Active Sitting or dynamic often refers to sitting on a ball or unstable surface. I'd like to redefine this term because I like my definition better. Active Sitting or Dynamic Sitting refers to movement variation while sitting. Active Sitting to me means to sit in several different postures at your desk during your work hours.
If you have a sit/stand desk even better, now you have even more postures to choose from.
Sitting for a long time is bad; here are a few reasons why:
1. Abdominal atrophy - this means your abs weakened and get smaller in size
2. Sustained postural overload on your spine. This decreases joint lubrication and spinal joint stiffness.
3. Circulation of blood and lymph decreases particularly in the lower extremities, thus contributing to leg complaints while sitting.
Why is Active Sitting/Sitting variations better
Loading your joints several different ways, all day long will have less of a negative impact on your body as a whole. You will be loading joints differently, as well as, muscles, tendons, ligaments and all of the cells in our body. 1 sustained rigid posture doesn't do this. So this means, cross your legs, cross your ankles, sit forward, sit back, sit with one leg up and then the other ( make sure to do all one legged postures on both sides) and then do this all day long.