At the rec center where we train, there is a 5', 100lb Everlast bag that is a wicked training aid. Nail it with a hard, perfectly floor-parallel roundhouse in the `sweet spot' of your instep and it moves a few millimeters. Follow that up with two or three more hard kicks and it starts swinging, with an amplitude of a few inches, then a few mor inches, ... and eventually it's packing so much momentum that it seems like it's going to be the kiss of death to kick it in its oncoming phase. If you do, you will be rewarded by instant, sincere pain, but you'll slow it down a bit; another couple of good kicks and you'll be back to square one with it.
This is my main problem with a standing back: they are either too light, and get moved holus-bolus, or they are massively packedĀsomeone mentioned such a bag in one of the previous postsĀand stand there like one of the uprights at Stonehenge. The nice thing about the hanging bag is that it simulates a live oppo much better. When you kick an oncoming assailant, you better be ready for something much closer to kicking an oncoming, heavy hanging back than any free-standing bag I've ever run across...