Bmi Floor Mats Why Do I Have Difficulty Squatting With My Feet Flat On The Floor?

Why do I have difficulty squatting with my feet flat on the floor? - bmi floor mats

What I mean, sitting as follows:
http://farm4.static.flickr.com/3341/3344 ...

Please note that your feet are bent to the ground and the legs are flat, slightly forward to balance the weight of his body. In addition, the feet will relax the muscles, without apparent effort.

That is exactly what they have problems ...

I always try to sit her heels on the floor, when to hold, I always end up falling backwards under the weight of my ass (and a weight of 105 kg and has a BMI of 18, I am of course very difficult. .. instead).

I know that, unlike the girl in the photo, my legs are not 80 degrees with the feet. You are at an angle of 90 degrees and hang when I try to push (with your back against a wall), my muscles are very tight and painful.

I have also tried our legs a bit, but my legs were not on the thigh, can not be reduced below a certain point, stretch, and it feels more like "adjourned meeting" rather than an occupied house.

So ...Why can not squat, feet flat on the floor? Why can not I do what the acute angle between the legs and feet? Is there something wrong with my muscles?

Thank you!