• Criterion and construct validity of an isometric mid-thigh pull dynamometer for assessing whole body strength in professional rugby league players

      Dobbin, Nicholas; Hunwicks, Richard; Jones, Ben; Till, Kevin; Highton, Jamie M.; Twist, Craig; University of Chester, Rugby Football League, Leeds Beckett University (Human Kinetics, 2018-02-31)
      Purpose: The purpose of this study was to examine the criterion and construct validity of an isometric mid-thigh pull dynamometer to assess whole body strength in professional rugby league players. Methods: Fifty-six male rugby league players, (33 senior and 23 youth professional players) performed four isometric mid-thigh pull efforts (i.e. two on the dynamometer and two on the force platform) in a randomised and counterbalanced order. Results: Isometric peak force was underestimated (P<0.05) using the dynamometer compared to the force platform (95% LoA: -213.5 ± 342.6 N). Linear regression showed that peak force derived from the dynamometer explained 85% (adjusted R2 = 0.85, SEE = 173 N) of the variance in the dependent variable, with the following prediction equation derived: predicted peak force = [1.046 * dynamometer peak force] + 117.594. Cross-validation revealed a non-significant bias (P>0.05) between the predicted and peak force from the force platform, and an adjusted R2 (79.6%), that represented shrinkage of 0.4% relative to the cross-validation model (80%). Peak force was greater for the senior compared to youth professionals using the dynamometer (2261.2 ± 222 cf. 1725.1 ± 298.0 N, respectively; P<0.05). Conclusion: The isometric mid-thigh pull assessed using a dynamometer underestimates criterion peak force but is capable of distinguishing muscle function characteristics between professional rugby league players of different standards.
    • Validity of an isometric mid-thigh pull dynamometer in male youth athletes

      Till, Kevin; Morris, Rhys; Stokes, Keith; Trewartha, Grant; Twist, Craig; Dobbin, Nicholas; Hunwicks, Richard; Jones, Ben (2017-11-16)
      The purpose of the present study was to investigate the validity of an isometric mid-thigh pull dynamometer against a criterion measure (i.e., 1,000 Hz force platform) for assessing muscle strength in male youth athletes. Twenty-two male adolescent (age 15.3 ± 0.5 years) rugby league players performed four isometric mid-thigh pull efforts (i.e., two on the dynamometer and two on the force platform) separated by 5 minutes rest in a randomised and counterbalanced order. Mean bias, typical error of estimate (TEE) and Pearson correlation coefficient for peak force (PF) and peak force minus body weight (PFBW) from the force platform were validated against peak force from the dynamometer (DynoPF). When compared to PF and PFBW, mean bias (with 90% Confidence limits) for DynoPF was very large (-32.4 [-34.2 to -30.6] %) and moderate (-10.0 [-12.8 to -7.2] %), respectively. The TEE was moderate for both PF (8.1 [6.3 to 11.2] %) and PFBW (8.9 [7.0 to 12.4]). Correlations between DynoPF and PF (r 0.90 [0.79 to 0.95]) and PFBW (r 0.90 [0.80 to 0.95] were nearly perfect. The isometric mid-thigh pull assessed using a dynamometer underestimated PF and PFBW obtained using a criterion force platform. However, strong correlations between the dynamometer and force platform suggest that a dynamometer provides an appropriate alternative to assess isometric mid-thigh pull strength when a force platform is not available. Therefore, practitioners can use an isometric mid-thigh pull dynamometer to assess strength in the field with youth athletes but should be aware that it underestimates peak force.