I have read that Lyme bacteria like to hide in parts of the body with low blood flow, to avoid the immune system and medication. Cartilage that connects the ribs to the breastbone is one of those areas, as are knees joints, which are known to get swollen and painful with Lyme. Exercise helps for several reasons, one of which is that it increases blood flow and therefore brings the immune system to bear on the bacteria. Some people seem to do better with more exercise than others. Walking works for me, but not running, which is too hard on my joints etc.
Muscle aches and pains can be for several reasons -- one of which may be that Lyme bacteria use up magnesium in the body, but it usually doesn't show as low on blood tests. I take magnesium supplements (magnesium citrate, aspartate, orotate, any version ending in '-ate') and it helps me a great deal.
Your Lyme doc may have some advice in these areas.
Newly diagnosed with Lyme, but for the past two years plus I have complained of sternum/rib/chest wall pain. It is odd because sometimes exercise helps and other times exercise will worsen the pain. Sometimes I can breeze through a work out and others I can barely make it before my bones and muscles start to ache and the next few days to weeks are hell as the pain increases. Sometimes just repetitive motion like scrubbing a bathtub will bring on the pain. I also work in the medical field and sometimes the motion of scrubbing up/washing my hands causes chest wall pain to begin. (BTW-the also believe I have fibromyalgia, but that hasn't been confirmed yet),
I have had on and off similar pains in the chest (sternum/rib area, right side). It also wraps around to my back in the similar spot in my back. It seems to come on when i do some activity that aggrevates these areas, like a throwing motion, or a pushing motion. I am finding relieve by instead of just sitting or lying around, in doing 2 things, doing those band stretches for my chest and back areas, then either walking or getting on the bike. if i do that without putting too much exertion into it, i start to feel better in those areas. It's like the muscles or ligaments constrict without doing that and just continue to feel lousy, but when they are stretched out (without over doing it), relief comes. :)
Sometimes I get a sore muscle that I have no idea what I did to strain it ... I think it's from inflammation by the 'bugs' -- but different people seem to have different levels of muscle soreness/pain/susceptibility.