Abstract
The combined effects of mechanical loading and maturation during adolescence are still not well understood. The purpose of the study was to investigate the development of the quadriceps femoris muscle-tendon unit from early adolescence (EA), late adolescence (LA) to young adulthood (YA), and examine how it is influenced by athletic training in a cross-sectional design. Forty-one male athletes and forty male non-athletes from three different age groups (EA: 12–14 years, n = 29; LA: 16–18 years, n = 27; and YA: 20–35 years, n = 25) participated in the present study. Maximum strength of the knee extensor muscles, architecture of the vastus lateralis (VL) muscle and patellar tendon stiffness were examined using dynamometry, motion capture, electromyography, and ultrasonography. Muscle strength and tendon stiffness significantly increased (p < 0.001) from EA to LA without any further alterations (p > 0.05) from LA to YA. Athletes compared to non-athletes showed significantly greater (p < 0.001) absolute muscle strength (EA: 3.52 ± 0.75 vs. 3.20 ± 0.42 Nm/kg; LA: 4.47 ± 0.61 vs. 3.83 ± 0.56 Nm/kg; and YA: 4.61 ± 0.55 vs. 3.60 ± 0.53), tendon stiffness (EA: 990 ± 317 vs. 814 ± 299 N/mm; LA: 1266 ± 275 vs. 1110 ± 255 N/mm; and YA: 1487 ± 354 vs. 1257 ± 328), and VL thickness (EA: 19.7 ± 3.2 vs. 16.2 ± 3.4 mm; LA: 23.0 ± 4.2 vs. 20.1 ± 3.3 mm; and YA: 25.5 ± 4.2 vs. 23.9 ± 3.9 mm). Athletes were more likely to reach strain magnitudes higher than 9% strain compared to non-athlete controls (EA: 28 vs. 15%; LA: 46 vs. 16%; and YA: 66 vs. 33%) indicating an increased mechanical demand for the tendon. Although the properties of the quadriceps femoris muscle-tendon unit are enhanced by athletic training, their development from early-adolescence to adulthood remain similar in athletes and non-athletes with the major alterations between early and LA. However, both age and athletic training was associated with a higher prevalence of imbalances within the muscle-tendon unit and a resultant increased mechanical demand for the patellar tendon.
Highlights
Human maturation describes the tempo and timing of the progress toward the mature state during growth (Mirwald et al, 2002)
Athletes were taller compared to non-athlete controls and early adolescence (EA) showed significantly smaller height compared to late adolescence (LA) and young adulthood (YA) (p < 0.001), but there were no significant differences between YA and LA (p = 1.0)
EA had smaller femur lengths compared to YA and LA (p = 0.002 and p = 0.028, respectively), but there were no significant differences between YA and LA (p = 1.0)
Summary
Human maturation describes the tempo and timing of the progress toward the mature state during growth (Mirwald et al, 2002). Kanehisa et al (Kanehisa et al, 1995a,b) reported an increase of the muscle anatomical cross-sectional area with age in parallel with muscle strength and, a pronounced development between age 13 and 15 years in untrained boys. There is evidence that the muscle strength in athletes increases most between 12 and 13 years in boys (Degache et al, 2010) and, potentially earlier compared to untrained counterparts. It seems possible that even early adolescent athletes already show indications of loading-related hypertrophy and muscle remodeling and that there is an interaction of maturation and superimposed loading that influences the temporal development of muscle during adolescence features (in terms of an earlier development) compared to untrained individuals
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