The term 'WORKOUT BY-PRODUCT' refers to the physical effects of exercise. The three solutions for this term are SORENESS (8 letters), SWEAT (5 letters), and ACHE (4 letters). SORENESS is a feeling of tenderness in the muscles that can occur after a strenuous workout. SWEAT is the body's natural response to physical activity, which helps to cool the body down. ACHE is a dull, throbbing pain that can be felt in the muscles after a workout. All of these by-products of exercise are normal and expected, and can be managed with proper rest and recovery.