When you attempt to ‘isolate’ muscles by performing single-joint exercises, you are actually creating a body that is non-functional and will be more prone to injury. Essentially, you are creating a body that is a compilation of body parts, instead of a powerful, functional unit that works together.
Now if you really want to end up hobbling around in a body bandaged up with joint problems, tendonitis, and excess body fat, then by all means, continue trying to ‘isolate’ body parts. On the other hand, if you would rather have a lean, muscular, injury-free, functional body that works as a complete powerful unit to perform complex movements (in athletics or even everyday tasks), then you need to shift your focus away from muscle isolation.
Believe me, focusing on how well your body functions will give you the side effect of a body that looks even better than it would have if you focused on muscle isolation.
Another benefit to moving away from the ‘muscle isolation’ mindset to a more ‘complex movement’ mindset is that you will find it much easier to lose body fat. The reason is that by focusing more on multi-joint complex movements as opposed to single-joint muscle isolation, you not only burn a lot more calories during each workout, but you also increase your metabolic rate, and stimulate production of more fat burning and muscle building hormones like growth hormone and testosterone.
Let’s look at an example. The machine leg extension is a single joint exercise that works mainly the quadriceps, can potentially cause knee joint instability in the long run, and doesn’t even burn that many calories. On the other hand, exercises like squats, lunges, step-ups, and deadlifts are all multi-joint complex movements that work hundreds of muscles in the body (including the quadriceps) as a functional unit, create more stable and strong joints in the long run (when done properly), and also burn massive quantities of calories compared to the single-joint exercises.