Cleveland Clinic uses the terms interchangeably. It's an involuntary muscle contraction. The body doesn't make a distinction between spasms/cramps, that's a human distinction. MayoClinic doesn't even have a dedicated "Muscle spasm" article, all they have is "Muscle cramp", and when googling for "MayoClinic muscle spasm", the first result is their "Muscle cramp" webpage.
Spasms: "Muscle spasms (muscle cramps) are painful contractions and tightening of your muscles. They’re common, involuntary and unpredictable".
Cramps: "Muscle cramps are sudden muscle contractions. Also called muscle spasms or charley horses, a muscle cramp can be a common symptom of many things, like exercise strain or a medical condition".
I don't seen any evidence for the claim that spasms are a unique, protective response to avoid further injury, or that spasms are fundamentally different from cramps. Reading what I can find online, it seems spasms/cramps are caused by a wide range of things - diet, hydration, overuse, lack of sleep, stress, and yes, in some cases structural injury.
So based on that, getting hit hard in the square of the back on a hot, humid day, seems like a perfectly reasonable explanation for back spasms (cramps) without any structural injury. Apparently the diagnostics confirmed as much.
Find out what causes muscle spasms and cramps and what you can do about them.
my.clevelandclinic.org
Find out what causes muscle spasms and cramps and what you can do about them.
my.clevelandclinic.org