Flexibility is crucial in reducing injuries and maintaining a healthy body. The yoga mantra of "I bend so I don't break" certainly has some merit. When your joints and muscles are more supple and flexible you have a greater range of motion available to you. This increases your body's "buffer zone". For example, if putting a saddle on your 16.2hh horse requires 80% shoulder mobility and 20% back extension (100% total movement), you can easily see how if you had poor shoulder mobility, say 60%, your back would have to extend to 40% (i.e. twice as much) in order for you to complete the task of tacking up.
These compensations happen in our bodies all the time, without us noticing. Sometimes they don't cause a problem, but more often than not, they do. In the example, the rider's back would fatigue more quickly because its working twice as much as it should. There may not be a problem at the time, but weeks or months later a simple task like reaching the top of the bridle may cause there back to spasm very painfully.
Having an awareness of your body's limitations and working on specific areas of stiffness and tension will help to reduce pain and injury in the future. There are only so many compensations that the body can make, and once it reaches a threshold, pain occurs. People often are confused at this point and wonder why they are in pain..... "I haven't done anything, I don't understand why it hurts" is a phrase I commonly here. The body has simply be overloaded.
So, for the next exercises
1) Downward facing dog - yoga pose where your hands and toes are on the floor and you push your bum up to the ceiling. There should be a straight line from your hands, through your arms, shoulder and spine to the top of your bum, and then from your bum straight to your heels. If this is too painful on your hamstrings and calves, bend the knees slightly.
2) Shoulder mobility - try to interlink your fingers behind your back, one reaching up your back and the other reaching down from your shoulder. Switch sides and see if your hands can meet this way..... is your writing hand worse? Can you do it at all? Is there a pulling in the front of your shoulder?
3) Seated Twist - sit on the floor, bend your left knee and put your left foot on the outside of your right knee. Twist to the left, keeping your back straight, using your arms to hold onto the left knee if necessary. Swap arms and legs and twist to the right. Is one way easier? Did you feel a block between your shoulder blades? Did you lean backwards in order to move further round?
Practicing these flexibility exercises or at least doing them as a stretch in the evening after you ride, is a great way of reducing stiffness and making sure you are equally supple on the left as on the right.