Other objective markers commonly used include the 6 minute walk test (6MWD), Incremental shuttle walk test and endurance shuttle walk test. All three of these tests are standardised, objective measures and have been validated (especially 6MWD which I believe is endorsed by the American respiratory society).

I think there is also a borg scale for muscle fatigue (can be handy especially for pulmonary rehab as fatigue has been shown to be higher in a COPD population.)

Hope thats some use,

Rach