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So, I've gotten into swimming again recently, racking up 1km three times a week. Which is about all I can manage, practically, to get to the pool.

I've started trying to train by carrying out some of the same movements just in air. Just the arms, either in front of me, or directly up in the air.

The trouble is, I've found this extremely fatiguing, after 10 or 15 strokes my arms ache and I'm not able to continue.

Has anyone got a clue why this is more difficult without the resistance of water?

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    I'd assume that while you're swimming, you're using other muscle groups to propel yourself forward, and, your shoulders move in a full range of motion. Standing vertically still and moving your arms in a swim motion changes the workload for your shoulders. – rrirower Feb 4 '17 at 16:26
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This is because water has a higher density than air which creates resistance**. The easiest way to notice this resistance is to run into water at the beach. The deeper you'll get into the water the harder it gets to keep running.

The same happens to your arms when you're in the water. It's easier to keep them up because the water below it causes more resistance than air would.

**Air is lighter because there are fewer molecules per unit volume compared with a unit volume of liquid water. A mole of water is 18 grams, so a liter of water contains about 55 moles (1000 grams).

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  • It seems you've answered the opposite of what the question is actually asking. – Arsak Dec 21 '18 at 11:00
  • I explained why it's easier to swim in water than in air, isn't that the same is answering why it's harder to swim in air than it is in water? My first line literally explains the difference between water and air. Should I refrase my last sentence to "it's harder to keep them up in the air.." instead of saying "it's easier in water..."? – MJB Dec 21 '18 at 14:29
  • Oh, then I misunderstood it. Especially the part "The deeper you'll get into the water the harder it gets to keep running." reads to me as if you're explaining why it was harder to swim in water than in air. – Arsak Dec 21 '18 at 15:44
  • @Marzipanherz I can see how that is confusing now that you mention it. What I meant to do was merely explain the difference in density between water and air, which helps understand why your arms seem to be easier to keep up in water (because of the density below it). – MJB Dec 23 '18 at 9:13
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while you are in water your arms are supported to a degree because they are able to float. making it easier for you to keep them in the swimming position.

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