There are both simple and complicated reasons why you should combine various workouts using light and heavy weights
. It goes without saying that lighter weights obviously mean more reps and even more sets if you are going to keep the current overall intensity of your workouts.
There are basically two types of muscle hypertrophy (muscle growth) that you can get from pumping iron on a regular basis. The first is the standard working out with reps less than ten in a set which is called "sarcomere hypertrophy" by the sports scientists and it means that the fast twitch or red muscle fibers will get stimulated primarily.
However there is also the slow twitch muscle fibers or white muscle fibers that need and should be stressed on a regular basis. This is going to happen when you do lots of reps like more than 12 and this means that you will be getting the mechanical stimulation in order to sufficiently stress these slow twitch muscles to get results.
We all know that to build muscle it takes progressive resistance and that basically means Mechanical Stimuli which leads to cell damage which then results in a clean-up and repair of the damaged cells and the result is cell growth. The reason why the cells grow is to be able to accommodate the stress the next time it is given.
There are basically two schools of thought on how these heavy and light workouts should be combined. The one school of thought will tell you that you should only train heavy a maximum of once a week and the rest of the week to train "light" which means no less than ten reps per set.
But there is another school of thought who believes that you will get the best benefits by combining light and heavy in the same workout. What this could mean is that you start off training 20 reps then 15 then 12 increasing the weight all the time. Then you start going seriously heavy as you add more weight and go to 8, 6, 5, and even 3 reps.
But after all the constant debate that is always going on about the best way to combine heavy and light, it seems that science has still not proved its effectiveness one way or another. What this means is that you should try both methods and see which works better for you and gets you the best results.
It should be noted that the people who say that you should only train heavy once a week actually mean that you should only train one body-part heavy once a week. This means that when you fail on bench-press with 5 reps or less that you then train back or legs heavy a week later and not chest again.