Before I dive into the specifics, you must first ask yourself why you train, and why functional fitness to be precise. If you know what you're doing it for, everything will start to make sense.
In my opinion, we train because life nowadays is too comfortable -- our body isn't getting enough stimulus to function optimally. Lifting weights keep you strong & capable -- it maintains bone density, strong muscles, stable energy levels and so forth.
To take this a step further, you have to ask yourself how our body is designed, and in particular how it is designed to move. Your body is what they call, a biological system, and in this case 'biological' simply means ''to respond to it's environment''. The functions of your biological system have evolved over millions of years. Both your physiology and psychology have had to problem-solve under pressure and your current biology is the result of this.
Now zooming into training, you can ask yourself, what aligns best with my biology, a barbell or a sandbag? What will allow your body to function at it's best, whilst mitigating risk, and all the while having a large carryover to life. Sandbags are messy and teach your body to stabilize under load, whereas barbells allow you to cheat and potentially hurt yourself.
Without getting too technical, when it comes to biomechanical forces being applied to the body, there is a fundamental difference between the two pieces of equipment; one shifts tension towards the inside, and the other towards the outside -- a sandbag creates internal tension and a barbell tends to open-up the body.
Now zooming out again; visualize yourself in a forest, having to survive for a month. Whenever you lift & carry a natural object, are you more likely to require the body to turn inwards or outwards? What happens to the tension in the body when you lift a block of wood, a stone, a dead animal or a jug of water?
Realistically, the only time in a natural environment that you will have your body open itself, is when you jump or throw. And when doing so, you move quickly and mostly concentric (lengthening of the muscle under tension) -- beyond that, you will very likely not be dealing with a heavy load.
Scrolling forward to the 21st century, your average gym bro will be using dumbbells, barbells, bars & machines which all miss a critical component; the internal compression that happens when you're trying to hold onto a big awkward object. Training in this way, results into tight hips, a locked-up lower back, and an overreliance on generative muscles (see other blog post).
Playing devil's advocate, if all you want is big muscles, and/or target very specific areas, using machines & barbells can be the better choice. If however you want to move well, pain-free and over a long-time, ditch the barbell and get yourself a sandbag.
