躺下比站立更不敏感于疼痛？新研究观察姿势与痛觉

一项关于姿势与痛觉的实验

这项研究发表在《心理生理学》（Psychophysiology），由西班牙哈恩大学的Gustavo A. Reyes del Paso等人完成，参与者是83名健康大学生，其中61名女性、22名男性，平均年龄约20岁。研究者让他们依次经历坐、站、平躺、再站四个阶段，用电脑控制的压痛仪在左手第三指指甲上施加逐渐增大的压力，记录两个指标：刚开始感到疼痛的“痛阈”，以及能忍受的最大压力“痛耐受度”。参与者还要对几档固定压力打分，报告主观疼痛强度。

结果与假设方向一致：平躺时的痛阈高于站立，痛耐受度也高于站立和坐姿。不过差异幅度不大，平均痛阈在各姿势间只在每平方厘米3.4到3.6公斤之间浮动，作者自己也指出这种小幅差异的临床意义尚不确定。主观评分上，在每平方厘米4公斤这一最高压力下，站立时报告的疼痛高于坐和躺；在3和3.5公斤时，站立也高于坐。低于3.4公斤的轻压力出现了与假设相反的结果——躺下时评分略高于坐姿，作者认为这些压力多数人可能根本没觉得痛。

为什么血压会影响痛觉

文章用压力感受器（baroreceptors）解释这条通路。颈动脉、主动脉、心腔和肺部的血管壁里分布着这类感受牵张的微型传感器。血容量增加或血压升高时，血管壁扩张，压力感受器被拉伸，向大脑发送信号，一方面减慢心率、放松血管，另一方面触发中枢神经系统的镇痛通路。因此血压较高的人往往对疼痛不那么敏感。文章引用了此前的一些证据：1994年的人体实验用轻柔负压刺激颈部压力感受器，降低了疼痛反射和主观疼痛；2009年的临床研究显示，给慢性低血压者用药升高血压后，其疼痛敏感性下降。

姿势之所以被纳入，是因为重力会改变血液分布。站立时血液向下肢聚集，回心血量减少，血压和压力感受器牵张暂时降低；平躺时血液更容易回到胸腔，每搏输出量增加，压力感受器被拉伸。研究中确实观察到这些变化：从坐到站，收缩压从平均109.36降到104.97毫米汞柱，每搏输出量从73.39降到63.61毫升；平躺时收缩压升到111.33毫米汞柱，每搏输出量升到89.47毫升，压力反射敏感性从站立的11.52升至30.35。

血压与疼痛的关联，以及男女差异

研究者还比较了坐姿下的基础心血管指标与疼痛反应。平均动脉压与疼痛强度评分呈反向关联：静息血压较高的人倾向于把压力刺激评得不那么痛。这一模式在随机顺序呈现刺激时出现在所有压力水平，在逐级递增时出现在六个水平中的三个。总外周阻力（血管收缩程度）与痛阈和痛耐受度都呈正相关。而随姿势变化最大的几个指标——每搏输出量、心输出量、心率和压力反射敏感性——反而与任何疼痛指标都没有关联。

性别差异也被记录：男性整体血压更高、血管阻力更大，痛阈和痛耐受度也高于女性，但男女在主观疼痛评分上相近。在男性中，血压与痛阈的关联直接且显著，相关系数在0.43到0.47之间；这一关联在女性中未达到统计显著，不过男性子样本只有22人。

作者提出的慢性疼痛假设，以及研究的边界

Reyes del Paso在采访中提出一个延伸想法：慢性疼痛患者常久坐少动，其中一些人存在运动恐惧（kinesiophobia），即因担心疼痛受伤而过度害怕活动。躺卧带来的镇痛效果可能通过负强化维持这种不活动，长期反而加重疼痛问题。他强调这是解释性的假设，研究本身没有检验慢性疼痛人群的躺卧行为是否真的被镇痛效果强化。

解读结果时还有几处限制需要注意。样本全部是年轻健康的大学生，静息血压偏低到正常，而血压的镇痛效应通常在血压偏高或高正常人群中更强，因此这里观察到的效应幅度未必能外推到老年人、高血压患者或慢性疼痛患者。血压与疼痛的关联只在坐姿下测量，且是人与人之间的比较，研究没有检验同一个人在不同姿势下血压变化能否解释其自身的疼痛变化。姿势顺序也没有随机化，所有人按同一顺序经历各阶段；第二次站立紧接平躺之后，血压未像第一次站立那样下降（平均110.30毫米汞柱），作者认为从平躺起身时血液向腿部转移更快更大，可能触发了快速反射，从而削弱了姿势对疼痛的影响。研究团队表示，下一步将在纤维肌痛或慢性背痛等临床人群中重复这项研究。

A new study published in Psychophysiology indicates that people are somewhat less sensitive to pain while lying down than while standing upright. The researchers propose that this reflects shifts in blood volume and blood pressure as the body adjusts to gravity, which may help explain why lying down eases acute discomfort. They also suggest the same relief could encourage inactivity in people with chronic pain, an idea the study did not test directly.

The human cardiovascular system does more than circulate oxygen and nutrients; it also helps regulate how the nervous system processes physical discomfort. Embedded within the walls of major blood vessels, such as the carotid arteries in the neck and the aorta near the heart, as well as inside the heart chambers and lungs, are microscopic stretch sensors called baroreceptors. When blood volume increases or blood pressure rises, the walls of these vessels expand, stretching the baroreceptors. Once stretched, these sensors send neural signals to the brain that slow the heart and relax blood vessels while simultaneously triggering pain-inhibiting pathways in the central nervous system.

Because of this built-in dampening mechanism, higher blood pressure tends to be accompanied by reduced sensitivity to physical pain. Early evidence from a 1994 human experiment demonstrated that artificially stimulating neck baroreceptors with gentle suction dulled pain reflexes and lowered how much pain participants reported feeling. Building on that work, a 2009 clinical study showed that giving medication to raise blood pressure in people with chronically low blood pressure reduced their pain sensitivity.

This link between the circulatory system and pain processing even extends beyond physical sensation. For instance, a study covered by PsyPost in 2019 found that higher resting blood pressure was associated with lower self-reported sensitivity to social pain and rejection. Closer to physical pain, a 2021 study of fibromyalgia patients by Reyes del Paso and colleagues found that individuals whose baroreceptors were less responsive to blood pressure changes experienced worse daily pain, fatigue, and anxiety.

Despite these established findings in psychophysiology, lead study author Gustavo A. Reyes del Paso, a full professor of psychophysiology at the University of Jaén in Spain, noted that “the general public is unaware that an increase in blood pressure has an inhibitory effect on pain: people with hypertension report fewer pain complaints, whereas those with chronic low blood pressure are more sensitive to pain.”

Recognizing that blood flow alters pain sensitivity, Reyes del Paso and his colleagues in Spain and Austria wanted to test whether everyday changes in body posture naturally move enough blood to change how much pain people feel. When a person stands up, gravity pulls blood down into the legs, reducing blood return to the heart and temporarily lowering blood pressure and baroreceptor stretching. When a person lies flat on their back, blood flows easily back to the chest, increasing the volume of blood pumped with each beat and stretching the baroreceptors, which the researchers hypothesized would dampen pain sensitivity.

“Changes in body posture alter blood pressure (it decreases during standing up and increases during lying down), so they may also affect pain,” Reyes del Paso told PsyPost. “We wanted to probe this hypothesis. Also, patients with chronic pain usually are very sedentary (i.e., have kinesiophobia) and are lying down much of the time. Is there a cause for that?”

In clinical settings, kinesiophobia refers to an excessive, debilitating fear of physical movement and activity resulting from a feeling of vulnerability to painful injury. To test how posture affects pain and circulation, the researchers recruited 83 healthy university students, consisting of 61 women and 22 men with an average age of about 20 years. None of the participants had cardiovascular conditions or chronic pain, and all abstained from pain medications for 24 hours and alcohol or vigorous exercise for three hours beforehand. Women taking oral contraceptives were excluded because synthetic hormones alter both blood pressure regulation and pain sensitivity, which could obscure the natural effects of posture.

During the experiment, the researchers measured cardiovascular activity and pain perception while participants assumed three postures: sitting, standing upright, and lying flat on their backs. The sequence included an initial sitting phase, a first standing phase, a lying down phase, and a second standing phase, with cardiovascular measurements recorded across four-minute intervals in each position. Pain testing was carried out during sitting, lying down, and the second standing phase, and the study protocol was publicly registered before data collection began.

To measure pain in a controlled way, the team used a computer-guided pressure algometer, a device that applies steadily increasing mechanical pressure to a one-square-centimeter area on the third fingernail of the left hand. The researchers recorded two behavioral thresholds: the pain threshold, defined as the pressure at which the participant first began to feel pain, and pain tolerance, defined as the maximum pressure the participant could endure. Participants also received five-second pressure stimuli ranging from 1.5 to 4 kilograms per square centimeter and rated how painful each press felt on a 10-centimeter visual scale from painless to extremely painful.

Throughout these postures, participants wore chest electrodes to record an electrocardiogram, which tracks the timing between heartbeats, and an impedance cardiogram, a non-invasive method that uses mild electrical currents across the chest to estimate blood flow volume. Continuous blood pressure sensors on the fingers of the right hand were calibrated every minute with an upper-arm cuff.

From these recordings, the team tracked blood pressure, heartbeat intervals, stroke volume (the blood pumped per beat), cardiac output (the blood pumped per minute), total peripheral resistance (how tightly constricted the blood vessels are), and baroreflex sensitivity, which measures how effectively the heart adjusts its beat timing in response to blood pressure shifts.

The results supported the hypothesis that lying down dulls pain sensitivity compared to standing upright. Participants exhibited higher pain thresholds while lying down than when standing, and their maximum pain tolerance was higher while lying down than during both standing and sitting. Across the sample, average pain thresholds varied only modestly between 3.4 and 3.6 kilograms per square centimeter across postures, and the authors noted that the clinical relevance of these small differences remains uncertain.

Subjective pain intensity ratings followed a similar pattern once the applied pressure crossed that threshold into actual pain. At the highest pressure of 4 kilograms per square centimeter, participants rated the sensation as more painful while standing than while either sitting or lying down. At 3 and 3.5 kilograms per square centimeter, standing also produced higher pain ratings than sitting. Contrary to the researchers’ hypothesis, for lighter pressures between 1.5 and 2.5 kilograms per square centimeter, ratings were slightly higher while lying down than sitting, though the authors noted that pressures below 3.4 kilograms may not have been perceived as painful by most participants.

The cardiovascular recordings confirmed that shifting postures produced substantial changes in blood flow and pressure regulation. When participants moved from sitting to the first standing period, their systolic blood pressure dropped from an average of 109.36 to 104.97 millimeters of mercury, and their stroke volume fell from 73.39 to 63.61 milliliters per beat. When they lay flat on their backs, systolic blood pressure rose to 111.33 millimeters of mercury, stroke volume climbed to 89.47 milliliters, and baroreflex sensitivity nearly tripled from 11.52 while standing to 30.35 milliseconds per millimeter of mercury while lying down, which the authors interpret as stronger heart-slowing nervous system activity.

When the researchers analyzed how baseline cardiovascular traits while sitting predicted pain responses, they found that mean arterial blood pressure was inversely linked to pain intensity ratings. Participants with higher resting blood pressure tended to rate the presses as less painful, a pattern that appeared at every pressure level when stimuli came in random order and at three of six levels when they increased step by step.

Total peripheral resistance, or the tightness of the blood vessels, was positively associated with both pain threshold and pain tolerance across the entire group, suggesting that blood vessel constriction may play a role in dampening pain. Stroke volume, cardiac output, heart rate, and baroreflex sensitivity, the measures that shifted most with posture, were not linked to any of the pain measures.

The researchers also observed distinct differences between men and women in both pain and circulatory measurements. Men had higher overall blood pressure, greater blood vessel resistance, and higher pain thresholds and pain tolerance than women, although men and women gave similar subjective pain intensity ratings on the visual scale. In addition, higher blood pressure was directly associated with a higher pain threshold in men, with correlation coefficients ranging from 0.43 to 0.47, whereas this specific association was not statistically significant in women, though the male subsample included only 22 participants.

The authors noted that lying down appears to be an intuitive, functional behavior for easing acute physical pain, which they attribute to greater blood return to the heart and stronger stimulation of pain-blocking baroreceptors. At the same time, they pointed out that this biological mechanism could have unintended drawbacks for people with chronic pain conditions.

“The reduction in pain during lying down could be a mechanism explaining the typical sedentarism and low physical activity in chronic pain patients, in which bed rest is considered counterproductive for pain rehabilitation, as this inhibitory effect can negatively reinforce inactivity by reducing pain,” Reyes del Paso explained.

“If you feel an acute pain, the maintenance of a lying down posture will help alleviate the pain,” he added. “However, if a person suffers from chronic pain, he/she should avoid the frequent use of a supine position, because it reinforces inactivity and in the medium and long term can aggravate the pain problem.”

As with all research, there are some caveats to consider when interpreting the results. The study sample consisted entirely of young, healthy university students who generally had low-to-normal resting blood pressure, and nearly three-quarters of the participants were women. Because the pain-dampening effects of blood pressure are typically strongest in people with elevated or high-normal blood pressure, the modest effects observed here might differ in older adults, people with hypertension, or patients living with chronic pain syndromes. The idea that pain relief while lying down reinforces inactivity in chronic pain is the authors’ hypothesis, and the study did not test it.

In addition, the links between blood pressure and pain were measured only while participants were seated, by comparing people with one another. The study did not test whether each person’s blood pressure changes across postures explained their own changes in pain.

Another detail to keep in mind is that the researchers did not randomize the order of the postures, meaning every participant went through the sitting, lying, and standing phases in the exact same sequence. When participants stood up for the second time, directly after lying down, which is when standing pain was measured, their blood pressure did not drop as it had during the first standing phase, averaging 110.30 millimeters of mercury. The authors explained that standing up from a flat position causes a bigger, faster shift of blood toward the legs than standing up from a chair, which likely triggered a rapid reflex that kept blood pressure from dropping. They suggest this may have weakened the posture effect on pain.

Looking ahead, the research team plans to directly investigate how these postural mechanisms operate in clinical populations. “We are going to replicate this study in individuals with chronic pain disorders, such as fibromyalgia or chronic back pain,” Reyes del Paso said.

The study, “Body Posture-Related Differences in Pain Perception and Their Mediation by Cardiovascular Factors,” was authored by Gustavo A. Reyes del Paso, Dolores Santiago, Belén Moreno, Casandra I. Montoro, Stefan Duschek, and Dmitry M. Davydov.