I’ve come across several subs who can in fact make injury happen by solo impact.
But let’s go with impact basics first.
Hand, flogger, plastic hair brush, wooden spoon, crop, whip, thinner 1/4” dia canes. Will result in a stinging surface effect. This is likely to not ever become injury. You will be able to break skin with excessive repeated work but you’ll be able to see that coming on and either stop or proceed. Just be sure to keep clean toys and clean off the cuts after play to prevent infection.
Thuddy items like heavy paddles, books, meat hammers, rolling pins, thicker solid wood canes 1”and up dia, metal bars, and things of that nature will do a deeper tissue bruising. You’ll feel more a solid deep hit as it contacts and less of the surface stinging as the prior toys. Use care to not hit the tailbone with these type implements. Do not us on lower back tops of feet etc. only soft fatty areas. Tits ass thighs. You don’t want broken bones from hitting in the no zones.
Risks?
Light stingy is just broken skin and maybe some day or two long light lines so surface marks. If you break skin then obv this healing will take longer.
Heavy thuddy. Bruising can last days weeks even months before fully cleared up. I have a sub who had light brown bruises on body 4 months post impact play with my hands on.
As far as how far can a sub go doing solo hits? It varies by person. I find in the right mindset and if one is directed or is doing this for a doms pleasure or hits subspace they can absolutely bypass the bodies natural instinct to not harm it.
Risk with heavy bruises is clots shifting so take care to not go over deep bruises again with thuddy heavy impact the very next day until it’s had time to disperse.
This is over all a RACK practice approach so just understand what you’re capable of and be sure you have aftercare in place.
One final caution tip. Don’t do impact with a metal buttplug in. Pinching and injury is easier with that interference.