Wondering why quadriplegic person has muscle spasm and what to do about it?

Muscle spasm: Even with quadraplegia you can had muscles spasms. If not severe then nothing to be done. If the spasms have become severe and affecting life style then a consultation with a neurologist would be helpful as there are some medications and injections that might prove helpful.

The disruption: Of signal from the brain, depending on where it happens, can lead to spasms in muscles. One of the tough things about being a quadriplegic, on top of so many other things, they can have really bad spasms from large disruption of "upper motor neurons", leading to these spasms. Oral medications like baclofen, botulinum injections, pumps can help. Look for other issues when spasms get worse, like uti.