Older brother nagging and making threats about money owed.

I'm an 19 year old student that is, about to start a high-paying entry-level job, paying for gas, and wakes up early everyday to study amongst other errands. I also have limited money and fund my bank account by mowing the lawn for the community and doing tasks for family and friends.

My older brother is 30, dropped out of college on his 3/4 year, made himself at home in my room, sleeps all day only to wake for WoW, and currently pays for our shared gym membership created under a promotional discount for the gym we go to.

My brother wakes up every other day at about 5:30 and gets me up to take us both to the gym. I could be busy with a online tutor, on the phone, or just relaxing, but he always finds a way to guilt-trip me into getting off my 'lazy ass' for a three-hour long workout. We go four times a week evening to night. On the car ride there, he'd nag me about not stopping a day's workout until you drop dead. If he continues, I either turn on the radio or keep driving while tuning him out.

The gym membership started in February and it's now July. During that time, I've steadily grown to annoy him on purpose and like give a rat's ass what he has to nag about. Just yesterday, a Thursday, he threatened me to give him $100 for the gym bill on Monday for the past few months. Note that we both agreed he would pay for the gym while I would pay for gas and equipment when we started February. He knows that I have limited money for emergencies besides gas and haven't started working yet. My brother is the type of man that demands respect. I speculate this is due to his mentality as the eldest.

Anyway, I brush him off and confirm that I haven't even started the job yet and he's making impossible debt threats. He acts like he doesn't hear me and gets back on his computer.

Also, he has more time for an effective diet and is currently a small-medium sized hulk. I'm on my way, but I've mostly focused on cardio and strength-training while he did heavy weight-lifting. When Monday comes around, I think I can take him if it comes down to another one of his man-child tantrums.

I forgot to mention that I'm still living with my parents until my graduation. A logical solution would be to tell them that my brother is acting ridiculous, but my parents, as caring and understanding as they are, would explode. My brother is on thin ice with them by only paying rent once in his 4 year stay since coming back. Also have to take into account that there are five other, younger, members of the family in the household. So I cut out that solution and decided to make a stand.

The problem is, what action do I take to finally put my foot down? I appreciate the answers to my problem.

P.S. I don't have the money he demanded at the moment. I start work in another week.