We all know moving too little is bad for us. Why exactly? So many reasons...
Our joints don't receive direct blood flow. Instead joints are encased in little sacks of facia that are filled with synovial fluid. Synovial fluid is a slippery nutritious substance (some what like grease in a hinge) that allows the joint ends to move on each other with out friction. This synovial fluid is also nutrition for the joint (like blood would be).
The only way this nutritions synovial fluid makes it to all the cells in the cartilage of the joint is through movement. So if you don't move, like in the case of paralysis, the joint will literally freeze up and become unmovable and degenerate. Movement stimulates the cells with in the joint to produce the synovial fluid that nourishes it...and movement again distributes that synovial fluid throughout the joint. Just moving a joint through it's full ROM (range of motions) will keep it healthy and nourished.
When we hold one stagnant position for hours, it is bad for out body. I don't care if it is standing at a cash register, squatting to pull weeds or sitting at a computer. Any fixed position held for hours, day in and out, will give you trouble. Why? So many reasons...
The brain tells the body what to do. What ever you do, you get better and better at. So if you sit all day, your body will adapt to being really good at sitting. The brain will decide certain muscles and facia only need to be a certain length to hold that position. The tissue will adapt and respond by shortening. Or maybe layering on some extra fibrils or growing a bone spur because there is always tension on that particular tissue and it needs to be stronger. Before you know it, your body has taken the shape of the chair. When you do stand up, your hip flexors are short and taught, your butt muscles are shut off, your pectorals and biceps are short and shoulder blades rolled forward. Your head has repositioned itself 4 inches in front of your spine. You feel lousy and the easiest thing to do is sit back down.
But you are smart and you know you should exercise. So you try and go for a run. This may end in disaster because sitting for 8 hours and then jumping up for a run with all of the dysfunction you have been cultivating day in and day out can result in an injury. For so many reasons... too many to explain here.
First of all don't hold any position for more then 1 hour. If you sit. Gert up and go on a 15 min walk. Come back and sit for another hour. Then get up and take a 10 min walk. Then sit for another hour. Then break and take a lunch time virtual pilates class. Then get back to it. Break up the sitting with legitamit activities. Maybe alternate between a sit and stand desk also. Make sure your exercise moves you through full ROM (pilates and yoga are good at this) and strengthens the patterns and body positions you rarely use.