I agree with the core exercises mentioned above. Here are a few more ideas that I thought of for you:
1. Your client will gain core strength from various free weight exercises done standing up. Getting a client to do more free weight exercises rather than machines will help them strengthen their core muscles. Often times we don’t realize that our stabilizing abdominal muscles are hard at work while we are doing other exercises such as squats, lunges, and even arm work sitting on a stability ball.
2. Some good core exercises like wood chops or torso rotations can be done standing up with a medicine ball, cable pulley system or resistance bands.
3. Planks may still possible. One idea would be to have the client plank at an incline. This takes some of the body weight off them yet strengthens their core. I had one client who was 257 pounds, and she was fine at an incline (for example she put her elbows on a bench instead of the floor for plank). Each client is different, though.
4. Simple kickboxing moves are great for the core! Throw in some simple drills for cardio like jabs and punches or maybe some knee lifts and this will help with core strength. I could give you some specific drills if you like–this is a format that I teach and it is amazing how much it works the core.
I hope some of these exercises help you and good luck!
Great answers by both Karin and Susan so far. Here are some I use:
1. Rectus Abdominal: You can have them do a crunch on a incline bench or stability ball where they would place their hips lower than shoulders; to increase the intensity you would have them sit further horizontal until they can be on the ground or parallel to the floor.
2. Transverse Abdominal: Another idea is to have them simply lay on the floor in a standard crunch position; then contract their abdominal muscles by simply drawing their belly button towards the floor, hold for X number of seconds, and then relax and repeat. They can even do this while seated at periodic times of the day to practice good posture while helping develop core strength.
3. Obliques: Next you can have them standing feet together holding a light resistance band attached to an anchor point perpendicular to their stance. To progress you can increase time, have them kneel, position them further from the anchor point, or use a heavier resistance band.
4. Spinal Erectors: Have your client hold a clipboard or other similar, light object and position them in a Standing Bent Over Row starting position; hips hinged, knees bent with arms outstretched, flat back and retracting their scapula as they hold the position for time. To progress you can add time or have them hold a heavier object. Alternatively you can have them hold the starting position on a 45-degree Back Extension Machine for time, arms outstretched starting with the clipboard and progressing from there.
in cases like that, I often use the stability ball or a dyna disc. Here are examples of entry level core exercises with the ball:
Have the person sit and hold the ball on her lap. Ask her not to let you push the ball away from her as you apply pressure on the ball in different directions. You can start with predictable and increasing pressure and then move on to random movement. This is also a lot of fun.
With a dyna disc, I have the person sit on it on a bench or chair, being otherwise unsupported. Then ask the person to move the arms from side to side to challenge the center of gravity. Eventually, you can add weight to that, a weighted ball, for example, or tubing.