研究目的：随着现在生活方式的改变，脑血管疾病发生呈现年轻化趋势，其原因之一是青年人身体活动不足。评价脑血管功能的一种有效方式是考察脑血流动力学以及脑血管储备。本研究聚焦身体活动（PA, Physical Activity），探讨不同的身体活动水平与脑血流动力学和脑血管储备之间的关系，以期为指导大学生增加身体活动改善脑血管功能提供参考价值。 研究方法：招募自愿参加实验的北京师范大学女大学生 265 人，其中排除了有严重呼吸系统疾病以及运动禁忌症的人群。对受试者进行实验室测试，包括：体格检查、生理机 能检查、身体素质检测。使用经颅多普勒超声（TCD，Transcranial Doppler）对受试者进行 左右两侧大脑中动脉血流动力学参数（血流速度、搏动指数、阻力指数等）检测以及通过 屏气实验检测受试者的脑血管储备。脑血流动力学检测和屏气实验均进行 3 次，结果取平均值。采用三轴加速度计 WGT3X-BT，监测受试者一周的身体活动数据。采用 SPSS 统计 学软件对实验数据进行处理分析，在 SPSS 软件的多重线性回归中使用等时替代模型来探讨每天 15min 或 30min 的静坐行为（SB，Sedentary Behavior）、低强度身体活动（LPA， Light Physical Activity）和中高强度身体活动（MVPA，Moderate and Vigorous Physical Activity） 时间相互替代对脑血流动力学和脑血管储备指标的影响。等时替代模型分为单因素模型、分配模型和等时替代模型三个步骤。以脑血流相关指标为因变量，以年龄为协变量，以 SB、LPA 和 MVPA 为自变量（不同模型纳入数量不同），建立三个模型，具体过程如下（以 15min 等时替代为例）：（1）单因素模型（模型 1）：因变量=(b0)某行为+(b1)协变量，协变量皆为年龄，探 究 特 定 行 为 与 因 变 量 的 关 系 ；（ 2 ） 分 配 模 型 （ 模 型 2 ）： 因 变 量 =(b0)SB+(b1)LPA+(b2)MVPA+(b3)协变量，该模型以 SB、LPA 和 MVPA 为自变量，以年龄 为协变量；（3）等时替代模型：以模型 3 为例，因变量=(b0)LPA+(b1)MVPA+(b2)加速度计 穿戴时长+(b3)协变量，该模型以年龄和加速度计穿戴时长为协变量，LPA、MVPA为自变量进入该模型，但 SB 变量本身并不进入该模型，即控制总活动时间（MVPA 时间+LPA 时 间+SB 时间）不变的条件下，b2、b3 表示分别用 15min 的 LPA、15min 的 MVPA 替代 15min 的 SB 对因变量的效应，模型 4 和模型 5 分别表示 LPA、MVPA 被其他两种行为替代后对因变量的效应。 研究结果：测得 238 名女大学生的完整有效数据。（1）结果显示女大学生普遍存在久坐行为，63.87%的受试者身体活动水平低于世界卫生组织的推荐值（每周至少进行 150 分钟的中高强度身体活动）。（2）女大学生的脑血流动力学参数和屏气指数（BHI，Breath Holding Index）均处于经颅多普勒超声诊断标准中的正常范围内。（3）不同身体活动水平的女大学生脑血流动力学 7 个参数（收缩期血流速度、舒张期血流速度、平均血流速度、 搏动指数、阻力指数、电导指数、电阻指数）仅电阻指数（CVRi，Cerebrovascular Resistance Index）差异具有统计学意义，身体活动不足组显著高于身体活动充足组（P＜0.05）。（4）不同身体活动水平的女大学生屏气指数差异具有统计学意义，其中身体活动不足组显著低 于身体活动充足组（P＜0.05）。（5）Pearson 相关分析结果得知，不同身体活动时间与脑血 流动力学各参数均无相关性，屏气指数与低强度身体活动时间具有弱正相关关系，其中 LPA 时间与大脑左侧屏气指数存在极弱的正相关关系（r=0.18，P＜0.01），LPA 时间与大脑右侧屏气指数存在极弱的正相关关系（r=0.168，P＜0.05）。（6）等时替代模型的结果得知，不同身体活动互相替代对脑血流动力学参数没有显著性的影响（P＞0.05）。LPA 与 SB 时间互相替代对屏气指数有显著性的影响，其中对于久坐行为来说，每日用 15 分钟的 LPA 替 代 SB，可以使左侧屏气指数增加 0.043（β=0.043，P＜0.01），使右侧屏气指数增加 0.041（β=0.041，P＜0.05）。每日用 30 分钟的 LPA 替代 SB，可以使左侧屏气指数增加 0.086 （β=0.086，P＜0.01），使右侧屏气指数增加 0.082（β=0.082，P＜0.05）。对于低强度身体活动行为来说，每日用 15 分钟的 SB 替代 LPA，可以使左侧屏气指数减少 0.043（β=-0.043， P＜0.01），使右侧屏气指数减少 0.041（β=-0.041，P＜0.05）。每日用 30 分钟的 SB 替代 LPA，可以使左侧屏气指数减少 0.086（β=-0.086，P＜0.01），使右侧屏气指数减少 0.082（β=- 0.082，P＜0.05）。而 SB 与 MVPA 以及 LPA 与 MVPA 互相替代，对屏气指数均无显著性的影响（P＞0.05）。 研究结论：女大学生普遍存在身体活动水平不足的情况，虽然身体活动不足会升高脑 血管电阻指数和降低脑血管储备，但由于女大学生处于青年时期，所以其脑血流动力学和脑血管储备仍在正常值范围内。通过等时替代模型分析表明，低强度身体活动替代静坐行为会对脑血管储备功能产生积极的作用，因此建议女大学生减少静坐时间，适量增加身体活动，促进脑血管储备。

Objective: With the change of current lifestyle, cerebrovascular diseases appear younger trend, one of the reasons is that young people lack of physical activity. An effective way to evaluate cerebrovascular function is to investigate cerebral hemodynamics and cerebrovascular reactivity. This study focused on physical activity and explored the relationship between different levels of physical activity and cerebral hemodynamics or cerebrovascular reactivity, in order to provide reference value for guiding college students increase physical activity to improve cerebrovascular function. Research methods: A total of 265 female students from Beijing Normal University were recruited to participate in the experiment, excluding those with severe respiratory diseases and exercise contraindications. Laboratory tests were performed on the subjects, including physical examination, physiological examination and physical fitness examination. Transcranial Doppler (TCD) was used to detect the cerebral hemodynamic parameters (Blood Velocity, Pulsatility Index, Resistance Index, etc.) in the left and right middle cerebral arteries of the subjects as well as the cerebrovascular reactivity of the subjects through the breath-holding test. Both cerebral hemodynamic test and breath-holding test were conducted for 3 times, and the results were averaged. The triaxial accelerometer WGT3X-BT was used to monitor the physical activity data of the subjects for one week. SPSS statistical software was used to analyze the experimental data. Using Isotemporal Substitution Model to explore effects of every 15 minutes or 30 minutes Sedentary Behavior (SB) ,Light Physical Activity (LPA) and Moderate and Vigorous Physical Activity (MVPA) substitution on cerebral hemodynamics and cerebrovascular reactivity with SPSS. Isotemporal substitution model is divided into three steps: single factor model, distribution  model and isotemporal substitution model. Three models were established with cerebral blood flow related indicators as dependent variables, age as covariable, SB, LPA and MVPA as independent variables (different models included different amounts). The specific process is as follows (15min isotemporal substitution as an example) : (1) Single factor model (Model 1) : Dependent variable =(b0) a behavior +(b1) covariable, covariables are age, explore the relationship between specific behavior and dependent variable; (2) Distribution model (Model 2) : dependent variable =(b0)SB+(b1)LPA+(b2)MVPA+(b3) covariable. In this model, SB, LPA and MVPA are the independent variables, and age is the covariable. (3) Isotemporal substitution model: Take Model 3 as an example, dependent variable =(b0)LPA+(b1)MVPA+(b2) accelerometer wearing time +(b3) covariable. This model takes age and accelerometer wearing time as covariables, and LPA and MVPA as independent variables, but SB variable itself does not enter this model. That is, under the condition of controlling the total activity time (MVPA time +LPA time +SB time) unchanged, b2 and b3 represent the effect of 15min of SB replaced by 15min of LPA and 15min of MVPA on the dependent variable, while model 4 and model 5 respectively represent the effect of LPA and MVPA replaced by other two behaviors on the dependent variable. Research results: Complete and valid data of 238 female college students were obtained. (1) The results showed that sedentary behavior was common among female college students, and 63.87% of the subjects' physical activity level was lower than the recommended value of the World Health Organization(Get at least 150 minutes of moderate and vigorous physical activity each week). (2) The cerebral hemodynamic parameters and breath-holding index (BHI) of female college students were within the normal range of the diagnostic criteria of transcranial Doppler ultrasound. (3) There was no significant difference in the 7 cerebral hemodynamic parameters (Systolic Velocity, Diastolic Velocity, Mean Velocity, Pulsatility Index, Resistance Index, Cerebrovascular Conductance Index, Cerebrovascular Resistance Index) of female college students at different physical activity levels except the Cerebrovascular Resistance Index (CVRi) (P < 0.05), and the CVRi of insufficient physical activity group was significantly higher than that of sufficient physical activity group. (4) The BHI of female college students with different levels of physical activity had statistical significance, and the BHI of insufficient physical activity group was significantly lower than that of sufficient physical activity group (P < 0.05). (5) Pearson correlation analysis showed that there was no correlation between different physical activity time and cerebral hemodynamic parameters, but there was a weak positive correlation between BHI and light physical activity (LPA) time, in which LPA time and left BHI had a very weak positive correlation (r=0.18, P < 0.01). There was a very weak positive correlation between LPA time and right BHI (r=0.168, P < 0.05). (6) The results of isochronous substitution model showed that the substitutions of different physical activities had no significant effect on cerebral hemodynamic parameters (P > 0.05). The replacement of LPA and sedentary behavior (SB) had a significant effect on BHI. For sedentary behavior, replacing SB with LPA for 15 minutes a day could increase left BHI by 0.043 (β=0.043, P < 0.01) and increased right BHI by 0.041 (β=0.041, P < 0.05). Replacing SB with LPA for 30 minutes daily could increase left BHI by 0.086 (β=0.086, P < 0.01) and right BHI by 0.082 (β=0.082, P < 0.05). For light physical activity behaviors, replacing LPA with SB for 15 minutes daily decreased left BHI by 0.043 (β=-0.043, P < 0.01), and decreased right BHI by 0.041 (β=-0.041, P < 0.05). Replacing LPA with SB for 30 minutes a day could reduce left BHI by 0.086 (β=-0.086, P < 0.01) and right BHI by 0.082 (β=-0.082, P < 0.05). However, there was no significant effect on BHI between SB and MVPA or between LPA and MVPA (P > 0.05). Conclusions: Insufficient physical activity level is common among female college students. Although insufficient physical activity will increase cerebrovascular resistance index and decrease cerebrovascular reactivity, the cerebral hemodynamics and cerebrovascular reactivity of female college students are still within the normal range because they are in their youth. The analysis of isotemporal substitution model shows that light physical activity replacing sedentary behavior will have a positive effect on cerebrovascular reactivity function. Therefore, it is suggested that female college students reduce sedentary time and increase physical activity appropriately to promote cerebrovascular reactivity.