1 - The Meathead stereotype is male hamstering.

I'm not sure how long the trope has been around, but jocks and athletes have been portrayed as retarded assholes in movies since at least the 80's (movies that usually include some kind of unrealistic Hollywood love story where the kind hearted loser wins a girl's heart).

In reality, fit dudes tend to be more content, and that makes them a lot more likely to be a cool dude. This is especially true among guys who are both fit and getting laid regularly; they're pretty much the opposite of a frustrated sexless fag plotting mass murder in his parents' basement.

People who exercise are also functionally more intelligent than people who don't. This is self-evident in that they are living correctly and putting their mental energy into bettering themselves instead of rationalizing why they don't exercise. Speaking of hamstering:

2 - The belief that everyone inevitably gets fat and unhealthy after their 20s is pure hamstering.

I hear constantly from older guys about how they don't work out because they're not as energetic/flexible/available/resilient etc. as they were in their youth. This is basically always bullshit, and a lot of times it's a chicken-and-the-egg senario, like when your back aches because you're weak because you don't workout, and that's simultaneously your reason for not working out.

What I realized is that, for most of these guys, they are exaggerating just how fit they were in their youth. While they may have looked trim in their teens and twenties, they were in fact weak and skinny-fat.

The older guys at my gym are beasts, and it's not even like they spend an inordinate amount of time there. There's a dude in his 40s who looks like he could compete as a bodybuilder, and multiple guys 50+ who still have very strong, muscular physiques.

Imagine being married in your 50s with a wife who actually likes the idea of having sex with you. It's totally attainable and yet it's really rare.