Freestanding punching bags use a reservoir as a counterweight to simulate the resistance and swing of a hanging bag. Theoretically, you could fill the reservoir with any substance heavier than air. In practice, most users choose to fill it with rock, sand or water. Each material has advantages and disadvantages.
In most cases, water is the most convenient material for filling a freestanding punching bag. However, rock and sand each have their own advantages.
How Punching Bags Work
A freestanding punching bag consists of a cylindrical foam pad that wraps around a hollow plastic stem. The base of that stem is a plastic reservoir that's wider than the bag itself. When that reservoir is filled, it provides enough weight to keep the bag in place when struck. Models have a screw cap on or near the top for adding or removing material to provide that weight.
Pros and Cons of Water
The original intent for freestanding punching bags was to fill the reservoir with water — which is still a common method. Water's biggest advantage over other options is its convenience. It's easy to pour in or out, and you have a source right there at your gym. However, water tends to leak out of the screw cap when the bag is knocked over, and also forms condensation during cold or humid weather.
Rocks Are an Option
Rocks are large enough that they won't leak through the screw cap — or even through cracks in an older bag. They also make a pleasant noise when the bag rattles after a particularly effective punch or kick. However, rocks can be harder to load into or out of the reservoir because they don't flow as well as water does. Using pea gravel instead of irregular stones can help minimize that disadvantage.
Sand Splits the Difference
Sand splits the difference between water and rock. It flows into the reservoir easily if dry, but is less likely to leak when the bag is tipped and won't cause condensation. The chief problem with sand — besides the hassle of going to a landscaping supply shop to get some — is its tendency to settle. If moisture gets inside the reservoir, sand can set into a form not much softer than concrete. This can make it very difficult to empty your bag to move to a new location.
Are the Bags Worth It?
If you're a boxer, the benefit of putting in time on a bag is obvious: The more you practice a skill, the better you get at it, and the more punches you throw, the more naturally they'll come. Heavy bags also do a good job of simulating the impact of hitting something solid — so your brain, bones, tendons and muscles all have a chance to get used to the idea before you actually step into the ring. Boxing can also be useful cross-training if you happen to fight in another style, like karate or jiujitsu.
But even if you never plan on going to battle in the boxing ring, taking some time to learn the "sweet science" still packs a number of benefits. It's great exercise to start with, working your entire body as a cohesive unit and building cardiovascular fitness. And, no lie, boxing is hard. So if you're looking for a way to dig in and burn lots of calories or maybe just sharpen your mental toughness, spending a little time on a heavy bag is a great way to start.
Just make sure you're using proper technique and protective gear; even one or two sessions with a good coach when you get started can help you avoid injury further down the road.