How to make loop animation and perfect loop.
1, let make two poses and that change repeat.
2, make first pose in frame-2, select all joints + (Spline Section)
3, frame-2 copy & paste to frame-3 (Ctrl+C, Ctrl+V)
4, frame-2 select all joints + (Linear Section)
5, make second pose in frame-20, select all joints + (Linear Section)
6, frame-50 select all joints + (Spline Section), and make key frame.
7, frame-3 copy & paste to frame-70 and 90, same frames.
8, frame-70 select all joints + (Linear Section)
9, change fps and total frames, Rate , Frame:  of 
export BVH this and upload to secondlife. checked Loop, LoopIn 0%, LoopOut 100%, EaseIn/EaseOut 0.5sec, others are default. start animation.. EaseIn 0.5sec change(start) animation is smooth. changing pose, first pose to next pose. when came to frame-20, you see second pose and that posing to frame-50. changing first pose frame-50 to frame-70 and posing to frame-90.. go to frame-2 and repeat animation. Linear Section, animation stops tight.
Make perfect loop processing before export BVH.
don't overwrite file, because original key frame is lost with this process.
1, increase total frames 90 to 300(x3), Frame :  of 
2, copy frame, frame-3 to 90 select all joints (Ctrl+C)
3, frame-90 select all joints and paste data (Ctrl+V)
4, data pasted frame-90 to 178, select frame-178 and paste data (Ctrl+V)
5, loop animation made in frame-90 to 178, use resample and build key frame.
top menu [Animation] Resample Key Frames, open dialog
(1) Make key frame every  frames
(2) Frame Range :  to  and click button [OK]
7, select all joints in frame-90 to 178, click button (Linear Section)
8, drag selected range (frame-90 to 178) and move to frame-2
9, change total frame, Frame:  of  and export BVH.
10, upload window, checked Loop, LoopIn 1.75/89x100% = 1.966%, upload.
copy range repeating, arrange three ranges. perfect subframe is made in middle range. make key frame and fix range, not need frame-2 and move range there. remove one frame.. when make loop animation, add one frame to total frame. this repetition has two same frames, first frame is same as last frame. that animation stops for instant. remove that part with LoopIn, 1.75(skip)/n(total frame)x100%=1.966%. if remove first frame or last frame, it's right way on poser. but animation structure fails on second life. two same frames which are not compressed create perfect loop.