That doesn't sound like spasticity. It sounds more like... hmm... clonus? Myoclonus? May be myoclonus.
At any rate, it's an involuntary movement of your leg caused by the quads, not the muscles behind the knee. Any spasm of the muscle behind the knee would cause it to bend up.
Most important thing is not to try to define your symptom before you visit the neuro. Tell him/her exactly what happened, and let them decide what it is.
Spasticity is essentially a resistance of the muscles against stretching out. For me, it manifested as unusually tight and sore muscles, in places where a sore muscle was odd. First it was a long muscle in my forearm. Then it was my thumbs. Then it was my calf muscle, and right now it's my calf, my torso, and the muscles right above my hips.
http://www.medhelp.org/health_pages/Multiple-Sclerosis/Muscle-Tone-And-Spasticity/show/159?cid=36
This is a great article that will help you understand spasticity and how it works.