The Myth of Muscle Isolation
By Christopher Chilelli RTSm, MATcs

One of the big pluses in having a fitness blog is the opportunity to dispel some of the more common training myths that I encounter everyday. There’s clearly a lot of deliberate misinformation out there. After all, in the absence of any real, comprehensive understanding fit pros and infomercials are free to invent whatever crazy, magical rationale they can dream up in order to convince you to give them money.

So let’s expose some of this nonsense. For my opening salvo into this territory I want to tackle the concept of “muscle isolation” and “muscle isolation exercises.” As in, “This piece of equipment will totally isolate your side abs!” or the converse, “Don’t use that piece of equipment because all it does is isolate your abs!” Ooh, scary. We apparently don’t want our abs getting lonely. If you haven’t guessed yet, depending on who’s talking, muscle isolation is either the best or the worst way to train, which is all the more amusing considering that as an anatomical reality it's entirely fictitious. But I’ll get to that.

First, a little industry history to frame our discussion. “Fitness” as both an industry and cultural phenomenon really took off in the 1980s with advent of modern bodybuilding. It just so happened that America’s soon-to-be obsession with fitness and physical appearance and the popularity of guys like Arnold Schwarzenegger and Sylvester Stallone coalesced at relatively the same cultural moment. As savvy investors scrambled to open health clubs and aerobics studios, they staffed them with the only people who had any working knowledge of exercise and gym equipment – bodybuilders. Of course that knowledge had in most cases not come from research of even common sense, but from experience or tradition. This is not a knock on the sport of bodybuilding. Bodybuilders have made enormous contributions to the practice of strength training. Before the 1990s “Exercise Science” had yet to gain any real currency within American universities and experience and tradition certainly hold some value. Bodybuilders had no choice but to improvise and because of this, much of the informational foundation of fitness training comes from the bodybuilding world.

This ‘gym science’ is unfortunately also the source for many of the more enduring exercise myths and certainly the origin for ‘muscle isolation’. See, a bodybuilders overarching goal is to increase the size of every muscle in his or her body. For this group the idea of ‘isolating’ the biceps or the deltoid or the quadriceps held quite an appeal and, through trial and error, they happened upon machines or movements that seemed to ‘work’ one or more muscles more than others. Hence, to this day you can walk into any gym and hear: “This exercise is the best for isolating your lower abs.”

But the bodybuilding world can’t get all the myth-perpetuating blame here. Ever see an infomercial for home exercise equipment? 9 out of 10 times the rationale for why the device they’re pushing is so much better than the 6 other useless pieces of junk collecting dust under your bed is because of its superiority in ‘isolating’ your chest, triceps, or side-abs, whatever those are.

And back in the world of health-clubs, more recent years have seen new generations of trainers, armed with entirely different philosophies than bodybuilders but equally little actual understanding of anatomy, crop up across the country. I don’t want to group everyone together, but by far the largest contingent of this set likes to call themselves “functional trainers.” Without launching into a full-on assault on everything that’s silly about that term, I’ll say that “functional trainers” by and large have no love for bodybuilding culture and distinguish themselves by criticizing all things related to it; supplements, exercise machines, and of course: “isolated exercises.”

So here we have a chorus of exercise professionals and equipment manufacturers either discouraging or advocating performing exercises that seem to isolate or ‘work’ individual muscles. The classic example of an isolated exercise is the leg extension. You’ve seen some version of this machine in every gym since the dawn of mankind. You sit down, a pad goes on the front of your shins and you extend the knee against resistance. Bodybuilders love it because it’s ‘all quad.’ Functional trainers hate it because it’s ‘all quad.’ The ‘biceps’ curl with a dumbbell is another classic offering. You hold a dumbbell at your side then flex the elbow. Again bodybuilders love it. It works the biceps. Functional trainers hate it. It only works the biceps. Who’s right?

Well, nobody. All of these camps – for, against, or otherwise – make the same mistake in assuming that ‘isolating’ a muscle or joint is even possible in the human body. Concepts such as this simply to do not do justice to the enormous complexity of both the muscular system and the demands placed on it.

Your body doesn’t ‘think’ in muscles, but rather in forces. The forces of the physical world are all around and within us. The muscular system – in fact the whole body – evolved precisely to negotiate those forces and ensure your survival as an organism. You can think of the muscular system as one big contractile field that’s tacked down to bone in different places. We call the divisions created by that tacking individual muscles, but these names – biceps, trapezius, etc – are really just artificial distinctions*. They’re quite useful in that we can identify and plot the specific mechanical ability of one muscle relative to another in overcoming a force, but in practice the body never uses just one or two or even 16 muscles at a given time. Rather, to some degree it’s all contracting all the time.

This constant contraction and relaxation does two key things for us. For one, it holds our joints together. Before our muscles can even begin to move us around they must maintain the integrity of the joints of the body. Each muscle is mechanically interdependent with every other muscle, especially those crossing the same joints. One muscle’s increase in activity will necessitate an increase in activity in other muscles. Were a muscle like pectoralis major to contract all by its lonesome – as many advocating or discouraging ‘isolation’ would have you believe – it would quite literally tear the shoulder joint apart. For this reason the nervous system is constantly dialing up or down activity across the entire contractile field based on the immediate force demands.

Sensing those demands is the other function that continuous, sequential contraction does for us. Muscle is extraordinarily sensitive stuff, dripping with special sensory cells. Each tiny contraction (a process together called tonus; where we get the term muscle tone) beams a little piece of mechanical information to the brain. The brain takes all those pieces in sum and then programs contractile activity – all within a span of time measured in the fractions of seconds. Complex activities like walking result from your nervous system intermittently increasing and decreasing the activity in sequential muscle groups based on information from those muscle groups and your own intentions.

So our individual muscles never, ever work alone. Even for the simplest, quickest task, involving a single joint and little force, the entire balance of muscular activity from head to toe must adjust. Let’s take the ‘biceps’ curl exercise from above to paint a clearer picture of what this process looks like in practice.

The second you pick up the dumbbell the game changes. Muscles in the forearm increase tension to maintain the radioulnar and elbow joints as do muscles in the upper arm and neck. As you begin to flex the elbow, creating an arcing motion up from the floor, already our notion of muscles working in isolation begins to crumble. The biceps is only one of 8 muscles crossing the front of the elbow that now all increase tension to produce this flexing motion at the elbow.

Whenever I make the previous statement in a fitness setting, someone is always quick to chime in that while its true that there are 7 other muscles crossing the front of the elbow, the biceps is the main muscle, or the prime mover doing all the work. While it’s accurate that the biceps has the greatest mechanical advantage in flexing the elbow through much of the range (based on its distance from the axis of motion), statements like this are equivalent to believing that acceleration is the most important part of driving a car. The gas petal is essential, but good luck driving without steering, suspension, or brakes. If it plays a role at all, it’s important. No area of the body is any less important than the other.

The biceps, by the way, also crosses the shoulder joint. That’s no small detail. A muscle does not increase tension at just one end, but rather pulls toward its center as it contracts, meaning that as the biceps ‘works’ to flex the elbow its also trying just as hard to flex the shoulder.

So what’s keeping your arm at your side? Well the weight of the dumbbell helps a lot, but it’s other muscles that fix your upper arm in place while the elbow moves. On the other side of the shoulder, the rear deltoid contracts to prevent any forward motion of the upper arm. The rear deltoid in turn attaches to the back of the shoulder blade, a bone simply swimming in muscle. Those shoulder blade muscles must now also contract to keep it from moving all over the place. Many of those muscles attach right into the spine and again, since they all pull equally at both ends, other muscles must now activate to prevent any spinal motion. As the dumbbell moves further into the arc and away from the body it creates a force that wants to flex the spine, so spinal extensors now enter the mix.

We could keep at this but I’m sure the point is clear. Even in this seemingly isolated motion occurring only at the elbow, virtually every muscle and joint is involved in some way. And the brain makes all these adjustments almost instantaneously.  Simply put: muscle isolation does not exist. Now, the larger question, what implications does this reality have for all those alleged professionals advising us to either to either always or never isolate our muscles? Think on that one.

*That may be over simplifying things a bit. Individual muscles do have distinct innervation from your nervous system and they are often otherwise defined by a relationship with surrounding connective tissue and individual histology – their cellular composition. In the brain itself, the situation is even weirder though. Areas of the motor cortex that stimulate contraction of one muscle often simultaneously stimulate the contraction of one or more other muscles. This implies that every time you move one part of your body, say your arm, other sections of your brain must work to actively prevent moving another part of your body, like your wrist. Clearly, even simple movements are the end products of incredibly complex neurological arrangements.