Is arm work important in running?

Absolutely! While running, the arms perform a function of balance and coordination, maintain an upright body position without unnecessary hesitation and twisting, set the rhythm and tempo and influence the economy of the run.
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What about technique:
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👉🏻 Shoulders down, relaxed, slightly pulled back.
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👉🏻 Elbows bent at right angles. A slightly sharp angle is allowed, but you should not open your arms more than 90 degrees.
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👉🏻 Focus on moving the elbow backwards and do not try to bring the elbow much forward beyond the body.
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👉🏻 Your hands should be relaxed. Imagine you are holding a sparrow. If you open your hand, it will fly away, and if you squeeze it into a fist, you will cut off its access to oxygen.
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👉🏻 Each hand works on its side of the body. Do not allow your hands to work in parallel, it will give the impression that you are sawing something.
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👉🏻 Keep upper body compact and solid. Unnecessary hands movements look scary.