Changes in Creatine Kinase and Hormones over the Course of an American Football SeasonStone, J. D.1; Kreutzer, A.1; Mata, J. D.1; Nystrom, M. G.1; Jagim, A. R.2; Jones, M.T.3; Oliver, J. M.1,*The Journal of Strength & Conditioning Research: April 20, 2017 - Volume Publish Ahead of Print - Issue - p doi: 10.1519/JSC.0000000000001920 Original Research: PDF Only Abstract Author Information The purpose of this study was to examine changes in creatine kinase and hormones over the course of an entire season of American football. A secondary purpose was to determine differences between starters and non-starters. Fasting blood samples were obtained from nineteen National Collegiate Athletic Association Division I (n = 19; 20 ± 1 years) football athletes over the course of a season beginning prior to the start of summer off-season conditioning (T1), before (T2) and after pre-season (T3) football camp, with remaining samples taken throughout the competitive season (T4-T8). A magnitude-based inference approach was used to define outcomes. Testosterone was higher in starters prior to the start of the season (T1, Effect Size [ES] = 0.8) and during pre-conference (T4; ES = 0.7). Post-Camp (T3) testosterone was lower in all players, though greater in starters (starters, 0.0%/0.3%/99.7%; non-starters, 0.2%/2.9%/96.9%). An increase cortisol relative to baseline (T1) was observed in starters early in season (T4, ES = 0.7; T5, ES = 0.5). Creatine kinase was elevated at all time-points in all athletes, with starters having higher circulating levels throughout season. These data demonstrate that changes in hormonal markers may be experienced over a season of football and differ by playing status. Differences between starters and non-starters may be indicative of greater damage and stress experienced by starters, which may result from a greater number of repetitions. 1 Exercise and Sport Performance Laboratory, Texas Christian University, Fort Worth, TX 2 Exercise and Sport Science, University of Wisconsin – La Crosse, La Crosse, WI 3 Health and Human Performance, George Mason University, Manassas, VA Corresponding Author: Jonathan M. Oliver, Ph.D. Sports Concussion Research Group Department of Kinesiology Texas Christian University PO Box 297730 Fort Worth, TX 76129 Email: firstname.lastname@example.org Copyright © 2018 by the National Strength & Conditioning Association.