About Heavy Armor (Sourced from Metmuseum)4. Armor is extremely heavy and renders its wearer immobile.—Wrong.
An entire suit of field armor (that is, armor for battle) usually weighs between 45 and 55 lbs. (20 to 25 kg), with the helmet weighing between 4 and 8 lbs. (2 to 4 kg)—less than the full equipment of a fireman with oxygen gear, or what most modern soldiers have carried into battle since the nineteenth century. Moreover, while most modern equipment is chiefly suspended from the shoulders or waist, the weight of a well-fitted armor is distributed all over the body. It was not until the seventeenth century that the weight of field armor was greatly increased in order to render it bulletproof against ever more accurate firearms. At the same time, however, full armor became increasingly rare and only vital parts of the body, such as the head, torso, and hands, remained protected by metal plate. 

The notion that the development of plate armor (completed by about 1420–30) greatly impaired a wearer's mobility is also untrue. A harness of plate armor was made up of individual elements for each limb. Each element in turn consisted of lames (strips of metal) and plates, linked by movable rivets and leather straps, and thus allowing practically all of the body's movements without any impairment due to rigidity of material. The widely held view that a man in armor could hardly move, and, once he had fallen to the ground, was unable to rise again, is also without foundation. On the contrary, historical sources tell us of the famous French knight Jean de Maingre (ca. 1366–1421), known as Maréchal Boucicault, who, in full armor, was able to climb up the underside of a ladder using only his hands. Furthermore, there are several illustrations from the Middle Ages and the Renaissance depicting men-at-arms, squires, or knights, all in full armor, mounting horses without help or instruments such as ladders or cranes. Modern experiments with genuine fifteenth- and sixteenth-century armor as well as with accurate copies have shown that even an untrained man in a properly fitted armor can mount and dismount a horse, sit or lie on the ground, get up again, run, and generally move his limbs freely and without discomfort.

There are a few exceptional instances when armor was extremely heavy or did indeed render its wearer almost "locked" in a certain position, such as armor for certain types of tournaments. Tournament armor was made for very specific occasions and would have been worn only for limited periods of time. The man-at-arms would have mounted his steed with the aid of his squire or a small step, and the last pieces of his armor could then be donned after securely sitting in the saddle.