A common piece of Victorian whimsy was that the knight in armour needed a crane to haul him up into his saddle. In reality a suit of plate armour might weigh around 20-25 kgs. This is actually less weight than that of the equipment that a modern soldier or firefighter might carry. Certainly I have carried rucksacs of this weight when necessary. The weight of a suit of armour would have been better distributed than that of a modern pack.
Many years ago I read about the feats of a knight called “Jean le Maingre” (ca. 1366–1421), aka “Maréchal Boucicault”. Wearing armour he could climb up the underside of a ladder only using his hands.
“Boucicaut or Jean de Meingre marechal de France who commanded the vanguard of the French army at Azincourt in 1415 and was there made prisoner and died in England in 1421 used to go up on the lower side of a ladder leaning against a wall without touching it with his feet but only by jumping with both his hands together from one bar to the other and that he would do armed with a steel coat and having taken off the armour with one hand alone he could ascend several bars and these things are true and by many other hard exercises of such sort he so hardened his body that his equal was hardly to be found….
Boucicaut at one time used to accustom himself to leap in armour on the back of a horse and often he would walk or long distances to give him long breath and enable him to bear fatigue. He also used to strike for a long time with an axe heavy hammer to harden his arms and hands and to accustom himself to raise his arms readily. By following such exercises he strengthened his body so greatly that in his time there was no gentleman to compare with him. He could throw a somerset completely armed except his basnet and would dance when armed with a steel coat In full armour and without putting a foot in the stirrup he would jump on the back of a war horse. He would also jump from the ground astride on the shoulders of a big man or a tall horse without other help than a hold of the sleeve of a man's jerkin. Holding with one hand by the pommel of a saddle placed on a high horse and with the other grasping the mane a little below the ears he would from the ground jump through his arms to the other side of the horse and he would ascend between two side walls of plaster at the distance of a fathom from each other and by the force alone of his arms and legs without other aid without falling either going up or coming down."
Extracted from his Life pour servir a I Histoire de France
An Introductory Course of Modern Gymnastic Exercises
By George Roland
Boucicaut was obviously an exceptional individual but it is evident that the wearing of armour was much less restrictive than many people assume. Most of us have seen Medieval illustrations that show armoured fighters ascending ladders during an assault on castle walls. Illustrations also exist of knights mounting their horses without need for assistance.
A friend of mine sent me this video which shows two re-enactors performing various movements in armour. Bear in mind that these gentlemen do not wear armour as often as a Medieval fighter would have, yet still can move around freely.
Here are some additional videos I found.