Theory of Body Tension

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I have not posted on the blog in the past two weeks because I have been doing a lot of study / writing about body tension, as well as coaching and course setting which has been a lot of fun. Anyway, I think I have come up with a good mechanical description of what body tension is; but this description is getting long and it includes several concepts not commonly discussed in climbing circles so I need to lay some groundwork prior to getting down to brass tacks. I am dividing my lengthy description into several posts. The first two posts provide the background for the later posts. If I can get the writing done I hope to have all the posts up by early next week.   

A Theory of Body Tension

As I mentioned in a previous post, its my understanding that a good definition of any facet of climbing movement, including body tension, needs to be able to describe what the climber is attempting to do to the Center of Gravity. This is the case because the primary purpose of climbing movement is to position or move the COG in space. So if we don’t understand the effects of movement on the COG we can’t really say we understand the move we are examining. In the case of body tension I want to emphasize four basic actions.

1)    Keeping / moving the COG inward and / or elevated in the base of support during the advancement of a hand.

2)    Controlling the outward movement of the COG during /after advancing a hand.

3)    Keeping the COG inward and / or elevated in the base of support when advancing a foot.

4)    Keeping / moving the COG inward and / or elevated in the base of support as the orientation of the body to the line of gravity changes. As in cases when the trunk is rotating.

When I say “inward” I mean the relative horizontal distance of the COG from the climbing surface. Similarly “elevated” means the relative vertical height of the COG in relation to the base of support.

If you notice that some of these four actions are similar to aspects described in the comments to the earlier posts, you are right. In addition, and this is the big one, if you note that climbers attempt to perform these four actions in situations that we don’t commonly think of as requiring body tension, you are also right.

As I studied a number of moves and worked it through, I could not find any positions or actions of body tension that are unique in what they seek to achieve in terms of moving or positioning the COG. Body tension situations seem to do the same kind of things that other moves do; what is different though is how we experience the physical sensation of the move, and additional variable at work in body tension moves.

To explain why this is the case I need to start by describing the forces at work on hand and foot holds.

The Forces That Keep Us on the Holds:

On every hold we use we experience the effects of the normal contact force and the friction force. These are the names given to the forces that are reactive to the forces we apply to holds with our hands and feet.  On a flat hold on a vertical wall such as in the photo below, the contact force resists the vertical force being applied by the climber’s mass to the hold, and the friction force resists the horizontal force applied to the hold.


Photo 1

Obviously we don’t just climb on flat holds on vertical walls. It’s important to take into account the wall angle, and hold shape. If we place the same hold pictured above in a roof and use it as a side pull the forces act in the opposite directions as we can see by rotating the photo 90 degrees.


Photo 2

Now the contact force is horizontal, and the friction force is vertical. This is important because the shape of the hold and its orientation in space effects which of these two forces is primarily responsible for keeping us on the hold.  In the first photo above, the hold surface is perpendicular to the line of gravity. This is pretty much the best case in climbing, as it essentially allows us to “hang” our COG from the hold and the force we are applying to the hold is usually significantly less than body weight, since each hold only bares part of our weight. On the other hand, holds with surfaces that are not perpendicular to the line of gravity the friction force plays a greater role in keeping us on the hold. The worst case is pictured in the rotated photo, in which the friction force is solely responsible for resisting the force of gravity.  The question is how do we generate enough friction to stay on the hold?

The Impact of the Center of Gravity on The Contact and Friction Forces:

It’s not just the orientation of the hold to the line of gravity that determines the balance of forces acting on hands and feet; It’s also the position of the COG in relation to the surface of the hold. For example, in the first photo above, assuming that the hold is over the climber’s head, and the climber’s body is close to the wall, the line of force from the hold to the COG comes within a few degrees of being parallel to the line of gravity. But lets say the climber is doing a long vertical dyno, so the COG is going to rise high enough that by the end of the move, the COG is at the same height as the hold. In this case the friction force would play an increasing role in keeping the hand on the hold and the contact force a decreasing role as the COG moves upward. Because as the COG moves upwards the line of force from the hold’s surface to the COG moves as well, until the point the COG is equal in height with the hold’s surface. At that point the friction force would be solely responsible for maintaining contact with the hold because the line of force between the COG and the surface of the hold would be parallel with the friction force.

In summary any situation in which the surface of the hold is perpendicular to the contact force, the climber has an important mechanical advantage. In other situations such as on slopers, or flat holds in over hanging climbs, the balance between the contact force and the friction force changes over the course of the move and there will be times in which the friction force is more important than the contact force.  Finally, its essential to point out that the contact force and the friction force are proportional. As the contact force increases so does the friction force. How this is related to body tension will be described shortly.

7 Responses to “Theory of Body Tension”

  1. ToddNo Gravatar says:


    Concerning your paragraph:
    It’s not just the orientation of the hold to the line of gravity that determines the balance of forces acting on hands and feet; It’s also the position of the COG in relation to the surface of the hold…

    Since I read “Self Coached Climber” I have been discussing this with my kids. I am trying to get them to understand two concepts:
    1: EVERY hold has a direction.
    2. How your COG moves in relation to that direction affects how the hold feels.

    I know that beginner lessons typically give lip service to get you to understand that the hold “feels” best when you are pulling against it. But, I have never heard them address that this affects EVERY hold. Also, they only think of it statically.

    Concerning #1:
    Even jugs have an optimal direction. We don’t typically think about jugs in this way because the difference is not always noticeable. But, I show them the example of trying to use a jug that is easy to hang from as a side pull.

    Concerning #2:
    I have never seen anyone else discuss how the feel of the hold changes as your COG moves in relation to that direction. The more your COG moves away from this direction the worse the hold is going to feel.

    They really start to understand the importance of this when they start seeing more slopers. Even your hold that you have in photo 1 begins to resemble a sloper for that hand as your COG moves upwards.

    Keep the good stuff coming!
    Love both books: SCC and RP!

  2. DouglasNo Gravatar says:


    Spot on, so there isn’t much to add. As for the hold in the photo. Yes, exactly, there is no lip, so as the COG rises in relation to the hold, the more important the friction force becomes in maintaining contact.

    Thanks for the thoughts. By the way, you mentioned, do you coach a kids team or otherwise do youth instruction?

  3. ToddNo Gravatar says:


    Yes, I do coach kids on a team. Currently, we are diligently preparing for the upcoming USA Climbing American Bouldering Series Regional Championships in December.

    Feel free to email if you would like to discuss in further detail.

    All the best,

  4. [...] can also read my attempts at a provisional definition of body tension here, here and [...]

  5. JeffNo Gravatar says:

    Just came across your blog while searching for your book. I’m still relatively new to climbing (about a year) and not very good, but I’m no stranger to force diagrams or kinesiology and this is the knid of stuff I’ve been looking for to help me learn how to apply it to climbing and be a smarter climber.

    However, if I’m understanding what you are saying correctly, I believe there is a mistake in your thinking in a few spots above.

    1) I don’t think the worst case scenario is the rotated photo, simply because if you continued to rotate the hold, or put on something that was more parallel to the wall (sloper, or a hold that is almost flat), the contact force would be much less useful.

    2) If I’m understanding the paragraph before the summary correctly, I’m not sure, on a vertical wall, why having my COG at the height of the hold necessitates that friction is the only, or even main force holding me onto the contact surfaces pictured above. When I do dips at the gym, my COG moves from below, to even with, and then above my contact surface, and I’m never helpd up primarily by friction. When I do what I believe is referered to as a mantle, I’m never held up primarily by friction. Not sure I’m following you here. The only case I can see where it would apply is on an overhang, but that change is due to a multitude of factors.

  6. DouglasNo Gravatar says:


    Thanks for the comments. There may be some ambiguity in my writing so I’m sorry about that.

    Regarding point #1 I’m not sure I follow exactly, but I agree that a sloper or a hold that is almost flat on a vertical wall would also be an example of a situation in which the contact force is much less useful. The examples aren’t exclusive, they are both describe the same principle.

    regarding point # 2. I can see from your comments that I probably need to be more clear about when I am describing a general principle and when I am describing a specific aspect of climbing movement. Also, its true that I do simplify a lot in these descriptions and I may be mis-judging what its safe to leave out. In this case the description is climbing specific, as you point out it does not apply to dips at all. But maybe my description needs more examining, because I may have over reached a bit. So let me ask your opinion on a specific example. Lets say a climber is on a vertical wall. and the only hand hold is a sloper on which the hold surface is 45 degrees in relation to the vertical wall. How do you describe the changes in the force at work on the hand hold as the COG rises in relation to the hold? Assume that the starting position has the climber as low as possible and as close into the wall as possible. As the climber moves I assume that the COG rises and in doing so comes out from the wall a bit, as is common. To boil down what I was writing about to its most simple form, the higher the COG rises the more important friction becomes, and at a certain point the climber can not apply enough contact force to the hold to maintain friction and so falls off. Again, this is climbing specific, the COG and the base of support are in different planes, and at no point is the hold surface perpendicular to the line of gravity or the contact force. I would say that these are two important differences between climbing and dips.

    In order to understand dips or a climbing move we want to look at the orientation of the body to the line of gravity, the orientation of the supporting surface to the line of gravity and the forces being applied to it, and the movement of the COG in relation to the supporting surface and gravity. Thoughts?

  7. JeffNo Gravatar says:

    For the most part I agree, although it may depend on things like where the hold, how convex the sloper is, etc. I’ll also add that a big part of the increase in friction force needed is this particular example is because the climber’s COG is moving away form the wall as you stated. He/she needs a contact force of some kind to pull it back the wall so it stays close to the feet and I would imagine that on most slopers it is this force that is more limiting that the friction needed to prevent downward displacement of the COG.

    Personally, I don’t have the grip for most slopers so I avoid them. When I need to do them in a situation like that above, I tend to fall backwards, away form the wall as opposed to slipping downwards. This is as opposed to when I am standing tall and using an overhead sloper – here my COG is close to the wall, the friction resists mostly downward displacement of COG and I slip downward.

    Also in reading and responding to these, as well as watching the videos, I’m thinking at some point down the line it would be good to have a mid level boulding problem that people of different levels attempt and we look at different choices they make and how it affects movement and balance.

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