Updated: Jun 5, 2020
Stiffness is almost always from not moving enough.
1. When we are sitting or sleeping, we typically have our knees and hips bent/flexed. This allows the flexor muscle groups (eg. hamstrings, groin muscles, calf muscles) to be in a shortened position. The longer they are in this state, they will adapt and the fibers will want to stay in a contracted state.
2. The heart pumps blood to the tissues via the arteries. However blood returns to the heart via the venous system which is dependant on muscular contraction. So if you aren't using your muscles, your blood isn't circulating to and from your tissues efficiently. This leads to lowered muscular hydration and metabolic waste build up.
3. Connective tissue in your body, such as tendons, ligaments, spinal discs, don't have a great direct blood supply. This is why joints such as knees shoulders and ankles take so ong to heal. They get their nutrition passively from the surrounding tissues. So if you aren't moving much, they will tend to dry out leading to poor joint lubrication.
Most of us find that if we've been sitting too long, by moving around and doing a bit of stretching, we get over the stiffness. We are effectively rehydrating/lubricating the muscle and joint tissue and stretching out the muscle fibers to an optimal length. However, with prolonged inactivity these changes such as shortened muscles, become more permanent and harder to change leading to poor posture and early joint degeneration.
So stay active and keep stretching!