When you're walking or running, your legs are doing most of the work, but your arms are involved, too. And how they move depends on your gait.
当你走路或跑步的时候，你的大部分工作都是由你的腿完成的，但是你的手臂也参与其中。 它们如何移动取决于你的步态。
As we walk, our arms usually hang naturally at our sides and are mostly straight. But when we run, our arms typically swing while bent at the elbow.
当我们走路时，我们的手臂通常自然地垂在身体两侧，而且大部分是笔直的。 但是当我们跑步的时候，我们的胳膊通常会在肘部弯曲的同时摆动。
Why is that? Researchers recently investigated how arm position affects energy efficiency, and they found that walking with bent arms was actually less energy efficient than walking with straight arms.
为什么？ 研究人员最近调查了手臂的位置是如何影响能量效率的，他们发现用弯曲的手臂行走实际上比用直臂行走效率更低。
A bent arm has a shorter arc than a straight arm; bent arms therefore require less energy to swing back and forth and should be more efficient for both running and walking, the researchers initially hypothesized.
弯曲的手臂比直的手臂有更短的弧度，因此弯曲的手臂前后摆动所需要的能量更少，对于跑步和走路应该更有效率，研究人员最初假设。
But if bent arms are more energy efficient, why don't walkers naturally bend their arms? To find out, the authors of the new study examined the movements of eight people — four men and four women — on treadmills. As the subjects walked and ran (performing both activities with straight arms and then with bent arms), the scientists used infrared cameras and motion-capture software to record the subjects' movements and construct 3D digital models of their bodies.
但是如果弯曲的手臂更节能，为什么步行者不自然地弯曲他们的手臂呢？ 为了找到答案，这项新研究的作者调查了8个人在跑步机上的动作，其中包括4个男人和4个女人。 当实验对象走路和跑步（先用直臂，然后用弯曲的手臂进行两种活动）时，科学家们使用红外摄像机和动作捕捉软件来记录实验对象的动作，并建立他们身体的3D 数字模型。
Two weeks later, the subjects repeated these treadmill sessions while wearing breathing masks, so the researchers could collect metabolic datarepresenting the participants' energy use.
两周后，研究对象戴着呼吸面罩重复这些跑步机训练，这样研究人员就可以收集代谢数据，显示参与者的能量消耗情况。
When the subjects ran with straight arms, they reported that it felt awkward. But there was no notable difference in energy efficiency, whether their arms were bent or straight, the researchers reported.
当受试者伸直双臂跑步时，他们报告说感觉很尴尬。 但是研究人员报告说，无论他们的手臂是弯曲的还是直的，在能源效率方面没有明显的差别。
However, the scientists found that when their subjects walked with bent arms, their energy expenditure increased by about 11%, likely because it required more effort to keep their arms bent while moving at a relatively slow speed. Their experiments shed light on why people naturally hold their arms straight when they walk, "but the reason for stereotyped bent arm running remains unclear," according to the study.
然而，科学家们发现，当他们的实验对象用弯曲的手臂走路时，他们的能量消耗增加了11% ，这可能是因为当他们以相对缓慢的速度移动时，需要更多的努力来保持手臂弯曲。 他们的实验揭示了为什么人们走路的时候会自然地保持手臂笔直，研究表明: “但是为什么人们总是固定地弯曲手臂跑步，原因还不清楚。”。
According to a 2014 study, arm swinging costs energy while running, but holding them steady takes even more energy. That's because arm swinging reduces the motion of the torso, that study, published in the Journal of Experimental Biology, found.
根据2014年的一项研究，跑步时摆动手臂消耗能量，但保持手臂稳定需要更多的能量。 这项发表在《实验生物学杂志》上的研究发现，手臂摆动会减少躯干的运动。
The relationship between arm movements and gaits could help explain how arm proportions evolved in the human family tree, the researchers of the new study added.
这项新研究的研究人员补充说，手臂运动和步态之间的关系可能有助于解释人类族谱中手臂比例的进化过程。
Our extinct relatives Australopithecus and Homo habilis, which lived millions of years ago, had arms that were longer relative to their legs than they are in modern humans. Australopithecus and Homo habilis forearms were also longer relative to their upper arms, according to the study.
我们已经灭绝的祖先南方古猿和能人生活在数百万年前，他们的手臂相对于腿的长度比现代人要长。 根据这项研究，南方古猿和能人的前臂相对于他们的上臂也更长。
But shorter forearms — and a shorter arm overall — swing less. Shorter arms would therefore benefitted modern humans during long-distance running; selection for this trait could have shaped the evolution of human arm bone length, the scientists wrote.
但前臂较短，整体上手臂较短，摆动幅度较小。 科学家们归纳，在20世纪90年代长跑，较短的手臂将有利于现代人类； 这种特征的选择可能已经形成了人类手臂长度的进化。
"Modern arm proportions emerged in Homo erectus, and coincided with the evolution of endurance running as an important hominin behavior," the researchers reported.
研究人员报告说: “现代人的手臂比例出现在直立人身上，与耐力跑作为一种重要的古人类行为的进化相吻合。”
The findings were published online July 9, 2019, in the Journal of Experimental Biology.
研究结果发表在2019年7月9日的《实验生物学杂志》的网站上。