Hi ! As the creator of the "eYo- equiYoga and Fasciatherapy for horses and riders" method, I do teach many stretching and toning exercises. You can learn more about it on my website : eyo.lachevalerie.org or on the Facebook page called " equiYoga ".
Hi ! I found this exercise that help my back a lot, you begin seated with legs out in front of you. You bend your left knee towards your chest and place over your right leg, but keeping your hips square to the front of the room. Then you place your right arm slightly behind your right hip for balance, and you can place your left hand on your knee or left ankle. Switch sides and repeat !
I hope it was clear ;)
I try to do 5 minutes of stretching before every ride, it really helps ! For your back, you can lie on your back with your knees to your chest. Wrap your arms around your legs just below your knees and pull your knees toward you. Hold this position for a few seconds and then relax. You can repeat it 3 to 5 times!