Look into subsummary reports. You will want to group by this field (arm / leg).
I took your advice and it looks like exactly what i want to do but something is going wrong. I followed the steps provided by this tutorial http://examples.oreilly.com/filemaker9tmm/screencasts/subsummary/screencast.html. It goes well except when i preview it after i put in a sub summary part, the sub summary grouping only applies itself to the first record and nothing more. Maybe its a way im sorting my records. Any ideas?
Fidn the set of records you want it to report on. Then you can run your script that takes the user to that report layout. You must sort by the break field. Have you looked it the report in preview mode?