Answer: If all you’re doing is running short and easy, it’s fine to hit the gym afterwards. But if you’re running long or doing speedwork, save your strength training for another day.
These particular running workouts stress the lumbar spine, pelvic area, knees, and ankles; lifting weights can strain already fatigued areas and may cause injury.
And remember: Don’t schedule a hard run the day after strength training; tired muscles need a day of rest to recover and rebuild.
— Tim Bishop is a certified strength and conditioning coach and author of Stronger Legs & Lower Body.