I am wondering if anyone can provide a strategy for managing lower back stiffness/mild pain
that usually occurs on the second day of a tournament.

Has anyone successfully changed their style to overcome back problems?

I have coached a number of players with various types of back problem.

Without seeing how you play, it would be almost impossible for anyone to give you advice with any confidence that it will be helpful and solve the problem.

The main thing I would suggest is that you play as many shots as you can while standing up straight - keep your body upright, your elbows reasonably straight, and swing from the shoulders. Learn to play shots such as the split from hoop 1 to hoops 2 and 3 while standing up and holding the mallet with both hands at the top of the handle, making use of forward slope rather than acceleration through the ball to send the striker's ball forward. (This probably will not work so well on a soggy lawn.) You may need to use a different grip in order to achieve this, and perhaps switch to a mallet with a longer handle. Try to avoid playing rolls with your hand well down the shaft near the mallet-head. It is surprising to some people that almost all strokes, with the exception of pass-rolls and certain hampered shots, can be played standing up and with the hands at the top of the shaft.

Of course, you may already be doing that. Most experienced players also learn to use their foot rather than their hand to position the balls for a croquet stroke. Measuring balls onto the yard-line can also be done in various ways that do not need much bending.

The problem is usually caused by the fact that most players lean forward for many shots, with the upper part of the body inclined forward at an angle of 30 or more degrees. When the mallet and arms move forward and have to be supported as well as the upper body in that position, it can put considerable strain on the back muscles, especially those needed to straighten up again before you move away. The more uptight you can remain, the better.

I agree with what John wrote, and can add some personal reflections too:

• when it's hurting, become aware of what you are doing and why, and in later practice sessions try standing more erect to achieve the same thing.
• think about changing grip so you can stand more erect — you may need a longer mallet. I tried a longer mallet but it does not really work for me because I have short legs. :)
• do some back-strengthening exercises (like sit-ups) — if you are go to a gym, discuss it with an instructor; if you don't go to a gym, consider it. :)
• I'm no doctor, but I suspect that the fact it hurts on day 2 means it's more muscular than skeletal so there's a better chance you can overcome the problem by better posture.

As John said, it's probably caused by leaning forward (your lower back muscles are having to pull to stop you falling forwards) — find a way of being more on balance when addressing the ball

The only thing I would add to what John and Dave have written is the benefits of stretching.

On rare occasions I have visited a sports medicine clinic with a muscle problem and their main solution has always been stretching exercises. One such that I find useful when my back bothers me is to raise one leg up onto a bench (seat level, or up on the back if you can manage it), then hold the position while gently reaching down along the raised leg with both hands to stretch your back. Don't bend farther than is comfortable. Hold the position for 30-45 seconds. Repeat with each leg 2 or 3 times. My understanding is that this is better for the back than exercises where you bend your back with legs together (as in sit-ups) because this puts more strain on the discs.

I don't have a back problem, but occasionally experience minor stiffness and mild pain as you do, and the above exercise helps every time.

Of course, if you wish to do this during a game, it may be necessary to urge your opponent to get going on a long break so you'll have enough time.

I use the following exercise for lower back pain. Lie face down on the floor with your arms bent as if you are going to do push ups. Then using the same arm action as for a push up raise your upper body, BUT ensure your hips and pelvis remain in contact with the ground. If possible do sets of 5 to 10 reps several times during the day. I would suggest that you start the exercises a few days before a tournament rather than wait until you experience lower back pain.

During a tournament when lying down may not be possible, try placing both hands side by side on your lower back and bend backwards supporting your lower back with your hands.

Both the above exercises were suggested to me by my physiotherapist.

I use lots of Arnica Gel. Start applying it morning and night a day or two before the event. Use it daily during the event, both morning and night. It won’t totally prevent the pain, but for me it keeps it manageable.