Could be from secondary mitochondrial malfunction due to Lyme/coinfections. Mitochondrial malfunction leads to loss of energy production for cells, and this impacts muscles a lot. If muscles don't have enough energy, you feel tired and wear out easily on exertion, leading to lack of exercise, which leads to a cycle of reduced muscle tone, which leads to even less activity, etc.
Also, if metabolism is messed up (particularly with hyperthyroid and/or fast oxidative states), or if there is malabsorption of nutrition, one's body may resort to catabolism (muscle breakdown) if insufficient calories and protein are eaten.
I also believe oxidative damage (from toxic byproducts of Lyme/coinfections) may contribute to muscle loss. Detoxing and supplementing with plenty of anti-oxidants are important for combating oxidatave damage to the body.
Chronic Lyme Disease, Gluten & Sulfite Sensitivity, Many Food/Inhalant/Medication Allergies & Intolerances, Asthma, Gut issues (dysmotility, non-specific inflammation), UCTD ("Secondary Lupus-Like Syndrome"), Osteoporosis, Anemia, Lymphopenia, intermittant Pancytopenia, chronic malabsorption/malnutrition, etc.; Currently TPN-dependent.
Meds: Zofran, Pulmicort, Vitamin B12 Shots, Heparin (to flush PICC line).