Exercise causes microscopic damage to the muscle fibres. While this damage is an essential part of building strength, it can lead to stiff and painful muscles - known as delayed onset muscle soreness (DOMS). Making sure you warm up before your workout, and cool down and stretch after,  can help to prevent muscle pain after exercise. If you do experience sore muscles, gentle exercises and stretches to get the blood flowing, applying heat, and massaging the area can help to relieve and manage soreness.