Why doesn't a skier fall when their center of mass is outside their base of support?

[FontysPhysioGuy]

Hi Everyone!

I am a physio and I'm currently working with some alpine ski athletes. I don't have a lot of experience with this sport, so I tried to understand some basics about the physics and biomechanics of the sport. (The athlete is in my practice for a knee injury).

Looking at the picture, I have some questions.

1) I want to understand how the athlete is able to keep his balance? His COG/COM is way out of his base of support, so why doesn't he fall?
2) I want to understand the forces acting on the knee joint? I tried to make a free body diagram, but I can't work it out...

Can someone please help me with this?

Thanks in advance!

 

[Bandersnatch]

1) I want to understand how the athlete is able to keep his balance? His COG/COM is way out of his base of support, so why doesn't he fall?

He's making a turn. Much like a racing motorcyclist he's leaning into the turn, so as to counteract the centrifugal force with gravitational component.

2) I want to understand the forces acting on the knee joint? I tried to make a free body diagram, but I can't work it out...

I'm pretty sure the resultant force should be directed along the axis of the leg, so that there's no unhealthy sideways twisting on the knee. Ideally, as far as the knee is concerned, the forces should be no different than when squatting. But then again, I'm no skier, so I've no idea how close to this ideal do they get in practice.

 

[russ_watters]

1) I want to understand how the athlete is able to keep his balance? His COG/COM is way out of his base of support, so why doesn't he fall?

He's making a turn. Much like a racing motorcyclist he's leaning into the turn, so as to counteract the centrifugal force with gravitational component.

That's only half the answer; He's likely also decelerating. Either way, the combined issue is the same: he's roughly vertical with respect to his g-vector, which is the combination of his acceleration (turning and decelerating) and gravity. (though with his legs apart, the COG could be over either, not necessarily exactly centered).

2) I want to understand the forces acting on the knee joint? I tried to make a free body diagram, but I can't work it out...

I'm pretty sure the resultant force should be directed along the axis of the leg, so that there's no unhealthy sideways twisting on the knee. Ideally, as far as the knee is concerned, the forces should be no different than when squatting. But then again, I'm no skier, so I've no idea how close to this ideal do they get in practice.

Yes. Another way to think about it is that for any column, the force is always along the axis of the column (even a bent column like on a scissor lift). Columns don't do well with twisting or off-axis forces. Because of the length of the ski, it is possible for the skiier's COG to be forward or behind (more likely behind) his feet, but it is bad form and puts extra stress on muscles/joints. So with proper form, the force vector points from the skiier's COG toward his ankles.

 

[A.T.]

2) I want to understand the forces acting on the knee joint? I tried to make a free body diagram, but I can't work it out...

With the knees bend like this, most of the knee joint force will come from the muscles (mainly quadriceps), and can be more than 4 times bodyweight. See Figure 5F here:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3171531/figure/Fig5/