You said you liked muscles, right? I know you usually get asked with bellies, but do you have any buff muscle head canons? If you do, let's go with Rin from Free, Ben Tennyson, and the twins from Ouran. (If you feel like you can't give a LONG explanation, then can it be a short list of what you'd want to see?)
Okay I can give these a shot though this is really out my wheelhouse and I’m not 100% sure you’re wanting. This could take some different routes with some of them especially because of differences in their states (Rin already has noticeable but Ben and the Twins don’t as much).
(As a weightlifter who at least TRIES to have good form
unless there is a swimmers technique that I’m not aware of this is the single most frustrating and annoying gif in all of Free. He’s pumping his legs and is cheating to get more reps but no actual tension on the pull up so he’s not actually working toward failure. If he needed to go for more reps they have assistance bands for that to help him.)
(At least this one seems to be with better form. Admittedly we don’t see his legs, but he’s properly holding for a split second at the top of the contraction. If he holds it there it can help build strength and muscle size)
(but oh my look at the separation of the head of the bicep. Beautiful)
Okay I’m not 100% sure where to start out with a muscle canon on Rin since there’s a long history. I’m sure part of it has to do with his desire to be like his dad and Gou is a factor as well. It’s hard to say if his working out came first, if she was influenced by her dad, or if Gou’s muscle kink has helped to push Rin on his muscle. One way or another, Rin’s love of muscle and trying to push himself has long been ingrained in him. The one difference with Rin is because he’s a swimmer is while he can get bigger and stronger, he has to be proportioned a particular way. He may get slightly bigger but he’ll always stay proportioned right.
While the gif may prove me wrong, I think that Rin would be a lot like Rei and want to make sure he’s doing things correctly and making sure he’s doing things with the proper form and with a focus on improving mobility. Especially once he knows and they try to fix Sousuke’s shoulder (which is a thing I understand well let me tell you that) they will both want to increase range of motion and reduce injury as well. He’s going to do it right and be smart about it
But I think Rin’s love of muscle would manifest early, probably at least in the end of junior high. He’d love seeing himself go faster, stronger, and seeing his muscles grow. I could see Gou helping (or forcing) Rin to measure his muscles to see if he’s gaining muscle and which things he need to work on. This also leads into why he’s more well proportioned. I think he’s more overall better shape when Makoto has a better back
and according to @dumdumdrawstumtums a Hank Hill butt, but Rin is more well rounded. It is possible because he started to work out earlier than Sousuke and Makoto that Rin isn’t as tall as them ( it can be possible that working out with too much weight too early can stunt growth which could explain me compared to others but I have never seen real studies on this, just older books I’ve read so could totally be a myth) but he’s going to have much higher flexibility (as evidenced by his tumblr rep as a stripper)
I do think that Rin likes to subtly show off his muscle by wearing tight clothes or revealing clothing. I also think with him liking muscle and probably always working towards putting on muscle and increasing strength as long as it doesn’t slow down his swimming since that’s his ultimate goal for now. I can definitely see Rin loving growing out of clothes when his arms are too tight for the sleeves or his shirt starts to ride up his back if he flexes too much or his chest when he moves makes his shirt bunch up or it starts to pinch around armpit. Once he loosens up I could see Rin putting on a display for Sousuke or Momo and Nitori (and they would all react differently if heaven forbid he actually tore a shirt). He might be convinced to flex in front of the team, and I halfway believe that they could teach the team more about anatomy by just having Rin pose in a swimsuit.
I think that there is definitely a possibility in the future especially after he’s done with swimming that Rin could turn professional model and could be someone that will end up on magazine covers or even do like physique competitions. I also think as he got older, he might try to develop his upper body a bit more so he will have a much more defined V shape (though he’s definitely not bad now) Do I still think Rin is an eater? ABSOLUTELY. My favorite is he’s a hard gainer so is having to constantly eat. He may try the technique of intermittent fasting where he only eats within a certain window or even just one big meal, but I like to think that he’s the type that would love to both have a great eating capacity but have the great bod to match.
One last thing though, is I think that if Rin knows you’re into him and muscle, he’s REALLY going to torture you and he’ll love it. He’d be the type that would throw an arm around Momo and make sure to subtly flex his bicep or squat so you see his butt or flex his back or SOMETHING to tease you about it. Even do something like flex his obliques or let his tank top strap fall off his shoulder to show off his shoulder or his pec or heck I even see him lifting up his shirt to flex his abs and even do the wave with them (I actually think he has a lot of muscle control there so he could even pretend to bloat out if he wanted or do the vacuum). He’s a doof that will exploit you if he knows your weakness is him.
Now Ben Tennyson is another interesting one because we KNOW for a fact that he grows into this:
Though I personally really hope he reconsiders on that facial hair.
But at some point he really does start to workout. Again I like to think he’s hard gainer and has to eat a ton to be able to put on mass but that’s a different canon (that I’ve already written lol). For Ben, I think that that basically comes out of the decision that he needs to get stronger and he’s constantly fighting. So he would have a much more different kind of ton and strength because it would be fighting based muscles but I do think between high school and college that Ben would really start to hit the gym both with weights and probably like agility training. Mostly likely with Kevin (RIP attractive buff Kevin) being his trainer/spotter. I could see them both spending time at the gym together and Kevin really helping Ben to push his limits and watch his form so he would improve quickly.
I think that with that kind of training and his eating habits, that Ben would bulk up very quickly with beginner gains and probably keep it up the whole time. Also like Rin I think he’d be one that would LOVE to show off his muscle once he starts. First his leather jacket would start to get tight, then his tshirts and I think Ben would LOVE to have his sleeves rip or bulge out. I could see him being the type that would wear a tight V neck tshirt and then love to bounce his pecs if someone was interested. Or really if Ben was just being really bad at flirting. Same with raising his shirt to show his abs or just finding an excuse to take off his shirt. Once he gets big enough, he would just peel it off for no real reason other than “comfort” and freedom of movement.
Ben would be the type that would try to strike poses when he was trying to show off and may even do it during battle if he thought someone was looking. I also imagine that he’d love to go swimming a lot more and like relax from a battle in a sauna or jacuzzi. Rook would have a lot of mixed feelings about this because it was a gratuitous display but he’s also probably really like being able to watch himself.
Honestly as long as he kept his mobility up, I really do think that Ben would be more likely to to get into legit bodybuilding probably thanks to Kevin’s influence. Maybe not enough to do anything competitive but he’d really like to have the power trip and know he’s looking good. I also think he did it on the added idea that the stronger his human body is, the stronger that the rest of his forms are so it is actually better for him to bulk up. But then I also had the strange thought of Ben doing a swimsuit calendar for the entire galaxy that will sell like hotcakes.
The Hitachiin twins would be an interesting bunch. If muscle captured their attention long enough they could be really interesting. Only problem is I could see them being fickle. HOWEVER that could be applied to them doing different types of work outs or sports so they are always showing off on what they are doing.
I have the feeling that if either Haruhi showed interest in muscle (which I have no problem with there) or a subset of the clientele had a muscle fetish, I think the pair would pounce on it. and by that I mean literally do it as part of the club. The twins would probably set up a gym in the corner of the club and workout for everyone, most likely shirtless just to show off to everyone.
You know that the pair would tease everyone too by doing something like “Hikaru, don’t you think my bicep is bigger?” “No Kaoru, don’t think my tricep is better defined?” “Well my delts are bigger!” “Oh yeah? My calves are AMAZING!” “What do you think Haruhi?” “Yeah what do you think?” *poses in front of all the girls and Haruhi to try to get attention.*
This doesn’t include when they start to come on to each other either.
Then they would always play the switching game so they would probably stay roughly the same size, though Haruhi would always be able to tell the difference and even point out some subtle muscle differences. I think once they got big enough it’s very likely that they would end up being a feature of the club and have some sort of muscle posing contest potentially only in colored posing trunks.
They would definitely flirt more with their bodies and they would put flexing as part of their hosting time. I could see if they got strong enough that they would lift or bench press some of the girls or pull them up by lifting their arms. I also think they would give lots of piggy back rides and flatter the girls saying how they weren’t heavy at all but everyone would love it because they would get to touch their broad muscular backs. Generally I think they would have some fun showing off their strength and fitting into form fitting clothes. I don’t think they would get SUPER buff, again more fitness model size but everything would be tailored to fit them really well. So everything would show off their figure and be a general tease. They most likely are going to have to have the type of pants that have some sort of elastic in them so that they can sit without having to tear the seat of their pants.
I also think that they would end up taking votes on which muscle group they should work on that day when they worked out in front of everyone. I think they would love to tease Tamaki with their muscle and he probably end up getting a personal trainer of his own too. But even then, they would flex right in front of Tamaki. I could see them sitting really close to him (like they were that in that RottenHeart pic with their bellies) and let their big shoulders muscles just be right there. And then probably pec bounce right in front of him just to get him flustered.