The study is a cross-sectional study in 963 independently living people, aged 20-40 years, who were randomly selected from Anhui, Hubei and Hunan province in the middle areas of China. They were initially selected to participate in this study carried out in Anhui National Physical Fitness Monitoring Center in Hefei, China, from February 2017 to May 2017. The age range was chosen for study based on the observation that CMJ test in the participants greater than 40 years of age would be hard to carry out.
Data on medical history and current medication use and psychiatric diagnoses were collected. Individuals with history of fractures in previous 24 months or significantly impaired renal or hepatic function, chronic kidney disease, type 1 diabetes, sustained periods of immobilization, amenorrhea (>6 months), use of hormone replacement therapy (>12 months), cortisone (>6 months) were excluded.
At last, 560 eligible participants (191 men and 369 premenopausal women, aged 20–40 years) remained for further study from May 2017 to December 2018. All participants provided informed consent and the rights of the subjects were protected. The study was approved by appropriate institutional research ethics committee. Likewise, the study methods met the ethical standards of the International Journal of Sports Medicine 
Measurements of CMJ performance
Compared with squat jump (SJ) test, the CMJ test showed higher relationship with the explosive power factor. In our study, CMJ was performed for the measurement of explosive impulse and the reuse of elastic energy during inversion of eccentric to concentric movement. Subjects were asked to stand with both feet on the ZTJ-II (Jian Min Corporation, China) and lower their body towards the ground by moving into flexion position at the trunk and lower extremity while extending upper extremity. Then subjects were asked to jump as high as possible while extending their legs. The jump was repeated 3 times, and the countermovement jump height (CMJH) was displayed on the screen.
The impulse in the CMJ phase (ICMJ) was assessed by equation (4), which was derived by equations (1)-(3). The potential energy in the CMJ phase (ECMJ) was obtained by the equation (5). (See Equations in the Supplementary Files)
, where g = acceleration due to gravity, BW indicates body weight, CMJH indicates countermovement jump height, Vtop indicates the velocity of body on the highest position, and Vup indicates the velocity of body leaving from the ground.
Measurements of body composition
BH was measured to the nearest 0.1 cm by a stadiometer (GM-I, Jian Min Corporation, China). Body composition was measured using multi-frequency body composition analyzer MC-180MA (Tanita Corporation, Tokyo, Japan). Subjects stood on bare feet with the heel and toe of each foot in contact with the metal footpads, with arms hanging to each side, lightly holding the analyzer handgrips. Coefficient of variance (CV) of the impedance measure was 0.4%. The Tanita software (v1.7.0) generated values for BW, FFM, FM and lower limb tissue composition including left lower limb muscle mass (LLLMM), right lower limb muscle mass (RLLMM), left lower limb fat mass (LLLFM) and right lower limb fat mass (LLLFM). Values obtained from bioelectrical impedance analysis (BIA) were supported by skinfold measurement using harpenden calipers. Previous work indicated that BIA method had advantages over anthropometry for measuring lower limb tissue composition in healthy individuals, and there was a high correlation between dual-energy X-ray absorptiometry (DXA) and Tanita MC-180.
Measurements of bone quality
Quantitative ultrasound (QUS) measurements have been found to reflect strength and architecture of bone[16,33][16,33].22, 23 In this study, QUS device (Achilles express; LUNAR, USA) was used to assess SI of the right calcaneus, which is a combination of speed of sound (m/s) (SOS) and broadband ultrasound attenuation(BUA) (dB/MHz) (SI=0.67×BUA+0.28×SOS–420). This measurement was calibrated daily in accordance with the manufacturer’s recommendations and before measurement. To examine the reliability of Achilles devices, short-term precision expressed as CV and standardized CV were calculated in another 25 individuals using duplicate measurement. The CV (%) were 2.2 (1.2–3.1) for BUA, 0.3 (0.2– 0.5) for SOS, and 2.3 (1.5– 4.1) for SI, respectively. The standardized CV (%) were 4.1 (2.7 –9.7) for BUA, 3.4 (1.5– 4.7) for SOS, and 2.9 (0.9 –3.7) for SI, respectively.
Subject characteristics, measurements including age, body composition parameters (BH, BW, FFM, FM, LLLMM, RLLMM, LLLFM, RLLFM） and CMJ performance were expressed as mean±standard deviation (SD). The level of significance was set at p<0.05. Student’s t-test was used for comparison of means and quantitative data. Bivariate correlation analysis and multiple linear regression analysis were used to investigate the relations among the CMJ performance, body composition parameters (BH, BW, FFM, FM, LLMM, and LLFM) and calcaneal bone SI. The statistical analysis was done using SPSS for Windows, Version 22.0 (IBM Corp., Armonk, NY).