Hamstring help. (Cramp)

[KenVar]

Hi guys.
Nee a little info on isolating my hammies.
Now I can hack squat (machine) 330lbs for 4x12 and I'm fine with stiff leg deadlifts, dbs and bbell, but when it comes down isolating them on a seated leg curl or a laying leg curl machine they just cramp up with severe pain and lock, no matter how light the weight is.
Is there a technique or an exercise I could do to stop this happening?

 

[GreaserPsycobilly]

I have the same issue. I got stuck on the gym floor once with a cramp I couldn't get out for 10 min. Left ham had a knot in it the size of a golf ball.
Try upping your potassium, bananas help. Also stretch, sounds lame but my old training partner had me talk to his chick who was a yoga instructor, not my thing but it really helped me lengthen out my hammies and avoided future knots.