What you're talking about is a cascading combo box. To be able to set that up you'd have to have tables like this:
Code:
tblBodyPart
BodyPartID BodyPartName
1 Legs
2 Shoulders
3 Arms
tblExercises
ExerciseID BodyPartID ExerciseName
1 1 Squats
2 1 Hip Bridge
Then you'd have to have a table for your customers and their exercise regimen. Customers on one table (customerID, name and other information), Exercises per Customer on another table (CustomerID, ExerciseID, reps, weight, etc) on another table.
WHen you do your data entry you'd have an unbound combo box relating to the body part which would then be a criteria for the contents of the exercises you could choose from.
There are a million threads about cascasing combo boxes on this forum and other places
http://fontstuff.com/access/acctut10.htm