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If you prefer other abdominal exercises like crunches, that’s okay too. Good riding is abdominal riding. This means that you need to really use your abdominal muscles to balance yourself, while clinching the horse with the legs when needed.  Doing sit-ups with a work-out ball is up to 20% more effective. Even if you don't practice yoga regularly, holding a few key positions, like the boat, half-boat, or warrior, for a few seconds a day can improve your fitness.   As an added bonus, doing these moves correctly can also strengthen your back and improve your posture! These also help open up the shoulders, which is good for people  who do a lot of slouching. It is easy to become complacent about posture when sitting at a computer or watching TV, but these are some of the best times to work out your back and shoulder muscles by simply sitting up straight and opening your chest. Riding is an aerobic activity, so the longer you are able to maintain your poster, the better equipped you will be for riding. This is a good exercise for the back and core, and it doesn't require as much coordination as yoga. Lie with your back on the floor and your knees flexed. Now lift your knees towards your head instead of the other way around. Repeat as many times as you would do a normal sit up. This move is better for your abs and doesn't shorten your hip flexors. It's important that those are loose for riding. This is often considered yoga move and it is great for your core muscles. Put yourself into the push-up position but instead of leaning on your hands, lean on your forearms. Push yourself up so that only the balls of your feet and your forearms are touching the floor. Contract your deep core muscles and keep your back flat, making sure to not let your lower back arch or your hips droop toward the floor.  Hold for about 45 secs and repeat a few times a day. (If you can't do an elbow plank without your lower back hurting, start with an easier version, which is just holding the upper position of a push-up.) Try the plank on horseback in the jockey position, and grab the neck for comfort to the horse and the rider. This will gain the forearm to help you stay balanced on the horse for 20 secs. This is also good for people who are into show jumping and approaching a jump.
Spend five minutes two or three times a day doing sit ups. Try using yoga poses. Practice good posture. Do reverse sit-ups. Try the plank position.