What I would do is after a few regular sets, do 1 to 2 sets of strictly negatives, 3 to 4 reps per set! I don't recommend that you do negatives in the same set as regulars because you will not have enough strength to really control the weight to get a good negative. And when doing negatives you want a weight that you can fully control on that phase of the lift. Let your spotter handle most, if not all, of the positive phase, so you don't want to pick a weight that is too heavy! lol But no matter how heavy the weight is, if you do negatives with a slow and control manner then your muscles will definitely respond! Good luck
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If you build it they will come