We particularly recommend using SdLS over the most frequently used SR not only because of its relatively high reliability and validity but also because of its postural properties. Finally, we conclude that most of the standard field tests of flexibility of the lower extremity based on using inexpensive field equipment could be both reliable and valid and, therefore, justified for use in routine testing.