how to save fields in a structure for all iterations from a processed video

Hi there, I am using the multiObjectTracking nested function in Matlab( http://www.mathworks.co.uk/help/vision/examples/motion-based-multiple-object-tracking.html ). Here is my code for saving all the tracks and their number of frames:

keep_tracks_ids= zeros(1,max(ids));
keep_tracks_framecount=zeros(1,max(ids));
 for i=1:size(ids)
keep_tracks_ids=[keep_tracks_ids step(reliableTracks.id)]
keep_tracks_framecount=[keep_tracks_framecount step(reliableTracks.totalVisibleCount)]
 end
save('keep_tracks_ids.mat','keep_tracks_ids');
save('keep_tracks_framecount.mat' ,'keep_tracks_framecount');

I get the following errors: Error using step (line 85) Not enough input arguments.

Error in ..."keep_tracks_ids=[keep_tracks_ids step(reliableTracks.id)]" Please what can I do? Without the for loop, I get the identity of the last track and its number of frames. So the main code is working. My coding for saving is not. Thanks for any help.