Breaststroke on the Pilates Reformer is one of my favorite exercises. It's a super challenging extension exercise in Pilates. But, it is possible for you to do it. And, often, you just need to start with the prep version. It is still a challenge to do the prep but it absolutely will help you to get strong enough to do the full Breaststroke. Before you do this prep, make sure you are practicing your Swan, Pull Straps, and Double Leg Kick. All of those exercises will make the prep and full exercise more possible. And, you also can do this prep without straps and use 1 or 2lb weights.
Reach your arms from your back, kick your legs from your seat.
Place one spring (heavy) on your Reformer and place your long box on. It would be on from the other long box exercises that come before it. Standing to the side of your Reformer, hook your thumbs in your handles and then reach forward to place your hands on the front of your box and then lay down on the box. Your knees should be at the edge of the box. Your hands can hold onto the edge of your carriage. This will help you connect your arms to your back. Press your pubic bone into the box so your back is long.
Kick your heels to your seat 3x. Then reach your arms straight ahead like you are one longboard. Open the arms to the side and replace your hands to the carriage. Repeat 2x. This is different from the full exercise because you are not lifting your chest. For the reverse, kick 3x and then reach your arms wide to the side and then forward. Replace your hands back down to repeat the kicks. Repeat 2x. Again, don't worry about lifting the chest. Just get the arms reaching from your back and practice reaching your legs from your center. When you're ready you'll feel the extension happen and can move into the full exercise.
Be sure to check out more of my online Pilates Reformer exercises. I want to help you continue your Pilates Reformer training!