Sitting on the ground on a self made animation/pose comes with several things to consider. These are:
Do you want to run the pose independently, without any “prim” to sit on? For example, shall the animation be run in an AO ?
Then, the height of your avatar can lead to different results when you give the animation/pose to avatars of different sizes.
Or – do you want to run the animation in a furniture or poseball?
Then you may want to check the sit scripts (more later)
If you want to run the animation independently, then you need to adjust the first triplet in your exported line. Example:
your export line is:
0.000001 0.000001 0.000001 0.1 0.1 0.1 0.1 0.1 0.1 32 0 0 -32 32 0 0 0 0 0 -12 0 -24 0 0 -60 0 0 0 0 0 16 0 -8 72 0 -72 84 -4 0 0 0 -32 0 48 32 96 0 0 0 0 0 -48 0 0 108 4 0 0 0 0
We focus on the first triplet, here in bold:
0.000001 0.000001 0.000001 0.1 0.1 0.1 0.1 0.1 0.1 32 0 0 -32 32 0 0 0 0 0 -12 0 -24 0 0 -60 0 0 0 0 0 16 0 -8 72 0 -72 84 -4 0 0 0 -32 0 48 32 96 0 0 0 0 0 -48 0 0 108 4 0 0 0 0
The second(!) value in this triplet is the Z-axis. When you copy/paste this line at the bottom of your bvh file, change that second value to a negative value. For example:
0.000001 -30 0.000001 0.1 0.1 0.1 0.1 0.1 0.1 32 0 0 -32 32 0 0 0 0 0 -12 0 -24 0 0 -60 0 0 0 0 0 16 0 -8 72 0 -72 84 -4 0 0 0 -32 0 48 32 96 0 0 0 0 0 -48 0 0 108 4 0 0 0 0
If you plan to make an animation with multiple frames then you need to make this change in all the frames that are kneeling/sitting.
This value can vary, on the pose you plan to make (for example crouch vs kneel vs lie down flat) and on the height of your avatar. Furthermore, not all viewers show the value in the upload preview correctly. So, failing uploads can sum up fast. To prevent this, I recommend to run tests in the preview grid. There, you can upload for free, until all works as intended, and then switch back to the main grid and upload the last version. If you are not familiar with the beta grid, please read here about it, in full: http://wiki.secondlife.com/wiki/Preview_Grid
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If you want to run the animation on a poseball or a prim, then the positioning task can (and should) be done by the lsl script. If you are familiar with lsl, then you know where to look. If you are not familiar with lsl, i recommend the tools "npose" or "MLP". both are free and open source. MLP is "Multi love pose" and provides the well known poseballs. npose is a system without poseballs, that allows multiple avatars to be seated on one object, for example on a furniture. Both projects can be used also by people with no lsl experience and are well documented.
If you can not dig them out inworld, i will happily provide you with those packs that i use myself.