Journal of Sports Science and Medicine
Journal of Sports Science and Medicine
ISSN: 1303 - 2968   
Ios-APP Journal of Sports Science and Medicine
Androit-APP Journal of Sports Science and Medicine
©Journal of Sports Science and Medicine (2019) 18, 65 - 72

Research article
Influence of Sex and Maximum Strength on Reactive Strength Index-Modified
George K. Beckham1, , Timothy J. Suchomel2, Christopher J. Sole3, Christopher A. Bailey4, Jacob L. Grazer5, Steven B. Kim6, Kasie B. Talbot6, Michael H. Stone7
Author Information
1 Department of Kinesiology, California State University, Monterey Bay, Seaside, CA, USA
2 Department of Human Movement Sciences, Carroll University, Waukesha, WI, USA
3 Department of Health and Human Performance, The Citadel, Charleston, SC, USA
4 Department of Kinesiology, Health Promotion, and Recreation, University of North Texas, Denton, TX
5 School of Health and Human Performance, Georgia College, Milledgeville, GA, USA
6 Mathematics and Statistics Department, California State University, Monterey Bay, Seaside, CA, USA
7 Center of Excellence for Sport Science and Coach Education, Department of Exercise and Sport Science, East Tennessee State University, Johnson City, TN, USA

George K. Beckham
✉ Department of Kinesiology, California State University, Monterey Bay, Seaside, CA, USA
Publish Date
Received: 25-08-2018
Accepted: 30-11-2018
Published (online): 11-02-2019
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Reactive strength index-modified (RSImod) is a measure of lower body explosiveness calculated by dividing jump height by time to takeoff. RSImod is different between stronger and weaker athletes and between males and females. The purpose of this study was to evaluate differences in RSImod between males and females while controlling for maximal strength and lower body explosiveness. Forty-three female and fifty-eight male Division-I athletes performed countermovement jumps on a force plate during unloaded (0kg) and loaded (20kg) conditions. We used an ANCOVA to test whether RSImod is different between sexes conditioning on relative maximum strength (PFa) and average RFD 0-200ms (RFD200) measured during the isometric mid- thigh pull (IMTP). Differences of 0.087 (95% CI: 0.040-0.134; p = 0.0005) and 0.075 (95% CI: 0.040-0.109, p < 0.0001) were observed for RSImod between sexes in unloaded and loaded conditions, respectively. A male with PFa of 186 (grand mean of the sample) and RFD200 of 6602 N/s (grand mean of the sample) is predicted to have 28% greater RSImod than a female of similar PFa and RFD200. Maximum strength development should be a primary aim of training in female athletes, in addition to other trainable factors, such as stiffness and RFD.

Key words: Countermovement jump, sex difference, jump technique, isometric mid-thigh pull, reactive strength

           Key Points
  • Despite controlling for the effects of maximum strength and explosiveness, differences in RSImod and its constituent parts exist between males and females.
  • The differences observed between males and females don’t seem to increase with additional load, with the exception of time to takeoff, which may be a function of use of a greater portion of females’ body mass.
  • Use of a 2/3 exponent in the allometric scaling equation for isometric mid-thigh pull PF results appears valid, at least for a sample of young adult athletes.
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