Abstract
Knowledge regarding the effects of athletic training on the properties of muscle and tendon in preadolescent children is scarce. The current study compared Achilles tendon stiffness, plantar flexor muscle strength and vertical jumping performance of preadolescent athletes and non-athletes to provide insight into the potential effects of systematic athletic training. Twenty-one preadolescent artistic gymnastic athletes (9.2 ± 1.6 years, 15 girls) and 11 similar-aged non-athlete controls (9.0 ± 1.7 years, 6 girls) participated in the study. The training intensity and volume of the athletes was documented for the last 6 months before the measurements. Subsequently, vertical ground reaction forces were measured with a force plate to assess jumping performance during squat (SJ) and countermovement jumps (CMJ) in both groups. Muscle strength of the plantar flexor muscles and Achilles tendon stiffness were examined using ultrasound, electromyography, and dynamometry. The athletes trained 6 days per week with a total of 20 h of training per week. Athletes generated significantly greater plantar flexion moments normalized to body mass compared to non-athletes (1.75 ± 0.32 Nm/kg vs. 1.31 ± 0.33 Nm/kg; p = 0.001) and achieved a significantly greater jump height in both types of jumps (21.2 ± 3.62 cm vs. 14.9 ± 2.32 cm; p < 0.001 in SJ and 23.4 ± 4.1 cm vs. 16.4 ± 4.1 cm; p < 0.001 in CMJ). Achilles tendon stiffness did not show any statistically significant differences (p = 0.413) between athletes (116.3 ± 32.5 N/mm) and non-athletes (106.4 ± 32.8 N/mm). Athletes were more likely to reach strain magnitudes close to or higher than 8.5% strain compared to non-athletes (frequency: 24% vs. 9%) indicating an increased mechanical demand for the tendon. Although normalized muscle strength and jumping performance were greater in athletes, gymnastic-specific training in preadolescence did not cause a significant adaptation of Achilles tendon stiffness. The potential contribution of the high mechanical demand for the tendon to the increasing risk of tendon overuse call for the implementation of specific exercises in the athletic training of preadolescent athletes that increase tendon stiffness and support a balanced adaptation within the muscle-tendon unit.
Highlights
Imbalances within muscle-tendon units affect the tendon safety factor [i.e., ratio of tendon ultimate strain to functional operating tendon strain (Ker et al, 1988)], because the ultimate strain of tendons cannot be significantly changed (LaCroix et al, 2013)
We found significantly greater jump height in athletes compared to nonathletes in both jumps, as well as greater mean mechanical power during the propulsion phase (f = 0.6, p = 0.004 for the squat jumps (SJ) and countermovement jumps (CMJ); Figure 1B)
The present study demonstrated that artistic gymnastic training during preadolescence is associated with increased plantar flexor strength normalized to body mass and jumping performance, without significant effects on Achilles tendon stiffness and absolute muscle strength compared to non-athletes
Summary
Imbalances within muscle-tendon units affect the tendon safety factor [i.e., ratio of tendon ultimate strain to functional operating tendon strain (Ker et al, 1988)], because the ultimate strain of tendons cannot be significantly changed (LaCroix et al, 2013). In individuals with higher muscle strength, the margin of tolerated mechanical loading is commonly increased by means of greater stiffness of their tendons (Arampatzis et al, 2007b). The types of loading that trigger adaptation and the time course of adaptive changes are different between muscles and tendons, and periods of imbalances in muscle and tendon properties can occur during training (Mersmann et al, 2016). It has been shown that an imbalance between muscle strength and tendon stiffness results in a higher mechanical demand for the tendon compared to non-athlete controls (Mersmann et al, 2017a)
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