The sensitive period of sports quality development refers to the time during which children and adolescents experience accelerated growth in specific physical abilities, based on their natural development. This phase is considered the optimal time for developing athletic potential. Identifying and utilizing this sensitive period is crucial for selecting and training athletes. Physical training should take full advantage of this window to enhance the specific qualities needed for high-level performance, laying a solid foundation for achieving outstanding results.
**Sensitive Period of Strength Development**
Strength is defined as the ability of the neuromuscular system to overcome or resist resistance during muscle activity. The resistance can be external, such as weight, friction, or air resistance, or internal, including muscle viscosity and resistance between muscles, joints, and connective tissues like ligaments and fascia.
The sensitive period for strength development in children typically occurs between **11 to 15 years old** for girls and **12 to 16 years old** for boys. This is supported by several factors. First, from around age 12, there is a sharp increase in muscle mass. Girls see a significant rise in muscle mass between **11 to 13 years**, while boys experience it between **12 to 14 years**. By **14 to 15 years**, the difference in muscle characteristics between children and adults becomes much smaller. The percentage of body weight accounted for by muscle also increases significantly: at birth, it's about **16.6%**, rising to **21% at 3 years**, **21.7% at 6 years**, **27% at 8 years**, **29% at 12 years**, and approaching **33.3% at 15 years**—close to adult levels of **43.5% for men** and **35% for women**.
Another key factor is the "longitudinal splitting theory" of muscle fibers. Research shows that adult muscle cells lose the ability to synthesize DNA and divide, but young individuals retain this capacity. Strength training during childhood can increase the number of muscle units and capillaries, leading to a higher density of muscle fibers through longitudinal splitting (Vorobiev, *Modern Training Ideas*, 1978).
**(1) Absolute Strength**
Girls' absolute strength develops rapidly until **13 years old**, with an increase of up to **46%**, especially in flexor muscles. From **13 to 15 years**, the growth rate slows to **8%**, then rises again slightly to **14%** between **15 to 16 years**, and finally increases by only **6%** between **16 to 21 years**.
For boys, absolute strength increases sharply during sexual maturity (**12–16 years**), with an average increase of **57.5%**. The growth rate decreases to **36.6%** between **17–20 years**, and only **9.6%** between **21–22 years**. By **25 years**, adults reach their peak strength (Devolk et al., *Changes in Muscle Strength and Speed Strength of Different Ages*, 1983).
**(2) Relative Strength**
Relative strength, or strength relative to body weight, develops more slowly. Between **12–14 years**, it increases by only **2–3% per year**, mainly due to faster weight gain and increased muscle cross-sectional area during periods of rapid height growth.
**(3) Power Speed**
Speed power develops rapidly between **7–14 years**. After **14 years**, boys continue to develop faster, while girls show a slower growth rate, especially after **16–17 years**. This is linked to hormonal changes during puberty (Yan, 1990).
**(4) Strength Endurance**
Girls’ strength endurance follows a straight-line trend before **13 years**, then begins to decline after **14 years**, largely due to the impact of sexual maturity. In contrast, boys’ strength endurance continues to improve steadily from **7 to 17 years**.
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