My dad is 50 and he has a stiff/tight pain behind his knee. no pain, not hot. he goes to the gym a lot. blood clot or muscle? help

Synovial cyst: It is not feasible to provide a meaningful opinion without examining your father, however, clot or muscle problems are unlikely. He may have an early synovial cyst, also called Baker's cyst. He may wish to consult a doctor at his convenience. For you: For good health - Have a diet rich in fresh vegetables, fruits, whole grains, milk and milk products, nuts, beans, legumes, lentils and small amounts of lean meats. Avoid saturated fats. Drink enough water daily, so that your urine is mostly colorless. Exercise at least 150 minutes/week and increase the intensity of exercise gradually. Do not use tobacco, alcohol, weed or street drugs in any form. Practice safe sex, if you have sex. Get HPV vaccine