Former Navy SEAL and Military.com fitness guru Stew Smith has some suggestions for improving core fitness. You can mix in core exercises throughout your workout; it's never a bad idea to mix extra core work into your week. But, don't just focus on abdominal exercises like sit-ups. Think about hip mobility, shoulder girdle movements, and lower and upper back exercises and stretches. For more details and some suggested workouts, visit the Military.com Fitness Center.