Characteristic of study sample.

Objective
The aim of this study was to investigate the association between sleep duration and muscle quality index (MQI) in middle-aged and older age groups, as limited evidence exists on this topic.
Methods
In order to assess the relationship between sleep duration and MQI, a cross-sectional study was undertaken, utilizing data from the National Health and Nutrition Examination Survey (NHANES) acquired during the period from 2011 to 2014. The study comprised a total of 4598 participants aged 20 years and above. To examine the association between sleep duration and MQI, sophisticated weighted multivariate linear regression models were employed. Additionally, smooth curve fitting techniques were applied to examine the possibility of any non-linear relationship between the two variables.
Results
The average age of the adults who were enrolled in the study was 38.48±11.69 years, and 46.75% of them were female. The results of the multivariable linear regression models showed that sleep duration had a positive correlation with MQI. However, when subgroup analysis was conducted, it was found that this positive correlation only existed among women (β = 0.09, 95% CI: 0.014 to 0.167). To further confirm the differences between sexes in the relationship between sleep duration and MQI, a weighted generalized additive model (GAM) was used.
Conclusions
This research study provides evidence that there is a positive correlation between the duration of sleep and MQI specifically in females, while no such association was observed in males. These findings shed light on the existence of gender disparities in the connection between sleep duration and MQI.

研究目的
本研究旨在探讨中老年人群睡眠时长与肌肉质量指数（muscle quality index, MQI）之间的关联，因当前该主题相关研究证据较为匮乏。

研究方法
为评估睡眠时长与肌肉质量指数的关联，本研究采用横断面研究设计，使用2011至2014年美国国家健康与营养调查（National Health and Nutrition Examination Survey, NHANES）的数据。本研究共纳入4598名20岁及以上的参与者。为分析二者间的关联，本研究采用了经过严格加权处理的多元线性回归模型；此外，还通过平滑曲线拟合技术探究二者是否存在非线性关联。

研究结果
本研究纳入的成年参与者平均年龄为38.48±11.69岁，其中女性占比46.75%。多元线性回归模型结果显示，睡眠时长与肌肉质量指数呈正相关。但亚组分析结果表明，该正相关仅存在于女性群体中（β=0.09，95%置信区间：0.014~0.167）。为进一步验证睡眠时长与肌肉质量指数间关联的性别差异，本研究采用了加权广义加性模型（weighted generalized additive model, GAM）开展分析。

研究结论
本研究证实，睡眠时长与肌肉质量指数的正相关仅存在于女性群体中，未在男性群体中观察到此类关联。上述研究结果揭示了睡眠时长与肌肉质量指数间的关联存在性别差异。