Oswald in the doorway: the blog of the Oswald Innocence Campaign, by Ralph Cinque

Why was Kennedy so stiff? He brought his hands up to his throat in response to being shot in the throat, and it seems he tried to cough to dislodge the bullet. But then, he couldn't put his arms down. He seemed to be breathing all right at that point, but he couldn't put his arms down. He remained cringed, like he was in a spasm. You see Jackie putting her hand on his arm to try to get him to lower it, to put it down, to relax, but it just goes on and on until frame 313 and the fatal head shot. 

Did they shoot something into him to put him in spasm? If not, what caused him to tighten up like that? You can't say it was the stress of the situation. Connally also got shot, and he didn't stiffen up like that. 

There are a lot of drugs that can cause muscle spasm. 

http://www.rightdiagnosis.com/symptoms/muscle_spasms/side-effects.htm