I was thinking about an exercise app but getting time to develop it is very difficult.

As schemas go I was thinking along the lines of:

1) User Personal info.
Height, age, id

2) Measurements
time/date, weight, bicep measurement, forearm measurement and so on..
(linked to user id)

3) Exercise
Name, description, id

4) Workout
id, exercise id, sets, reps, description

5) Workout_performed
workout id, user_id, performed id, description, time/date performed

6) Performed
workout id, user id, target set, actual set, target rep, actual reps, heart rate, calories burned

Actually "(6) Performed" is actually two separate tables.

A User can have many measurements.
Exercises and Workouts are system-wide (not tied to specific user).

Workouts can contain many exercises.

A user selects a workout by populating workout_performed (linked by the user id and workout id)
Then each exercise is completed by populating the performed table (linked by user id and workout id)

I think this makes sense.