Well, there’s the part where any injury will end her track career. Bruises will slow you down, to say nothing of broken bones. This is before you consider the risk of stress fractures down the line. If she breaks any bone she’s actually using, which is to say, “all of them,” her track career is over.
She can come back after she’s healed, but, that’s lost time she’ll never get back. As a teen, coming back later is too late, she’ll have missed the windows for advancing to higher levels of competition. As an adult, coming back later means her body is already in decline, so she’ll never be back to where she was.
We’ve talked about this in passing before, but anyone who engages in violence pays a severe opportunity cost. You cannot be a champion athlete and get into frequent fights off the field/court/whatever. There’s no such thing as, “so good you don’t even get hurt.” That’s a fantasy. If you fight, you will get hurt. That may be at the hands of your opponent, but there is a decent chance you’ll do it to yourself.
A lot of the time, your most painful combat injuries will come from pulling muscles. This is actually preventable, if you do your stretches reliably. The problem is, these won’t be the same stretches a track runner would need to engage in. I don’t know if the stretches are somehow incompatible, but it is something that can, and will, mess you up.
For a character who’s career is violence (or includes violence), injuries are an accepted part of the job, and something that they can work around (though there are always other costs), but for someone like a track star? That opportunity cost will be the end of her career.
From what I know of Muay Thai, it actually makes this more likely. The style is very aggressive. It originated as a pit fighting style, and that still shows. I don’t know if the American schools that market Muay Thai as self defense have rectified this, but at least in it’s original form, this was a style that protected the martial artist by reducing their opponent to meat pudding before their foe could do the same to the practitioner. It’s brutal to both the victim and the attacker.
This is also exactly why no reputable martial arts school would leave your character’s violent impulses unchecked. It’s a huge liability issue for the school. So, as a martial arts student, getting into fights will get you expelled. Actual martial artists are not going to be going in and solving their issues with violence. Again, “violence as a last resort.” This is non-optional, because it’s a necessary component of the training. It depends on the type of training they’re engaging in, but violence outside the school will eventually get noticed. This can lead to legal liabilities for them and damage them financially. This character can only get into so many “self-defense” fights before it becomes a pattern. Generally, street level violence is something they’ll attempt to ward her off of.
Of course, her martial arts school isn’t the only one that would get rid of her over violence. If we’re talking about a teenager, she’ll get cut from Track the instant these fights start. Which may have been her goal all along. If she has someone in her life that’s pushing her towards sports, then violence may be her means of lashing out. Training in Muay Thai exacerbates the situation by making her more dangerous, and it’s worth remembering that this isn’t considered “evening the playing field”. As an adult, then serious fights will result in criminal charges, another level is added based on her martial training. Even if she can demonstrate self defense, that’s still time lost due to legal entanglements which will interfere with her ability to train (I’ll come back to this in a minute), and of course, a conviction could easily end up being a couple years in prison, which would end all of her athletic aspirations on the spot.
When we’re talking about a track star, we’re looking at someone who is dedicating a lot of time and energy into being good. This may be their only way into a good college or their ticket ride to a great one, they may have their sights set on the Olympics. All those are in danger of being dashed by an injury pulled from a street fight or a criminal charge. This means a pattern that might be considered slightly irregular in someone else increases into genuinely self-sabotaging/self-destructive behavior. They are setting out to destroy their future on all fronts and I’d think long and hard about the psychological reasoning behind why.
If we were talking about a sport, where violence is an acceptable part of the game, and it makes the school serious money, like
concussionball football, then the administration has incentives for turning a blind eye, but, for someone in track and field? Not so much.
There’s also a “not enough hours in the day” question. While training to actual combat proficiency in Muay Thai is possible, that’s not really compatible with someone who’s committing enough time to competitive grade track and field training. Competitive, high level sports are all day every day. They take priority. She’s getting into her Muay Thai school, maybe, three times a week for an hour. It’s recreational and she’ll be skipping days to focus on track when competitions come up. She could be more regular during the off season, but a lot of Track and Field members do double up for Cross-Country in order to stay competitive. So, that’s where her Track and Field teammates are.
If your character is training for any higher level track competition, that’s going to be their life. All of their free time is going to go right back into training. As an adult, that’s all of her time. Failing to do so will put her at a disadvantage against anyone who’s getting those extra four hours of training and conditioning in, that your character is spending learning Muay Thai.
This is long before we get into other life aspects like social pressure. Her coach will notice if she’s routinely showing up to practice with a split lip or a black eye. If she’s limping because someone connected with her thigh. If her knuckles are cut up. She will be bruised and she will be bloody from her fights and the adults will notice. If she tries to pawn it off on the Muay Thai, her coach will either talk to her parents about it interfering or go to them directly to have a discussion on the subject. By working together, they’ll figure out about her extracurricular fights even if they didn’t immediately jump to that conclusion.
It’s not as easy as it is in Hollywood to hide the fact that you’ve been fighting. The more in the spotlight a character is, then the quicker it outs.