1) I think acupuncture must hurt but it can be effective.

2. doing a triathlon

Doing a triathlon might be a good exercise.

3. smoking

Не знаю

4. being with pets 

Being with pets must be fun.

5. doing yoga

I'm afraid that doing yoga could be boring.

6. research into the causes of arthritis 

I'm sure that research into the causes of arthritis must be difficult.