One of the most effective tools in a "functional training" arsenal are
They are basically big rubber bands. Resistance bands come in many varieties and sizes but they all have the same qualities in common. I use them often because of their versatility and effectiveness. I use the when I train individual clients and in a group/bootcamp setting.
Resistance bands offer the flexibilty (sorry for the pun)to train in so many ways. I like them because I can train standing up thereby engaging more muscle groups then I could sitting down. If I want to do a band row then I can do that while I am standing in a deep split stance. I get the benefits of training my arms, back and shoulders but get the added benefit of training my legs and core.
You can wrap them around your ankles or knees to train your legs and hips for a workout that will have smoke coming out of your lower extremities. I use them often for hip strengthening and agility work.
Band training can have an incredible impact on your core musculature as well. You can add trunk rotations that you would be hard presses to do on any fixed machine.
Another benefit to resistance band training is that you can vary your training speed. If you want to be fast then you should train fast. Bands allow you to do that in a safe environment.
Bands are also a great way to create some instability and perturbation. They can force the athlete to regain his/her balance and stability which in itself has a great transfer onto the playing field. I often loop them around legs or the trunk and pull or pulse the bands to force my client to "readjust". This is challenging and is great for proprioception and kinesthetic awareness.
Resistance bands are extremely cost effective(you can buy a variety of bands at a fraction of the cost of a set of barbells)and they are space effective(you can stick them in your closet and they won't take up much space at all).
One of the biggest difference between resistance bands and free weights like dumbbells is the resistance curve. A dumbbell will feel heaviest at the beginning of the lift and easiest at the end range(think of a dumbbell curl).
A resistance band on the other hand will get more difficult at the end of the range and be easiest at the beginning. Some strength coaches feel this is unnatural and you should not train this way. I disagree. I feel that the more training stimuli you can overlap the better off, and stronger, you will be.
Resistance bands will last a long time but they will not last forever. When you see wear and tear anywhere along the band please replace it. The pain that you will experience when a band snaps and hits you in the chest, or worse in the head, will linger for a while. You will not be happy(I speak from experience).
If you train alone you can attach most bands to a door or some stable object. Sometimes you can step on a band to anchor it to the ground. The possibilities are many, just use common sense.
I put together a series of band combo exercises for you to watch. These exercises are not for beginners. They will train your core along with your upper body and legs. Try them to get an idea of the possibilities of resistance band training. This is just the tip of the iceberg. Add resistance band training to your routine to create new opportunities to improve strength and overall performance. Enjoy yourself and train responsibly.