Substitute exercise for tuck ups

I'm currently doing the Foundation 1 series.
I have been diagnosed with chondromalacia patella and fully bending the knees at the top of the tuck up causes me pain.
Is there any substitute exercise to the tuck up that doesn't require the full bend of the knees?

Thanks

Hi Diogo, simply reduce the amount of bending in the knee, is impossible to substitute this variation.