Slastix training is such an effective system for building strength and tone. They come in all shapes and sizes and can be used to create resistance in literally any movement. You can even attach them to different parts of your body for sport specific conditioning.
As an example when you lift a dumbbell, the bottom of the lift is hard and the top of the lift is easy. That's due to lever length and how our bodies work. Problem is the dumbbell weighs the same all the way up. That means the weight works my muscles hard at the bottom of the rep but not so much at the top.
If I use a slastix system then as I stretch the bands the load increases so I get a more progressive resistance through the movement. That means you can overload the muscle all the weigh through the range of movement. Try it and you will see how effective slastix are for strength training.