We recommend engaging your muscles with an isometric hold for 7-10 seconds at 60%-80% of your maximum effort. Follow your isometric hold with 10 slow and concentrated repetitions. The isometric hold will tear your muscle fibers and the reps will continue to work your muscles with further pump.

You can also hold the last rep or hold before and after reps. These are both great ideas.  

Reps without isometric holds are beneficial by engaging different muscles and having a rhythm. Mix up your routines and concentrate on your muscles being exercised. Form is even more important than reps and hold time.