Results from the present study confirm some previous findings on morphological differences of trunk muscles in different populations and provide preliminary evidence of significant alterations in mechanical muscles’ properties assessed with a non-invasive and reliable technique such as TMG. Expectedly, males performed longer during static side (in particular on the left side) and front plank endurance tests; however, no differences were observed between males and females during the flexors, extensors, and dynamic endurance tests. These findings seem to be consistent with previous findings in adolescents showing males had higher lateral torso endurance than females [
28]. Males also presented larger m.OE and m.OI muscles, whereas no significant differences were found in other abdominal or lumbar muscles. Abdominal muscles thickness evaluated with ultrasound was found to be consistent with previous literature suggesting a transverse abdominis < external oblique < internal oblique < rectus abdominis pattern, with sex-linked differences [
24]. Nevertheless, in the present study, this difference was significantly evident in abdominal oblique muscles. A significant side × sex interaction was reported for m.OE, showing that compared to females, males had significantly thicker m.OE on the left side, consistently with the findings on lateral endurance, and this might be hypothesized to depend on the specific activation of this muscle during sex-specific gymnastics exercises [
29]. The importance of abdominal oblique muscles in gymnastics and the difference in their thickness between males and females might be explained by the fact that compared to females, the male gymnasts from our sample performed more turns and rotations (including off-axis jumps) during their training activities. Our findings suggest that sex differences might be present in ES TMG parameters, and in particular Dm can be larger in males compared to females (∼172%), in line with previous findings in healthy participants [
14]. Although it did not reach statistical significance, females were found to present smaller lumbar muscles thickness compared to males, and this might have affected the mechanical responses to the electrical stimulus, as previously suggested [
14]. In addition, the findings from the present study, suggesting a lower ES Dm in females than males, confirm the previous observations from Lohr and colleagues [
14], and considering the various hypothesis explaining the such difference, it should be considered that regional adipose tissue distribution might have affected the TMG responses causing larger noncontractile tissue oscillations after the contraction in the female participants [
14]. Nevertheless, other factors should be considered, as despite we found a general correlation between body fat percentage and ES Dm, when corrected for sex, it was not significant anymore. Other TMG parameters of the ES have been found to differ between males and females, as Ts and Tr; nonetheless, more studies are needed to provide a better understanding of the physiological significance of such parameters and, therefore, it is not possible to hypothesize if these differences depended on different muscle contractile properties, if they depended on variability [
12].
It should be noted that the modest sample size of this study was not sufficient to exclude possible bias arising from interindividual differences, and in particular, we reported that males from this sample trained at higher volumes compared to females, and this might have affected the results. In addition, different gymnastics apparatuses are used by male and female gymnasts, with peculiar characteristics and required motor skills that might explain the observed difference [
16,
17]. Participants were sub-elite gymnasts, and differences might be present compared to elite athletes; nonetheless, it should be important to consider sub-elite athletes as representing the majority of the sports population and also be at risk of musculoskeletal injuries. In addition, despite more research being needed, females might be characterized by different responses depending on the phase of the menstrual cycle they are tested [
30], and, therefore, future studies should focus on such differences. Other measures of stiffness, such as myotonometry or shear wave ultrasound might have provided additional insights into sex differences in this interesting parameter [
13,
31,
32]. However, the sex-based differences in low back TMG parameters are consistent with previous findings suggesting significant differences in trunk extensors muscle stiffness and contractile characteristics between males and females [
14,
31], and this might be relevant for the increased risk for musculoskeletal injuries and overuse, as in low back pain [
33,
34].
This study provides preliminary evidence of sex-based differences in trunk flexors and extensors characteristics in adolescent sub-elite gymnasts. In particular, it is suggested that females can present lower lateral torso endurance, smaller lumbar multifidus thickness, and reduced erector spinae radial displacement, which might be an indirect measure of increased muscle stiffness. Gymnastics is a sport that presents similar fundamentals, such as highly dynamic movement, strength and postural control tasks, although males and females perform different exercises and, therefore, might present muscular morphological and functional differences. Despite the moderate sample size, this study encourages future research to globally investigate trunk muscles’ characteristics and sex-linked differences as they offer the opportunity to better tailor training and rehabilitation programs in this sport.