Why might this person have bruises ?

I would expect this if she’s a Kendo practitioner (although that’s usually the wrists)…

Pets, medical conditions and yes (potentially) consensual sex would come to my mind as well. But I would definitely ask a close friend if I saw these kinds of bruises.

You seem to know a lot of details about her sex life so she's clearly not embarrassed

I bruise very easily and have experienced people asking me before where I got them and if everything is ok and it’s made me feel uncomfortable and not believed as I protest that everything is fine, really!

Definitely ask in a 'omg what happened?' way. Her reaction will tell you if you need to worry.

I would say that too

Low iron levels cause easy bruising!

I've had similar marks from martial arts, as well as bruised forearms also from martial arts and kettlebell training. I do sometimes wonder if people assume it's something else!

Just ask her.

Not always. I've had those bruises from sex, but I've also had the same type of bruises from being grabbed and thrown across a room by my violent (ex) husband.

If they're friends, she should really ask, just to check she's ok.

I don't see why it's wierd to ask. I would think it's weirder to see them and not ask. Hey you've lots of bruises, did you know they were there?

To be honest I don't really get what you expect the outcome to be. If she is being abused then she knows it and is allowing it to happen for whatever reason. You asking about a bruise isn't going to make her suddenly realise. She will probably lie to you anyway in that case. I just feel it's very unlikely she will break down and confide in you. I also find it strange that that's where your mind went straight away, people get bruises for lots of reasons. I feel there must be other concerns regarding her behaviour that make you think DV is an issue.

I have asked my mum that question.
It is up to you. Maybe she’ll tell you, maybe she won’t. Maybe she’ll lie, maybe she’ll tell the truth.
Nobody here knows. What’s the point in speculation?
Honestly, it’s really weird to be afraid to comment because they “might be from sex.” I don’t think it’s common to have bruises from consensual sex. Unless by “liberal” you mean you know she is into bdsm or something (in which case you know her well enough to express genuine concern surely), or have some other reason to think she is being abused, I can’t understand why your mind has gone here.
Weird thread.

“I really don’t mean to pry, and tell me to shut up if you like, but I can’t help noticing that you’ve had bruises on your arms a few times. I’m just concerned in case that’s a sign of something bad happening. If you need to talk, I’m listening.”

My upper arms, legs, back, bum and generally everywhere bar my head, feet hands and lower arms are riddled with bruises. Im about as vanilla as they come: I just bruise really really easily. Some people bruise easier than others.

I think this is for the best, OP. If it is (consensual) sex-related, you can acknowledge and move on. If it’s not, you’ll have shown her that you’re someone who she can turn to if she needs to.

I have bloody evil door handles , I’m hurrying up to do things I get horrible bumps and bruises on upper arms .
i really want to change each and every pigging door handle when I have spare £
I also have a dog that when he’s getting himself comfy , he’ll stomp about on my thighs ..then sit on my lap .
But if you feel you have a close enough relationship to ask , then ask her

And there you go
Kinky brusies.
Just ask already.

Same. Everyday brings a comment of "where the fuck has THAT come from" when I get dressed. I currently have a very large one on my outer thigh that I have absolutely no idea about. It wasn't there yesterday.

Some people bruise like a peach (me included!) If you are concerned maybe just ask her but try and not make that big a deal of it, something like ‘oh my goodness look at these bruises are you ok’ and see what she says, there could be loads of reasons for them.

Everyone is giving 1000 reasons people have bruises. And they all exist. But so does domestic violence. Just because you're a kickboxer on bloodthinners, doesn't mean someone else isn't. OP should make her mind up. Not be convinced it's nothing when it could be something.

I’m covered in bruises on my arms. No one has ever assaulted me.

Thin skin and blood thinners are a bad combination. 🤷‍♀️

I remember my mum having a black eye and saying the baby hit her with a rattle (which was true) no one believed her and thought my dad had hit her, much to her embarrassment.

I bruise easily and get asked about them if they show. On the rare occasion that there’s a reason I’d rather not discuss, I’ve covered up. If she’s not hiding them, it’s not unreasonable to ask. But be prepared that some people get off on talking about their sex life, and the conversation could be far more uncomfortable for you than for her.

That said, she wouldn’t be the first person to mistake sexual liberation with abuse, so I’d probably ask anyway.

I always have massive bruises but I’m in a BDSM relationship. My choice.

I currently have bruises ob the tops of both my arms and on both of my lower arms just above the wrists. It's from kettlebell training but looks like i've been grabbed violently! If it was a friend I would ask though!

could it be a contact sport? It sounds exactly like roller derby!! Teammates often hold their pals in place while someone pushed them from behind. My wall often leave handprint bruises on each other!