As someone who has actually backpacked in the original sense of the word, for over 30 years I think I have a fair amount of knowledge and experience on the subject.
First, carrying anything on one shoulder is wrong. It may be fashionable but it's stupid. The purpose of any carrying system is to distribute the weight as much as possible. So two shoulder straps are better than one and two straps and a hip belt are better than 2 straps. Look at any good backpack and look at the hip belt ( It is not a waist belt, your waist is above your hips although guys with beer bellys don't want to know that.). Do you see how wide it is, how it's shaped and padded? This is for a reason obviously and the only reason is to get the weight distributed onto the hips and decrease the weight on the shoulders. Look at the sternum strap on a good backpack. Why do you think that is there? To look good? To simply keep the shoulder straps from slipping off your shoulders? No, it is there again for a very good reason, to keep the shoulder straps in the correct position.
Second, a backpack must fit the individual. Each person has a longer or shorter torso. The best backpacks have a means of adjusting the distance between the shoulder straps and hip belt to account for that difference. Suppose a backpack does not have an adjustable system. You put the pack on with the hip belt in the correct position (resting on the top of your hip bones) and the shoulder straps go upwards from the top of your shoulders to the pack. That is a pack that is too long for your body. Similarly, if you put the hip belt in the correct position and the shoulder straps meet the pack down by the bottom of your shoulder blades the pack is too short. (you may have seen a lot of school kids carrying a pack with the shoulder straps half way down their back. It's the fashion. In their case the pack is actually too big but they are just letting it dangle down past their ass.)
The hip belt should rest on the top of your hip bones (good ones will have contoured belts that curve around your hip bone) and the shoulder belts should meet the pack at the top of your shoulder blades. Many packs are not adjustable or not adjustable enough for someone who is taller or shorter than average. I cannot stress too much how much difference it makes to comfort and efficiency to have a pack that fits properly. Unfortunately, a lot of sales clerks have no idea of how to fit a pack properly and of course most consumers buy based on price and looks.
I do not know what you actually mean by a 'rucksack'. It has a specific meaning to me but I know many people use the word to mean anything from a small daypack to a large backpack. If I go by the weight of 16kg. you mention it still doesn't tell me much since nowadays people can have that much in a small daypack if they are carrying a laptop, camera, etc.
While a daypack can easily carry that much weight it is in fact not designed to carry it efficiently. They rarely have a hip belt since they are generally too short to reach from shoulder blade to hips. Some of the 'Camelback' type packs are long and slim to reach from shoulder to hips but even they don't normally have a proper hip belt. It's usually just a small waist strap that holds the bag close to your body but does not transfer any weight off your shoulders and onto your hips. They're more suited to running with water and an energy bar in them. In other words not a lot of weight. The less weight you carry the more you can get away with as far as efficiency of the pack in transferring the weight to your body properly.
I would not reccommend carrying 16kg. in a daypack for any length of time (over an hour). At that kind of weight you should be using a properly fitting backpack, particularly if you have a back problem of any kind.
You can get into all kinds of technical details on sites like this one: http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=2
Or find all kinds of websites by Googling for 'backpack suspension' that discuss pack fit and weight distribution. In backpacking (wilderness) circles this has been a major topic for decades. So read to your heart's content if you want.
But I'll give you my number one best piece of advice on backpacks. Learn to carry LESS WEIGHT. In backpacking (wilderness) circles there is a common definition of the ultimate backpacker. It applies equally well to the tourist with a backpack I think.
"He would carries the most comforts with the least weight." In other words if you can't feel comfortable without your french cafetiere, make sure it is the lightest weight cafetiere there is. I know people who cut half the handle off a toothbrush and drill holes in the remaining half to reduce weight. If you think I'm kidding, read here: http://www.backpacking.net/tips.html#toothbrush
Read through the tips to see just how fanatic people get about reducing weight. In particular though read the tip about footwear. It is in fact a widely held belief that losing 1lb. off your fee is the equivalent of 5lbs. off your back. Basically, the theory is mathematical based on the amount of energy needed to lift the weight on your foot with each step you take. Less to lift, less energy used, the better you feel.
