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I was watching the speed skating event, and was wondering, why do the athletes use their right hand while accelerating? If they want to increase the moment of inertia, then they should move the hand away from themselves, but they don't do that.

Is it due to the shifting of the Centre of Gravity to the right (leading to an increased torque and more chances of toppling over) that they don't move their left hand at all?

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    $\begingroup$ Do they go round the rink clockwise? Or counter clockwise? $\endgroup$ – innisfree Feb 18 '14 at 16:19
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    $\begingroup$ @innisfree Counterclockwise $\endgroup$ – Michiel Feb 18 '14 at 16:53
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    $\begingroup$ Are you describing this commons.m.wikimedia.org/wiki/… $\endgroup$ – innisfree Feb 18 '14 at 20:11
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    $\begingroup$ yes, precisely. $\endgroup$ – Saurabh Raje Feb 19 '14 at 16:45
  • $\begingroup$ Not really sure what you are asking here. Speed skaters use both hands when accelerating in a straight line. At sustained cruising speed they may retract one (the left) or both hands behind their backs. In corners the right arm will swing and the left either stays on the back or makes "short" swings (a bit like: from the elbow). And in short-track, during cornering, they might/do extend the left arm towards the center of the ring-even touching the ice-, but not swing it. So, which part of which event are you referring to? $\endgroup$ – Keep these mind Feb 25 '14 at 18:40
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If I remember correctly they only do this in the turns and they use both arms in the straights. It is the outer arm that is active. This helps them turn in two ways. It helps accelerate the outer side more than the inner which is what is what turning really is. The reaction force at the shoulder also helps them lean into the turn which helps them stay stable through the acceleration of turning. This the same reason why bicycles and motorcycles lean into turns and why race tracks for some sports are banked.

The mechanics of swinging arms in skating are similar to those in running and even walking. The main benefit is during the back/down part of the stroke. Since the arm is moving back, down & also out, there is an opposing forward, up and inward reaction force on the torso at the shoulder. The forward component helps with acceleration, upwards helps extend the stride and the inward helps with balance. Doing this only on one side helps turn to the opposite side as described above.

There are (at least) two reasons why the return part of the arm stroke does not completely undo the benefit of the first part. One reason has to with timing. The active part of the stroke occurs while the foot on that side of the body is off the ground and the other foot is planted so it is helping propel the side of the body that is up moving and there is a longer lever from the shoulder to the fulcrum at the planted foot on the far side. During the return stroke that side of the body is on the ground so it does not slide back due to returning forces. Also the leverage from the shoulder to the planted foot is shorter. When the opposite foot is off the ground the return stroke actually helps move the opposite site of the body forward. A simple experiment to demonstrate these effects is to swing just one energetically arm back and forth while standing on one foot and then on the other. Observe the different ways your body responds to swinging the same arm in while standing each foot.

The other possible reason the return strong does not undo the benefit of the initial stroke is that the arm can be in a more relaxed position during the return stroke so it has smaller moment of inertia and thus the reverse effect is lesser. Also return stroke can be less aggressive. This is similar to the way falling cats turn themselves in mid air without violating conservation of angular momentum. I am not certain how big a role this mechanism plays in skating or running though.

EDIT

My explanation is only valid in cases where the skaters move as I described. According to @Pulsar below, this may not be universally true. However, arms do go out of phase with legs

out of sync

and both arm are used

both arms

at least some of the time

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  • $\begingroup$ "....and they use both arms in the straights." Why is that so? What actually provokes them to do that? $\endgroup$ – Saurabh Raje Feb 28 '14 at 16:45
  • $\begingroup$ the return stroke so it ha... ? :( $\endgroup$ – Emilio Pisanty Feb 28 '14 at 18:37
  • $\begingroup$ "the active part of the stroke occurs while the foot on that side of the body is off the ground" No, it's the opposite. If you watch a skater making a turn, you'll see that their right arm moves in sync with their left leg. Also, they usually don't use their arms in the straights. $\endgroup$ – Pulsar Feb 28 '14 at 18:52
  • $\begingroup$ @EmilioPisanto, I lost my connection before. $\endgroup$ – Daniel Mahler Feb 28 '14 at 20:55
  • $\begingroup$ @Pulsar Interesting I will have to look into that. I think this may vary across different skating disciplines. I am fairly sure I have seen skaters using both arms in some situations, but I have also seen the fold both arms behind their backs for longer distances. Also the techniques may have changed, I have not actively watched speed skating since sometime in the last century :) $\endgroup$ – Daniel Mahler Feb 28 '14 at 20:55
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This doesn't exactly answer the question, but it does suggest that the key explanatory link is the step frequency.


I would like to propose the following "theory".

The use of one or both arms allows for an increase in "step" frequency. (Why? Don't know. Perhaps for the same reason that speedwalkers flap their arms. Biomechanics. The Dutch Wikipedia refers to a slight increase in "tempo" due to the use of arms.)

Immediately after the start you'll see skaters use both arms, and this is also when we see - I think - the highest step frequency, apparently required for acceleration.

During cornering fast skaters need to increase their step frequency in order not to miss the turn. This would explain the use of at least one arm in cornering. (Some will use both arms.)

Just before the finish you might see skaters trying to accelerate for a final push, also using both arms.

During the straights, skaters will use one or both arms when they feel they are going too slow and need to keep accelerating. Especially using both arms is usually commented on as being a sign of tiredness. But I think the more direct cause is that the skater knows that his/her speed exiting the corner wasn't sufficient and he/she needs to speed up.

So why do many of the top-skaters not use their arms on the straights during longer events? My "theory" might explain this as follows: it would increase their frequency beyond sustainability. Above cruising speed, too much work will go into maintaining the frequency itself as opposed to being used for forward acceleration. A lower frequency (with longer, more powerful strides) is preferred. Hence: arms on the back.

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  • $\begingroup$ I get that u use arms to accelerate, but what exactly makes you accelerate when you use your arms? Do you agree with the previous answer? $\endgroup$ – Saurabh Raje Mar 1 '14 at 2:45
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    $\begingroup$ @SaurabhRaje The other answer doesn't explicitly refer to frequency. So, I merely suggest, it may miss a key point. $\endgroup$ – Keep these mind Mar 1 '14 at 2:52
  • $\begingroup$ To increase frequency you must make the individual movementss either faster, shorter or both. I do not expect increasing frequency just by making the moves shorter will increase overall speed. You can make movements faster by applying greater force. So a mechanism that amplifies forces will likely also increase frequency. $\endgroup$ – Daniel Mahler Mar 1 '14 at 6:42

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