The term 'cause of muscle ache' can be summarized by the crossword solution 'STRAIN'. STRAIN is a type of injury to a muscle or tendon, which is caused by overstretching or over-contracting the muscle. It can also be caused by repetitive motions or sudden movements. Symptoms of STRAIN include pain, swelling, and tenderness in the affected area. Treatment for STRAIN usually involves rest, ice, compression, and elevation. In more severe cases, physical therapy may be necessary.