The Soleus as a Knee Extensor, ACL tear prevention

I recently purchased a spider squat bar. You see, ever since I found out these things existed, I've wanted one. Well, I finally bought one, and it is now hanging proudly in my power rack in my bedroom-gym.

Only the coolest bedroom in the world...

Spider squatting is quite different from your typical squat (<--- best squat article ever, by the way), as the bar always seems to want to pitch you forward, making this extra awesome if you want to train core stability.

Anyway, I started practicing doing some reps past parallel with this 80-pound beast, and I noticed something that I had forgotten about when reversing direction coming out of the hole... the fact that the soleus muscle (the deeper of the two calf muscles) can be used in close chain activities as a knee extensor.

How is this possible?

You might be wondering a few things right now. 

1. Did I really mean knee flexor, not extensor?

No. The gastrocnemius, the superficial calf muscle that crosses both the ankle and knee joints, can be recruited as a knee flexor (bends the knee). I am correctly referring to the soleus as a knee extensor (straightens the knee).

2. How is that possible? The soleus only crosses the ankle joint, not the knee joint, right?

If you thought this, congratulations, you know your soleus anatomy. The soleus does only cross the ankle joint, taking origin from the leg below the knee joint.

The gastrocnemius (cut) crosses the knee joint, while the soleus does not.

If this is true, shouldn't it mean that the soleus can only affect the ankle joint? The answer to this is no, but you'll have to allow me to clarify. If you scroll back up to the original point I made, you will find that I said that the soleus can be used as a knee extensor during certain closed chain activities.

A closed chain exercise is a physical activity where the hands or feet are placed on a surface that doesn't move (like in a squat or pushup).  This is the opposite of an open chain exercise such as the bench press where the hands are feet are placed against a surface that is meant to be moved (e.g. bench press or leg press).

Anyway, the soleus is a plantar flexor. This motion normally looks like one of the following:

Dorsiflexion (bringing the toes up) and Plantar flexion (pointing the toes)

Calf raise finish (plantarflexion, on left)

In these situations, the calcaneus (the heel bone) is pulled upward, which causes the foot to point downwards. In other words, the calcaneus moves relative to the rest of the body. 

But what would happen during an exercise such as the squat, where the entire foot is supposed to remain in contact with the ground?

Well, in those cases, the calcaneus would be fixed. In the squat, the ankle would slightly dorsiflex during the movement, but it would not look "normal," as the toes would not rise off the ground. Rather, the tibia and fibula would move forward relative to the foot instead of the other way around. 

On the upward movement of the squat, similarly, the soleus would in fact be causing plantarflexion, but rather than the calcaneus moving around the tibia and fibula, those bones would be moving around the fixed calcaneus!

So, due to the pull from the soleus, these bones would be driven backwards and as a result, would be extending the knee!

Functioning in this manner, would it be plausible to suggest that the soleus would also be active in preventing ACL tears? I think so, and there is evidence to support this.

Anyway, I hope this shed some light on the not-so-obvious role a muscle can have when conditions are changed.