Since all the TMG parameters had a main effect by sex, the results of the two-way ANOVA highlighted the sex-based differences of the TMG parameters in healthy adolescents. This results were similar to the evaluation of muscle function frequently used in clinical practice, such as muscle mass and strength. Based on the multiple comparison test, upon examination of the dominant and non-dominant feet, the DM was the only TMG parameter that was different between males and females for the non-dominant foot, indicating that Dm is the most likely indicator of sex difference in TMG. In addition, as seen in Table 3, the 95% CIs of Dm, Td, and Tc for both dominant and non-dominant feet overlap between males and females. However, the values for Dm and Tc among females are generally lower, which highlights the possibility that in healthy adolescents, females have a smaller amount of vertical displacement during muscle contraction due to electrical stimulation and the slower contraction rate from the onset to the end of the muscle contraction. In addition, Td’s wider 95% CI among females may result to the muscles’ slower responsiveness of to muscle contraction by electrical stimulation. Therefore, when using this TMG parameter as an evaluation battery, it is necessary to consider the sex-based differences when choosing subjects.
Overall, there was a weak correlation (correlation coefficient: 0.27-0.45) among the items, with a significant correlation between the TMG parameters and each parameter. Specifically, there was a correlation between the multiple TMG parameters and lower limb muscle mass. The Dm and lower limb muscle mass of the non-dominant foot in men (correlation: -0.29) and the dominant foot (correlation: -0.45) in women were negatively correlated, indicating that those with high muscle mass in the lower limbs have low Dm and high muscle stiffness. In addition, there was a correlation of 0.42 for the dominant foot Td and 0.33 for the non-dominant foot Td in women. It is possible that in healthy adolescent females and in those with low muscle mass of the lower limbs, the muscle responsiveness to contraction is quicker, whereas those with a large amount of lower limb muscle may require longer times to muscle contraction from electrical stimulation. The evaluation of muscle functions, such as lower limb muscle mass and joint torque, does not include individual muscles but involved multiple muscle functions working in combination. This is thought to reflect the amount of activity of daily life 14). In the fields of sports and medicine, muscle strength, thickness, fatigue rate, and risk of injury, including the muscles’ physiological and pathophysiological, are substantially affected by sex 15, 16). Similarly, in this study, the amount of activity of daily living, including the frequency of exercise and the physiological aspects of muscles, differed between men and women. It is possible that lower limb muscle mass was related to the neuromuscular response of muscles in healthy adolescents. Although many subjects in our study did not exercise on a daily basis, they were generally healthy adolescents.
At present, information on the application of the TMG parameters in healthy adolescents and females is limited, making it difficult to determine their characteristics by comparing with findings of previous studies on TMG for athletes. Since the reason for the different correlation between the dominant and non-dominant feet by sex is unclear, it will be necessary to investigate sex, exercise frequency, and motor function in the future. The results of this study provide basic information for future application of TMG in subjects with such diseases.
In the report on the validity of the TMG parameter, the maximum muscle strength and the muscle fatigue rate are often verified. A high correlation is reported 17,18), but in this study, the maximum muscle strength by the manual muscular strength meter and femoral circumference had little to do with TMG parameters. This may be due to differences in muscle strength measurement methods or differences in subjects who do not play sports on a daily basis. There was a weak correlation between the TMG parameter and lower limb muscle mass, but not with other muscle function assessments. As such, the TMG parameter may be a unique evaluation method for assessing neuromuscular function.