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.
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.
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.