After looking in the manual, as well as in the Digital manual in the car, I'm still not 100% sure what the answer is. For the drivers seat, my assumption is that the button in the front (Left in picture below) is adjusting the length of the tight support?
Mine is not doing anything.....

 

yeah, it is basically an air bag that inflates under the backs of your knees. At the front of the seat.

Do your other air functions of the seat work? Like the massage and active side bolsters?

My 2010 is setup just like that, i have had instances where all air functions of the seats stopped working. Turning the car off and back on brought these functions back to life.

Even with the front of the seat "fully extended" or "fully aired up" as you could possibly say, it's not incredibly noticeable. you have to watch it and see, which i am sure you have done so.

 

Yes, the other function are working, and esp the dynamic bolsters are quite nice when driving spirited.
That said - the bottom (to the right in the picture) functions are also a bit weird. There is definitely an airbag inflating but the vertical movement of the lower back support is hard to notice.
The "Pulse" (e.g. massage) function is working very well,

I expected the thigh support to actually extend so maybe I will have to check even closer....

 

That don't sound right. I would have the dealer look at it.

 

Yes, the other function are working, and esp the dynamic bolsters are quite nice when driving spirited.
That said - the bottom (to the right in the picture) functions are also a bit weird. There is definitely an airbag inflating but the vertical movement of the lower back support is hard to notice.
The "Pulse" (e.g. massage) function is working very well,

I expected the thigh support to actually extend so maybe I will have to check even closer....

 

Here's a video of mine. Its not very noticeable, but on long drives, it is nice. It more so just stiffens the front of the seat up more than anything.

Ah, this video makes me realize how badly i need to clean my interior

 

Thank you very much!
Yeah - mine is moving that small amount as well. So that's the good news. Compared to BMW or Audi seats that is close to no extension, so hence my confusion.
Again - THANK you very much for taking the time to make the video!

 

Not a problem sir.

Yeah, my dad used to have a '03 M5 and that one would go out very far. I thought mine was not working as well on the E63. I also have a friend with an Audi a5 and hers extends out pretty far as well. But, the mercedes seat would have to be redesigned for it to function like the BMW and Audi one.

On those, a whole section of the seat extends where as the Mercedes one is just an air bladder behind the front of the seat, i suppose.

No problem again. Sometimes a video helps more than trying to explain via text