Weird bicep problem

[WannaBPowerful]

I dont know why this is happening, maybe someone on here can figure this out for me. I normally start my bicep workout with barbell or cambered bar curls up to 105 x 10 reps VERY strict and controlled. Now i avoid preacher curls because whenever i do them, i cant get 8 reps with 50lbs. Its so weird. Im all over the place with the weight, my armpits are locked into the pad, and i feel the tension let up at the top of the movement no matter how hard i squeeze. Any idea at all about what is going on?

 

[gymtime]

There's a big difference between standing curls and preacher curls. For example, you say you feel the tension let up at the top of a pc right? Well, look at what's supporting the weight at the top of that lift. Is it your bicep? no. It's the forearm bone. At that point, your muscles are only being used for balance.

This would not be true at the top of a standing curl.