|Human beings are wonderful but fragile creatures. To function at all, we must have oxygen available to breathe at sufficient pressure so that our muscles used in breathing are strong enough to expand our lungs to inhale that oxygen. To function best there are other requirements too: temperature control, micrometeoroid protection, communications between each other and with planet Earth to name a few, but let's talk about breathing.
The suit was pressurized to 3.7 pounds per square inch with 100% oxygen. That pressure provides us with about the same partial pressure of oxygen we all experience right here on planet Earth right now. This 3.7 pounds per square inch pressure, 100% oxygen environment provided a healthy work environment but makes the suit hard to bend. To get a feel for the forces involved, consider that a small toy balloon has an internal pressure of less than one half of a pound per square inch and we know how stiff it can be to reshape.
Enter the suit engineers. They came up with a brilliant solution. By incorporating an ingenious array of constant volume joints and cables and pulleys the rigid suit was transformed at just the right places. It could be made to bend with manageable forces at the shoulders, elbows, wrists, hips, knees and ankles.
Running from worksite to worksite was fun because our marvelous spacesuit design made the best of our very demanding requirements as humans.