If anybody has lower back pain when doing standing barbell curls or pushdowns, this tip may help you out:
Use a dip belt with a couple of heavy plates hanging from your waist. This will dramatically increase your body mass and make the weight that is putting the torque on your back much less heavy, relative to your bodyweight.
For example, if you weigh 180 lbs and you’re curling 110 pounds, that’s more than 60% of your body weight out in front of you. But if you add 90 lbs of plates to your bodyweight, that drops to 40%. This is a LOT less torque being directed onto your lower back.
The same is true for pushdowns. Heavier “body” mass means less torque on your lower back.