Purpose: Muscular strength is suggested to be dependent upon muscle characteristics. Yet, sex-specific relationships of muscle characteristics to strength in the resistance-trained require investigation. Therefore, the purpose was to evaluate sex differences in muscle characteristics and isometric strength in the elbow extensors, as well as their respective associations.
Methods: Resistance-trained men (n = 15, mean ± SD 22 ± 4 years, 87.5 ± 12.8 kg, 16.9 ± 2.9% body fat) and women (n = 15, mean ± SD 25 ± 5 years, 59.3 ± 7.3 kg, 22.4 ± 4.2% body fat) were tested. B-mode ultrasound images assessed muscle thickness, pennation angle, and echo intensity. Muscle volume and fascicle length were estimated from previously validated equations. Maximal voluntary isometric contraction measured elbow extensors isometric strength. Independent samples t-tests and Fisher's r-to-z test examined differences between sexes.
Results: Sex differences existed in all muscle characteristics (p < 0.05). Men's absolute strength (27.86 ± 3.55 kg) was significantly greater than women (16.15 ± 3.15 kg), but no differences were noted when controlling for muscle volume (men 0.069 ± 0.017, women 0.077 ± 0.022 kg/cm3). Sex differences did not exist in the relationships of muscle characteristics to strength with muscle size having the largest correlations. However, the relationship between echo intensity and body fat was different in men (r = - 0.311) and women (r = 0.541, p = 0.0143).
Conclusions: Sex differences in isometric elbow extensor strength are eliminated when expressed relative to muscle volume. Relationships of echo intensity and body fat were different between men and women and may be indicative of greater adipose infiltration in women.
Keywords: Echo intensity; Gender; MVIC; Strength; Ultrasound.