Hi Ray,

Dont mind me but I am going to go out on a limb here and throw out my text books to answer this. (I have a feeling you are looking to define the characteristic of the contraction that you see in terms of "cause" as opposed to symptoms)

To me it all comes down to who is communicating to whom and what is the message.

A spasm in my opinion is one of those crazy defensive mechanisms where the brain gets carried away when it applies a splint to a joint in order to prevent range of motion. The increased tone will decrease blood supply to the deeper portions of the muscle (which is due to pressure as opposed to capliary distribution) and create a painful cramp. The brain can splint a joint without spasming as long as the pain/spasm cycle is not initiated (as in the thaw stage for Wikipedia reference-linkfrozen shoulder).

An involuntary movement is a reflex only response where the brain is bypassed when initiating the movement. Here the spinal cord directs the contraction and not the brain to remove the muscle/body part from the stimulus.

They are both used to prevent injury to the body. The spasm prevents injury caused from within and the involuntary movement prevents injury caused from without.

Now we can go on into every exception to the above statements but what we see on a day to day basis with normal healthy adults can be put into one or the other category.

Adamo