Forward Knees and the Lower Leg

[KevinH]

In SSBBT3, a part of the analysis regarding knees forward at the bottom does not make sense to me:

“A knees-forward position produces a more acute knee angle...It also means that there is more moment force against the ankle/mid-foot balance point because of the more horizontal angle of the tibia.”

1) If we assume the COM to be over the midfoot, then shouldn’t the horizontal moment arm acting on the ankle be the same regardless of tibial angle, since the ankle is a set distance from the midfoot?

2) Shouldn’t the vertical moment arm between the ankle and the origins of the ankle plantar flexors be shorter at a more horizontal tibial angle? This is obviously unfavorable, since it shifts some of the load from the slackened hamstrings to the weaker ankle plantar flexors.

This is just a minor quibble about the analysis. I understand rest of the analysis and the importance of not letting the knees slide, but I wanted a little more clarity.

[Mark Rippetoe]

1. Good point, an error to correct in the next printing. Should read: “A knees-forward position produces a more acute knee angle...It also means that there is more moment force against the shank segment from the knee because of the more horizontal angle of the tibia.”

2. Vertical moment arm? If gravity is the force, moment arms in this system are horizontal.

[KevinH]

I guess I wasn't clear. Of course, gravitational forces are oriented vertically, so will only produce horizontal moment arms. Muscle contraction is not limited in the vertical direction.

The force by the ankle plantar flexors produce a increasingly horizontal force when the knee angle closes. This is a moment force, and its moment arm shortens as the knees get closer to the ground. It's a similar concept to that you and Steve Hill presented in "You Must Understand the Gravity of Your Situation," although with less profound implications.

[Mark Rippetoe]

The force by the ankle plantar flexors produce a increasingly horizontal force when the knee angle closes.

When the knee angle closes AND the ankle dorsiflexes, the horizontal distance between the ankle and the knee increases. The force applied by the contracting muscle is Tension.

This is a moment force, and its moment arm shortens as the knees get closer to the ground.

I guess I'm dense, you're going to have to explain this.