Longitudinal changes in strength, power, speed, and body mass in American football players: Diminishing returns as training age increases

The aims of this study were to: 1) detail the strength, power, speed, and body mass (i.e. physical qualities) of National Collegiate Athletic Association (NCAA) Division 1 American football players by playing position across a four-year collegiate career, and 2) quantify the rate of change in physical qualities as athletes progress through their eligibility period. 2628 observations from 512 NCAA Division 1 American football players were collected during a standardised testing battery that took place across a 15-year period. One-repetition maximum bench press and back squat, 40-yard sprint, vertical jump, standing broad jump, and body mass were analyzed. Year-on-year changes in physical characteristics by the entire cohort and positional groups were analyzed using linear mixed models with Cohen's effect size (ES) ± 95% confidence intervals (CI). Improvements in physical qualities occurred over the four years, although the largest changes were evident in the first year, with the improvements becoming less pronounced each year. For example, across the cohort, large effects in the bench press (ES ±95%CI: 1.63 ± 0.18) and back squat (ES ±95%CI: 1.62 ± 0.18) occurred in the first year but only small changes occurred between years 3 and 4 (ES ±95%CI: 0.45 ± 0.16 and 0.26 ± 0.16, respectively). Collectively, this study demonstrates the longitudinal changes in strength, power, speed, and body mass of collegiate Division 1 American football players across different positional groups. Furthermore, it shows that physical qualities improve throughout the four years, but the year-on-year change becomes smaller each year and therefore suggests physical qualities become harder to increase/improve.