During intervals I usually try to be way up in HR during the interval, and then during the recovery I drop down. Your intervals should be spent upwards of 100 cadence, and recovery down around 80. My intervals tend to run about 90-95% Max HR. You dont want to be mashing at 75 cadence during an interval. Usually the whole point to interval training is to increase your VO2 max, so you need to be breathing heavy and running a high HR.