Maximum Force Capacity of Back Extensor Muscles in Healthy Women and Men: Not Different, if Anthropometrically Normalized

Background: In this study, trunk extension strength was compared between healthy female and male participants. Methods: Overall, 115 women and 124 men underwent isometric maximal voluntary contraction (MVC) testing in an upright standing position. Upper body weight was additionally assessed. The primary outcome variables were maximal torque, upper body weight expressed as torque, and the MVC-to-upper-body-weight ratio. In view of the large sample size, interpretation of the results was based not only on statistical significance but also on clinical/practical relevance, as reflected by effect size (ES) and the relationship between the difference in mean values (DMV) and the minimal important difference (MID). Results were classified as relevant only when all three predefined criteria were met (p < 0.05, ES > 0.5, and DMV > MID). Results: Men showed significantly higher MVC values than women (241 Nm vs. 162 Nm; p < 0.0001; ES = 1.974; DMV > MID). Likewise, upper body torque was significantly greater in men than in women (115 Nm vs. 80 Nm; p < 0.0001; ES = 2.119; DMV > MID). However, after normalization for upper body torque, the between-sex difference in relative strength was no longer considered relevant (2.16 vs. 2.00; p = 0.0055; ES = 0.364; DMV < MID). Conclusions: These findings indicate that although absolute trunk extension strength differs markedly between sexes, relative strength adjusted for upper body weight is broadly comparable. Both women and men demonstrated a physiological strength reserve of approximately 100% relative to upper body weight.