Countermovement Jump Performance and Sprint Speed in Chilean Adolescent Athletes: Does the Association Differ by Sex?

Background: This study examined the relationship between countermovement jump (CMJ) performance and sprint speed in adolescent athletes, considering anthropometric variables and potential sex differences. Methods: A descriptive-correlational study was conducted with 140 athletes (43 females and 97 males, aged 14–17 years) from the Promesas Ñuble program in Chile. Body composition, biological maturation, flexibility, vertical jump performance, and 10 and 20 m sprint performance were assessed using standardized protocols. Results: CMJ performance was strongly and negatively correlated with 20 m sprint time (r ≈ −0.76), indicating that greater jump heights were associated with faster sprint times. In the multiple linear regression analysis, CMJ emerged as the strongest predictor of 20 m sprint time, with the overall model explaining approximately 61% of the variance (R2 = 0.608; adjusted R2 = 0.594). A significant CMJ × sex interaction was observed in the primary analysis (B = −0.0217 s·cm−1, 95% CI [−0.0414, −0.0019], p = 0.032). Sex-stratified analyses showed a negative association between CMJ and sprint time in males (B = −0.0267 s·cm−1, 95% CI [−0.0343, −0.0190], p < 0.001), whereas the female-specific estimate was less precise and its confidence interval included zero (B = −0.0240 s·cm−1, 95% CI [−0.0542, 0.0062], p = 0.116). However, influence diagnostics identified one valid but highly influential observation in the female subgroup (Cook’s D = 1.238). In a sensitivity analysis excluding this observation, the CMJ × sex interaction was attenuated and no longer statistically significant (B = −0.0051 s·cm−1, 95% CI [−0.0226, 0.0125], p = 0.568), indicating that evidence for sex-specific differences was sensitive to this observation. Conclusions: CMJ performance was strongly associated with 20 m sprint performance in adolescent athletes. Although the primary analysis suggested that this association differed by sex, sensitivity analysis showed that the interaction was substantially influenced by one valid influential observation in the female subgroup. These findings support an association between CMJ and sprint performance but indicate that sex-specific differences require confirmation in larger and more balanced samples.