How important is it? Turns out – it’s everything! If you’re not varying the weight and/or the actual exercises every once in a while you will likely plateau. The body becomes accustomed to a regular, unchanging workout routine after about 4 weeks. At about that time you won’t see the same results you may have been seeing when you started.
There are many ways to change up your workout. There is no need to throw the baby out with the bathwater. To ensure there is enough variation, I change up the workout loads both on a daily and weekly basis. Every four weeks I change my workout from a body building routine to a strength routine so some of the exercises and set/rep ratios change. How you should adapt your workout will depend on your fitness goals.
Variation ensures the body is continuously adapting. When the body adapts, it builds muscles!