The possible causes of muscle atrophy on the left side of the waist include long-term inactivity, lumbar dorsal fasciitis, cervical spondylosis, and so on. 1. Long-term non-exercise: it may be due to long-term non-exercise, resulting in wasting muscle lesions, the left side of the waist muscle atrophy. 2. Lumbar dorsal myofasciitis: due to long-term chronic muscle strain, resulting in pathological changes in the fascia and muscle tissues of the lumbar back, lumbar dorsal myofasciitis is a kind of aseptic inflammation of the fascia, which will give rise to the symptom of lumbar left-sided muscle atrophy. 3. Lumbar spondylosis: muscle atrophy on the left side of the waist may be due to lumbar vertebrae herniation and compression of the nerve, resulting in muscle atrophy and numbness and weakness of the arm. The left side of the waist muscle atrophy may also have other reasons, it is recommended to go to the hospital in a timely manner, improve the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.