Electric Muscle Stimulator
When I first started having light weakness in my legs I brought then what was a top of range Electric Muscle Stimulator (Compex 8.0)
Physiotherapists use them and athletes.
Cost around 1k at the time but can pick them up around £700,even cheaper ones probably do same at around £200
When it starts up it does a muscle respons to electric impulses test to see what level to start off at. I always noticed my left bicep and quad was around 50% less responsive.
Turns out this little machine was already telling me I had muscle damage as mri scans later showed exact issues with muscle breakdown on those areas.
I find it really useful and use it on all muscles at different settings to contract the muscle and even build up muscle strength and conditioning.
I highly recommend it