<p>Fitness has been dominantly defined in terms of physical conditioning components. Under such definition, males tend to outperform females in strength, speed, aerobic or anaerobic capacity when compared at the same age and training status. However, females have a higher life expectancy, which in humans is related to higher biological fitness. Using the paradox of sex differences in fitness—where males have higher physical fitness but do not have a higher life expectancy—the aim of this opinion paper is to (a) highlight the multidimensionality of fitness, and (b) redefine health-related fitness, drawing on key fitness goals in biology: adaptability and survival. The redefinition of health-related fitness as the “adaptive ability to foster survival possibilities” encompasses synergies across physical, mental, psychological, emotional, social and subjective dimensions, while embracing the diversity of human characteristics, including sex, gender, age, somatotype, vital state, disability, disease and wellbeing, among others.</p>