How to classify the human postures of sitting and standing from video input?

Currently i have done human detection on input from webcam. It puts a rectangle bounding box around the person and the centroid of the box is shown too. How to proceed further to detect the sitting and standing posture?

vid = videoinput('winvideo', 1);
for i=1:10
    misc = getsnapshot(vid);
end
bg = misc;

X = im2double(misc);
misc = (X(:,:,1)+X(:,:,2)+X(:,:,3));
misc(misc==0) = 0.001;
X(:,:,1) = X(:,:,1)./misc;
X(:,:,2) = X(:,:,2)./misc;

DlgH = figure('units','normalized','outerposition',[0 0 1 1]);
H = uicontrol('Style', 'PushButton', ...
                    'String', 'Close', ...
                    'Callback', 'delete(gcbf)');

while (ishandle(H))

      mov = getsnapshot(vid);

      subplot(1,2,1), imshow(mov);  %1. show current frame%
      title('Current Frame');

      X1 = im2double(mov);
      misc = (X1(:,:,1)+X1(:,:,2)+X1(:,:,3));
      misc(misc==0) = 0.001;
      X1(:,:,1) = X1(:,:,1)./misc;
      X1(:,:,2) = X1(:,:,2)./misc;

      A = imabsdiff (X,X1);
      A (A<0.27) = 0;           %threshold for background subtraction figure 2%
      A (A>=0.27) = 1;

      Res = imdilate (A,ones(9));

      GX = rgb2gray(bg);
      GX1 = rgb2gray(mov);

      FD = im2double(imabsdiff(GX,GX1));

      %subplot(2,2,2), imshow(FD);  %2. show foreground%
      title('Foreground');

      FD(FD<(0.10)) = 0;            %threshold for figure 3%
      FD(FD>=(0.10)) = 1;
      DR = imdilate(FD,ones(9));
      re =  imfill(DR,'holes');

      finalRes(:,:,1) = Res(:,:,1).*re;
      finalRes(:,:,2) = Res(:,:,2).*re;
      finalRes(:,:,3) = Res(:,:,3).*re;

      %subplot(2,2,3), imshow(finalRes); %3. show processed foreground%
      title('Foreground2');

      boundaries = bwboundaries(rgb2gray(imclose(finalRes,ones(3))));
      bndrSize = size(boundaries);
  %     subplot(2,2,1:2)

      subplot(1,2,2), imshow(mov);    %4. show final frame%
      title('Human detection');
      hold on
      for k = 1:bndrSize(1)
          bnd = boundaries{k};
          RB2 = max(bnd);
          LT2 = min(bnd);
          w = RB2(2)-LT2(2);
          h = RB2(1)-LT2(1);
          x1 = LT2(2);
          y1 = LT2(1);
          x2 =  LT2(2)+w;
          y2 = LT2(1)+h;
          x3 = (x1 +x2)/2;
          y3 = (y1 + y2)/2;
          if (max(RB2-LT2)>100)       %with rectangle over them.  adjust this to cover only large objects, small values will cover smaller objects % 
              rectangle('Position',[LT2(2),LT2(1),RB2(2)-LT2(2),RB2(1)-LT2(1)],'EdgeColor','g','LineWidth',1);
              rectangle('Position',[x3,y3,10,10],'EdgeColor','r','LineWidth',1);
                  end
      end

end
close