The fibular muscles fit on the fibula, bone on the outer side of the leg. They end in tendons that fit on the basis of the 1st and 5th metatarsals.
The chronic tendinitis of the fibulars, manifests itself by swelling at walking, pain in rough terrain (pebbles), feelings of instability.
Your surgeon will diagnose you by causing pain at the tendon loading.
The treatment is medical with sports discontinuation, painkillers, anti-inflammatory, physiotherapy, and soles (ankle centered inside, or cavus foot). Surgery is rarely necessary (synovectomy for thickening or tendinous rupture management).