Chances are you were lifting the toddler or sandbag with ho hum form. You aren't going to hurt yourself lifting 50 pounds like that. You try ho hum form with 400lbs on a barbell and your body is not going to be very happy with you.
Does ho hum mean bad? Couldn't really find it, just that it means routine/dull.
But if good form, then I find it unlikely that you hurt yourself with 400lbs if you progressed to that.
In this case I mean OK form, not proper form since you are just thinking about moving a sandbag on the truck bed and not the angle of your back and where your weight is positioned over the knees. Weightlifters get bad injuries even if they know good form in and out, it only takes a small movement to fall out of form and seriously hurt yourself while lifting heavy weight and can happen under supervision, which you should have already in the form of a spotter for heavy lifts.
