International Journal of Sports Physiology and Performance, 2015,10, 542-545 http://dx.doi.org/10.1123/ijspp.2014-0255 ©2015 Human Kinetics, Inc.
Eft Human Kinetics £Qll ORIGINAL INVESTIGATION
Reliability of the Dynamic Strength Index in College Athletes Christopher Thomas, Paul A. Jones, and Paul Comfort Purpose: To determine the reliability of the Dynamic Strength Index (DSI) in college athletes. Method: Nineteen male col lege athletes performed the squat jum p (SJ) and isometric midthigh pull (IMTP) to determine peak force, on 2 separate days. Reliability was assessed by intraclass correlation coefficient (ICC), typical error (TE), percentage change in the mean, smallest worthwhile change (SWC), and coefficient of variation (%CV). Results: Peak force for the SJ was 2137 ± 499 N and 2781 ± 435 N for the IMTP, resulting in a mean DSI of 0.78 ± 0.19. Peak forces in the SJ (ICC = .99, TE = 57.22 N, change in mean = 0.2%, SWC = 4.7%, CV = 2.6%) and IMTP (ICC = .95, TE = 104.22 N, change in mean = 0.5%, SWC = 3.1%, CV = 3.8%) were considered highly reliable between sessions. However, IMTP peak force was the only variable with an overall TE 4.67%, 3.13%, and 5.13%, respectively, identifying meaningful changes in response to training or competition.
Keywords: peak force, isometric midthigh pull, squat jump The assessment of physical capabilities such as strength and power plays an important role in the evaluation of training effects. Performance testing can provide a rationale with which to individu alize strength-training programs to improve specific physical quali ties required for successful sports performance. The term strength diagnosis refers to the assessment of an athlete’s current strength level, aiding the creation and development of athlete profling.1 Frequent assessment of athletes helps strength and conditioning coaches and sports scientist monitor acute training responses, measure chronic training effects, identify strengths and weaknesses, individualize training programs, and compare data with norma tive measures found in peer-reviewed publications.2 Strength and power are key determinants in sporting success,3 but the degree to which these qualities are important depends on the sport and event. Therefore, when assessing the aforementioned qualities, a thorough needs analysis of the sport should be conducted so that the tests performed measure sport-specific qualities, and these qualities can be trained through various modalities to elicit adaptations in the required physical qualities.4 When selecting performance tests, 2 important factors must be considered to assess physical capabilities: validity and reli ability.5 Validity concerns determining the degree to which the measuring instrument measures what it is intended to measure. Reliability relates to the consistency of the measure, controlled for by the protocol design, clear definition of procedures, equipment calibration, and consistent testing through practice and the use of pilot studies. Typical error of measurement (TE) provides a direct measure of the amount of error associated with the test, which can also be represented as a percentage of variation in an athlete’s mean score (%CV).5 These measurements allow strength and condition ing coaches and sport scientists to make sound conclusions about
The authors are with the Directorate of Sport. Exercise and Physiotherapy. University of Salford, Salford, UK. Address author correspondence to Christopher Thomas at
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whether the changes seen in response to a training intervention are meaningful or not.5 Smallest worthwhile change (SWC) refers to the ability of the test to detect the smallest practically important change. When interpreting test results this value should fall outside the TE to determine whether changes in testing results are of benefit to athletic performance.6 Retest correlations such as intraclass cor relation coefficients (ICCs) refer to the reproducibility of variables for repeated tests, with a value of >.80 considered highly relaible.7 However, a test deemed reliable using ICCs does not necessarily suggest it is effective for monitoring performance in athletes.8 High reliability is important not only for testing procedures but also when monitoring changes in individual and team training effects over extended periods of time. Comparing the results of isometric and dynamic testing allows strength and conditioning coaches and sport scientists to determine the dynamic-force capabilities of athletes in relation to their maxi mum-force capability. This has been termed the Dynamic Strength Index (DSI).910 The DSI has been expressed as a ratio of ballistic peak force to isometric peak force (ballistic peak force:isometric peak force). A variety of methods have previously been used to test lower-body strength and power. Vertical jumps such as the squat jump (SJ), countermovement jump, and jump squat have been widely used to measure lower-body dynamic strength and power capabilities due to their high reliability and validity across a range of populations.4-9-11 Recent research has shown the isometric midthigh pull (IMTP) to be a reliable method of maximum-force capabifity in a range of sports.12- '4 Relation of IMTP performance to dynamic performance still remains unclear due to contrasting biomechanical and neuromuscular characteristics.3-15 Although isometric-strength testing has produced contrasting relationships with dynamic performance,3 1546 comparing an athlete’s isometricforce capabilities against their dynamic-force capability may give strength and conditioning coaches and sport scientists information regarding the training status of strength qualities. Early research by Wilson and M urphy17 obtained a Dynamic Strength Index ratio (DSI) using rate of force developm ent
Dynamic Strength Index
(RFD) and peak force during an isometric back squat (isometric RFD:isometric peak force) in recreational males. The authors con cluded that a DSI ratio of >3.1 warranted an increase in maximal strength training, whereas a ratio of
