I have a love-hate relationship with "up and down planks". Well, I mostly hate them. But I do them because they are so effective in building core strength. They also work your arms as well because it is almost like doing a pushup.
If you want to add an extra level of intensity, attempt to get through these as fast as you can. You can break them up into sets of 10 and do 3 sets before going on to another exercise.
Cheers Eights &Weights!