Lets start with cupping, because I think it'll help lead us into IASTM/Graston/Gua Sha. Cupping has been around for a very long time! They used to use bamboo cups or clay cups before we moved to glass, then plastic and silicon. The idea was that you had to bring sickness to the surface in order to get rid of it. Think like blood letting or using leeches to suck out tainted blood. Chinese medicine had the element of helping with stagnant Chi flow (our life energy that flow through our body).
As we got into more modern times though we learned that we have natural cleaning systems like our kidneys, spleen, and lymph nodes that are constantly working to clean out any problematic things in our body. The idea of needing to bleed out toxins gets voided out by this knowledge. Now, we do have superficial lymph vessels that help clear things out so this is some peoples attempt to justify the idea of needing to bring stuff to the surface, but we also have lymph vessels all throughout the different layers of our body so that doesn't apply either.
The next step was to try and integrate Eastern and Western medicine and say that instead of working with Chi flow we're gonna work with blood flow, so cupping helps "stagnant blood". Again, based on modern information we have a closed system of blood vessels where blood is continuously pushed through the system. You shouldn't have stagnant blood anywhere, unless you have a blockage which leads to tissue death like in a heart attack. There goes that idea too.
So now onto the bruises! People who are deep into cupping will usually make the claim that they're not bruises, it's different. Well it's not. Its actually another form of stagnant blood because the cup has ripped the blood out of the vessels into the intercellular spaces to form a bruise. Instead of preventing or helping stagnant blood they've actually created it. Now the body has to deal with whatever the original complaint of the client was plus it has to clean up the spilled blood. Which leads us to the next part where people will claim that the process of cleaning up the bruise has medicinal effects. There is an enzyme called Heme oxygenase 1 or HO-1 for short. It's job in this scenario is to help clean up the mess. Studies have found that there may be an analgesic effect of Ho-1 or even possibly it's counterpart that shuts down HO-1. But in those studies they are doing injections of massive amounts of HO-1 into an injured area, way more than would be present in a cupping bruise. Not only is it still theoretical and being studied but the dosing would be wrong anyway.
This leads us back to our question of does cupping need to leave a bruise. My opinion is absolutely not. The focus should be on the decompressive effect of the cupping and the neurological stimulus it provides. If during that time the client/patient has a bruise appear then oh well. But the focus shouldn't be on creating a bruise. The amount of suction seems to be directly related to how dark and full the bruise becomes, but a high amount of suction isn't required to have the effects we want. a low to moderate amount is plenty depending on if its static cupping or if you need a little more because you're going to have your client move around while they have the cups on.
Hopefully that makes sense. We need to shift our mindset since we've learned a lot about the human body and it's physiology so the narrative of what the therapy is doing needs to be updated.
Same goes for IASTM/Graston/Muscle Scraping/Gua Sha. Gua Sha is the chinese medicine version of the modern Graston or IASTM. The belief is similar to that of cupping where you have to cause bruising in order to help people clear out the toxins or sickness. The biggest difference between scraping(lets use that term to cover all the different names) and cupping is that is compressive versus decompressive. But the same effect can happen, it forces blood out of the vessels and capillaries into the intercellular space leaving a visible bruise. "It's not a bruise, it's petechiae!" They say. It's blood that is no longer in the vessels. Same argument as cupping, same HO-1 enzyme brought up for the same reason, same logic to refute it. But this time I don't think it's ok to leave a bruise. You can provide someone a scraping treatment and stop if things look like they're starting to bruise. Cupping its a little more of a guessing game. Scraping can be a lot more controlled so there's no reason to leave marks on someone. Treatment philosophy is similar with cupping, the reason the treatment works isn't because we are changing the anatomy of what's going on in the area but its a physiological response to the stimulus you're providing. Because of that you don't have to go crazy on someone in order to get the response we want, which means no bruises.
If you are a practitioner, give it a try for yourself. Think of it as providing the minimal amount of of stimulus to create a change. Don't go overboard "just in case". I think you'll find like I did that we can get the exact same response without being as aggressive. Give it a go and let me know!