best way to define assocations...

I think this is an easy question, but i just don't know the best way to
do this.

I have the following tables:

- Exercises
- Biking_details
- Running_details.

as you can see i have 2 kinds of Excercises: running and biking

so i could do this:

Exercise:
  has_one :biking_details
  has_one :running detail

running_detail
  belongs_to :exercise

biking_detail
  belongs_to :exercise.

of course this could work, but these models would allow that an exercise
would have both running and bking details, which is not possible

when i would like to generate a list of exercisess with a column
"exercise type", i would have to loop though a the list of exercises and
for each record check if there is biking detail OR a running detail,
generating a lot of queries

i could solve this by adding a field exercise_type to the exercises
table, but wouldn't this be kind of redundant?

so now i am having troubles to decide what to do

who can give me some insight?

thanks