Unlike four-legged mammals that generally move with a horizontal spine, the human spine is uniquely shaped to allow us to move in multitudes of activities while standing or sitting upright. With the muscles and tendons providing structure, the spine can bend and twist and oscillate. It can also relax and allow us to sit for long periods. Unfortunately, our spine also has a good short-term memory about what it has been doing.
The tissues hold on to what we have been doing. What does this mean? It can take our spine as long as half an hour to get off the couch. Now this is not a mental problem, it's physical. After we've stood up from sitting, the tissues of our spine hold onto what we have been doing, making it hard to change activities.
For example, if we are sitting slouched at a desk or in front of the TV, our disks will slide comfortably into a round backed sitting posture. Our muscles relax and our ligaments become lax, making us very satisfied to stay slouched. We are turned off.
When we get up we need time to prepare our spine for sitting upright and taking the concussive forces of the trotting horse. The redistribution of the disk material takes time to adapt to an upright posture--up to half an hour.
The viscosity of our tissues also changes as we go through the day. Stillness brings about frictional resistance to motion within the spine and torso. This happens when slouching around and when sleeping. This is the reason, after sitting all day, we may feel stiff, have a hard time hoisting the saddle, or feel uncomfortable sitting the trot or canter. It is a good reason to warm up by walking, either on or off the horse. Once we begin to move and the ligaments have tightened and the "viscous friction" has been reduced by easy motion we can begin to take more load on our spine. Which means, take a few moments to go from bed to running, or from office to active rider.