Associations between physical fitness components with muscle ultrasound parameters in prepuberal children.

BACKGROUND/OBJECTIVES: Muscle ultrasound is a convenient technique to visualize normal and pathological muscle tissue as it is non-invasive and real-time. This technique is related to several physical performance parameters and body composition components in adults; however, this relationship remains unexplored in early aged. We aimed to evaluate the association between physical fitness components with muscle ultrasound parameters in prepuberal children. METHODS: A sample of 282 prepuberal children aged 5-9 years (144 boys) participated in the study. A trained sonographer obtained six B-mode images from femoral rectus for muscle thickness, subcutaneous adipose tissue (SAT) and area of the muscle of interest, were captured, and muscle ultrasound parameters (echo-intensity: EI uncorrected, EI correct equations) and intramuscular adipose tissue (IMAT) were extracted. Lean muscle tissue has low EI, whereas intramuscular fat and connective tissue have high EI. Physical fitness components (cardiorespiratory fitness, upper and lower muscle strength, speed-agility, and overall fitness levels) were also evaluated. Children were categorized as fit or unfit for each specific fitness test. RESULTS: After adjustment for sex and age, higher physical fitness components and overall fitness (z-score) levels were negatively associated with EI, IMAT, and SAT (cardiorespiratory fitness ß range = -0.264 to -0.298; upper-muscular strength ß range = -0.389 to -0.457; and lower-muscular strength ß range = -0.202 to -0.279; and speed-agility ß range = -0.257 to -0.302). Children categorized as fit according to four physical fitness components had lower EI uncorrected, EI correct equation 1-2, IMAT, and SAT than unfit children for each respective tests (all Ps < 0.001). CONCLUSION: Physical fitness components are inversely associated with EI, IMAT, SAT after adjusting for potential confounders, including sex and age, in prepuberal children. The present study strengthens the idea that muscle and adiposity parameters is affected by physical fitness even from early childhood.