A convulsive movement is jerky and uncontrollable. A convulsive sob is the kind where your body shakes when you cry, and convulsive dance moves at a party might scare your partner away.
Sometimes our bodies move in ways we can't control — like a convulsion, which is a quick, jerky muscular contraction. These contractions are also described as spastic and spasmodic. Movements that are like convulsions can be called convulsive. When you throw up, your body makes convulsive movements. But not all convulsive movements are bad — convulsive laughing is the really good kind. Convulsive movements are powerful; they're much bigger than tremors, which are also involuntary movements but are small.