There's two extremes I notice when it comes to armour. There is the one extreme where armour is portrayed as being bulky and hard to move in such as the knights armour. And there is the other extreme where since armour was made to fit person for persona and to be distributed evenly so that even a 100lb armour would not feel heavy and be so light that you can do cartwheels, hand stands, jumps, run, and even fancy acrobatics. That armour is so light that someone who's not conditioned would feel its like wearing a T-Shirt.
So when I found my sister's weighted vest that totals to about 20 lbs, I decided to test it out. At first it did not feel heavy at all and it felt so light I can jump around it and even walk 2 miles without feeling exhausted. So I thought real armour must be as light as the other extreme is, so l thought plate armour was lighter than a shirt.
However once I started crouching and doing other prone movements to test swordsmanship and aerobics I began to feel pressure. In fact I was surprised as hell how tired I got just doing squats and practising low level attacks. In addition when I tested running, it suddenly felt so heavy. Not as heavy as Hollywood portrays mind you but I began to wonder if some of the tests such as the link below had validity.
http://www.bbc.com/news/science-environment-14204717
As I was finishing my first mile and I reread the above article while I was resting, everything was so spot on.
Also trying to do high level acrobatics such as jumping over hurdles in a track field and some of those fancy gymnastics was almost impossible.
So it makes me wonder how wearing an armour would be like. I know its a running vest I used that had pockets filled with metal bars that totaled 20lbs, far less than a typical breastplate so its a different tool. In addition I'm not exactly a nerdy waste as I lift weights enough that I can curl 2 sets of 50 lbs dumbells casually and benchpressing a barbell with 50 extra weights on both side for 100 reps ain't hard. So does that explain why wearing the vest was initially not difficult?
I am so curious how armour felt like but don't have money right now to buy it so I ask people with experience here!
Medieval armour was relatively light compared even with your belief in its weight (85lbs would be the top for a tournament foot harness, while 60-66lbs is more like what a field harness would have weighed. See here for more details), and it definitely didn't restrict the wearer's movements to any significant degree. This has been recognised for a considerable period of time. The Metropolitan Museum of Art made a film showing how light and flexible a suit of plate armour was in 1924, and modern re-enactors have shown as much - including cartwheels.
Of course, with that said, you can't expect to wear over 60lbs of armour without suffering some repercussions. A simple example is Daniel Jaquet's recent testing of a man in full plate, a fireman and a soldier, all carrying similar loads and percentages of bodyweight. With full equipment, the fireman's time was increased by 1.87, while the man in armour's time was increased by 1.96 and the soldier's by 2.32. The latter is quite a significant increase in time, and both the fireman and soldier had moments where the weight distribution of their equipment made the obstacles less easy than for the man in plate.
Regarding the news article, and the paper that inspired it, there are a couple of issues with the test. First and foremost, the armour was too heavy. The Churburg armours were generally non-representative of medieval field harnesses, and Churburg 19 in particular was heavier than many tournament armours, despite being designed for the field. The two Tower harnesses listed were much more in line with extent harnesses, but these weighed less than any of the armours involved in the test. Of those, only the suit of armour labelled "AB" came close to a historical field harness weight.
Another major issue is the weight of the leg armourand how it was attached to the body. The armour "AD", for example, had close to two kilograms more weight in the greaves, sabatons and cuisses than the historical equivalents the armour was being compared against. This is vitally important because, as this study by S. J. Legg and A. Mahanty shows, carrying weight on the feet or lower legs is significantly more strenuous than carrying it on the body. This increase in weight was further exacerbated by the leg harnesses being attached to a modern belt, instead of purpose made attachment points on the arming doublet. This increased the strain on the bodies of the test subjects.
A similar test was conducted by Daniel Jaquet et. al., and the increase in energy expenditure was found to be an average of 66% when walking in armour. This is roughly half the maximum increase found in the other test. Although the authors do not believe field harness was as heavy as those tested by Askew et. al., they used a harness of similar weight to provide a proper comparison between the two tests. The biggest difference was the appropriate attachment of the leg harness (taking a good deal of weight off the legs) and taking extreme care in replicating the original suit of armour.
In addition to /u/Hergrim 's excellent answer, I'll just add that your 20-pound vest is about equivalent to the mail shirts worn through much of the Middle Ages. ~60 pounds of plate represents the upper end of armor weight in the period.