How does moving your legs forward propel you in swimming?

[CuriousBanker]

Ok, so I know why your arms move you forward in swimming. You are pulling the water back which pushes you forward a lot because there is a lot of resistance, and then you are moving your arm forward in the air which offers little resistance to push you back. So the amount you push back in the water is greater than the amount the air pushes you back when you move your arm forward.

 

But what about the legs? There are two things I am confused about:

 

1)How does moving your legs vertically propel you forward at all? I don't know how a downward kick can move you forward. If I were to kick the ground right now, it would not move me forward.

2) If kicking your legs down propels you forward, shouldnt bringing your leg back to the original position move you backward? For instance, if I were to floor the gas pedal in my car while it was in drive for 5 seconds, it would move me a distance forward. If I were to then throw my car in reverse and floor the pedal for 5 seconds, it would move me back to my starting point. So how come kicking down propels you forward, while kicking up does move you backward?

 

Sorry if I did not explain my question clearly enough.

[Phi for All]

No matter how rigid you keep your legs and ankles, I think there is some flexion that helps the feet propel you like paddles. Don't the legs do more to provide stability and lift for the swimmer than propulsion though? The scissoring of the legs pulls you to the surface as your arms try to dig you deeper in the water.

[Iggy]

1)How does moving your legs vertically propel you forward at all? I don't know how a downward kick can move you forward. If I were to kick the ground right now, it would not move me forward.

2) If kicking your legs down propels you forward, shouldnt bringing your leg back to the original position move you backward?

Would it help to think about wearing fins on your feet to help you swim when you're entirely submerged?

 

It's more analogous to a snake slithering across the ground than a person jogging.

Edited August 18, 2012 by Iggy

[ACG52]

The motion of your legs propels water backwards, which pushes you forward.

[CuriousBanker]

Yeah the motion of your legs propels water backwards moving you forward. But what about when you have to bring your leg back forward. Wouldn't that propel you backward?

 

As for the snake analogy I don't really get how snakes move either

 

Also if the legs scissoring pull you up wouldn't they push you down when they go the opposite way?

[JMJones0424]

Yeah the motion of your legs propels water backwards moving you forward. But what about when you have to bring your leg back forward. Wouldn't that propel you backward?

(Seems like here you're referring to the kick used for strokes like the breast stroke, similar to how a frog's legs kick) When you bring your legs in, you keep them tight inside the wake created by your body moving through the water. This part of the kick does slow you down a bit, but the tighter you keep your legs inside the line of the body, the less drag you create. When you begin the propulsion phase of the kick, you extend your legs out and then snap them together, propelling the water between your legs forcefully behind you, giving you thrust. The same principle is used with your arms in the breast stroke, but it is more effective because your arms are able bend more appropriately than your legs. The arms extend straight forward, giving little drag, then snap around to the side of the body to provide propulsion, then are drawn up along the body and extended forward again along the line of the body moving through the water. The wikipedia article on breastroke has a decent gif that illustrates both the arm movement and the frog kick for reference.

 

Also if the legs scissoring pull you up wouldn't they push you down when they go the opposite way?

When doing a flutter kick like the motion that is common in the crawl, the legs are almost entirely stiff with very little knee bending. A small amount of thrust is created with each leg when they move up and when they move down because the legs are flexing at the hips. The effect is the same as when you wave a handheld fan back and forth to propel air. You aren't necessarily propelling yourself up to the surface with your legs, rather you are maintaining forward movement which keeps your lower body level with your torso and minimizes drag.

[CuriousBanker]

(Seems like here you're referring to the kick used for strokes like the breast stroke, similar to how a frog's legs kick) When you bring your legs in, you keep them tight inside the wake created by your body moving through the water. This part of the kick does slow you down a bit, but the tighter you keep your legs inside the line of the body, the less drag you create. When you begin the propulsion phase of the kick, you extend your legs out and then snap them together, propelling the water between your legs forcefully behind you, giving you thrust. The same principle is used with your arms in the breast stroke, but it is more effective because your arms are able bend more appropriately than your legs. The arms extend straight forward, giving little drag, then snap around to the side of the body to provide propulsion, then are drawn up along the body and extended forward again along the line of the body moving through the water. The wikipedia article on breastroke has a decent gif that illustrates both the arm movement and the frog kick for reference.

 

 

When doing a flutter kick like the motion that is common in the crawl, the legs are almost entirely stiff with very little knee bending. A small amount of thrust is created with each leg when they move up and when they move down because the legs are flexing at the hips. The effect is the same as when you wave a handheld fan back and forth to propel air. You aren't necessarily propelling yourself up to the surface with your legs, rather you are maintaining forward movement which keeps your lower body level with your torso and minimizes drag.

 

I understand what you are saying about breaststroke but not about flutter kick. Maybe I'll never understand. When you use a handheld fan you push air in one direction then you push air in the other direction. Net, you are moving back and forth but not going anywhere. How does it help you maintain forward movement? Obviously I'm missing something. I don't know what. Thanks for trying

[JMJones0424]

When you use a handheld fan you push air in one direction then you push air in the other direction. Net, you are moving back and forth but not going anywhere.

This is incorrect. Try it yourself with any stiff, flat object and "fan" yourself with it. The movement of the air is not perpendicular to the face of the "fan". Try it the normal way, where the pivot (your wrist) is inline with the fan and away from your face, and then try it with the pivot out in front of your face rather than inline.

 

 

Why is the airflow this way? To be honest, I'm not sure. Perhaps someone else can help us both with that.

[MigL]

Have you ever seen a centrifugal scroll or fan as in a turbocharger or early jet engine ???

 

Its Newton's laws of motion. There is no centripetal force constraining the air to the radius of the fan

so the air is free to move in astraight line after its initial acceleration.

[Iota]

Basically, the water is exerting force back against you, as you do to it.

 

When you stand up, you are pushing down on the ground, and the ground is pushing up against you. With water It's the same, just lesser.

[Iggy]

Yeah the motion of your legs propels water backwards moving you forward. But what about when you have to bring your leg back forward. Wouldn't that propel you backward?

 

As for the snake analogy I don't really get how snakes move either

 

Also if the legs scissoring pull you up wouldn't they push you down when they go the opposite way?

 

This might help:

 

 

The idea is to push more of the white dots down than up. Underwater swimming involves a similar undulatory motion by alternatively bending toes, ankles, knees, hips, and good swimmers even get their torso involved and start to look like a dolphin.

Edited August 18, 2012 by Iggy