Pad Placement Explained
Pad placement is very important for effective EMS training. Many people feel like it is more an art than a science. However, it doesn't have to be. Many people also, when trying EMS on a muscle they've never stimulated before, feel stumped and ask for expert advice. Many ask for the exact pad position for that muscle, and fear that should the pad position not be exact, their EMS training will not work. Understanding a couple of principles will help with pad positioning for any muscle.
The main points to keep in mind are:
Position relative to the muscle anatomy;
Direction between pads;
Size of pads.
Polarity in my opinion, and that of several respected researchers, is not important, but I will mention an issue with it later.
Generally speaking you want to electrostimulate your muscles when they are in an intermediate position between full extension and full contraction. For the legs, for instance most experienced athletes position their legs slightly more distended than 90 degrees (100-110). If you place the pads before reaching the correct geometry, the ideal pad position risks to move with respect to the muscle motor plate you want to hit.
One pad goes on the belly of the muscle. This is also called the active pad, and the belly of the muscle coincides with the motor point, i.e. the point where the innervation arrives into the muscle. For example, on the vastus lateralis, i.e. the outside part of the quadriceps, this is the bulge in your muscle, several inches away from your knee.
The other pad, called the inactive pad, goes on the proximal end of the muscle. This is the side of the muscle closer to its innervations, i.e. closer to the head or to the spinal column. You want the pad to still be on a fleshy part of the muscle; you don't want this pad to be where the muscle turns into its tendon, because there are no innervations on the tendon. Therefore to make sure you are on the muscle side, you tend to err on the side of the muscle. In our example of the outside quad, this is a couple of inches below the groin line, obviously still on the outside part of your thigh.
The best way to understand these positions is to flex your muscle: the video presented in this website (© 2008 Gianpaolo Boschetti) shows this very well. For help with the correct position, it's best to check the anatomy of the muscle you are going to stimulate.
I found the on-line resource accessible from the link on this site very handy. But there are plenty of other anatomy resources on the web. You can also buy an 8.5x11 muscle-anatomy chart to carry with you when you do not have a PC nearby. I mention two I own: Muscular System, Spark Charts by Barnes & Noble; Muscular Sytem, by BarCharts Inc., at www.quickstudy.com. Keep in mind that anatomy changes from one person to the other. One very visible example is that of the calves: there are people with long calves like a fuse, and people with short compact bundle-like calves.
It's up to you whether to use standard one-input pads, or double-input pads. It doesn't make a difference, because separate channels output the current not at the same time. From a practicality point of view, double-input pads are faster to apply (you need to stick only one pad instead of two), and probably save some money. However, the position of the double-input pad is necessarily a compromise between the two positions for the two muscles that you are trying to stimulate, and as can be seen in the video posted in this site, professional athletes prefer two separate pads on the quadriceps.
The imaginary line connecting the center of one pad to the center of the other pad has to follow more or less the direction of the muscle fibers. This is, generally speaking, the direction along which a muscle does its pulling.
A muscle will twitch and have the so called tetanic contractions, even if its pads are not aligned in this fashion. That is because the pads eventually end, closing the circuit somewhere through the muscle, and in this circuit some nerves will be hit. However, to be most effective in hitting most nerves you have to collaborate with the geometry of the muscle and work along its fibers.
Also if the current circuit is properly closed along many innervations and fibers, the current will encounter less resistance in its path. This is very important to obtain a strong contraction, as it directly affects comfort. EMS users generally increase current up to their comfort level, which is, obviously, immediately below the pain threshold. And pain is caused by high voltage traversing your skin.
Let me explain how current, voltage and resistance are interrelated. Current flows in a circuit affected by the voltage applied and by the resistance it encounters. If you increase the voltage more current will flow and vice versa. If you increase the resistance less current will flow and vice versa.
The Globus Premium is a constant current generator: this means that the device tries to keep the level of current stimulation constant. You need a certain level of current to stimulate the muscles. You do this by directly acting on the current control keys for the particular channel. As you do this the stimulator brings up the voltage to obtain the particular current level of current that you commanded. If the resistance increases in this circuit, the stimulator will increase the voltage to keep the current from decreasing; if the resistance decreases in this circuit, the stimulator will decrease the voltage to keep the current from rising.
However, voltage is directly associated with the tingling feeling. As you increase voltage, the tingle becomes more intense, until it becomes painful and intolerable. This explains why during or immediately after warm-up you are able to bring the level of current up: warm-up increases blood flow through your muscles, and more blood means less resistance to the passage of current; less resistance in turn means that the stimulator needs to generate a lower voltage to maintain that level of current; in turn less voltage make your tingling feel less intense. You are able to bring the current intensity up, and your contraction becomes stronger.
Getting back to why the direction along the two pads is important. If the current has to cross the muscle fibers, rather than flowing along the fibers, the following will happen:
the resistance will be higher,
which will cause the voltage to be higher to keep the current constant,
which will make you hit your discomfort level earlier, and
you will not be able to reach as strong a contraction.
Unfortunately a lot of literature and images of pad position available seem to disregard this principle. The most disregarded is the position of pads for pectoral muscles, where many pictures show one pad above the other, rather than one closer to the armpit, and the other in the direction of the center of the chest.
There are two schools of thought. One school of thought advocates pads as large as they can fit in a particular body area. The other advocates different sizes to be able to concentrate the flow of electricity in the most appropriate direction.
The school of thought advocating the largest pads available, assumes that the larger the pad,the easier it will be to distribute current into the muscle, the higher the comfort level, the higher you will be able to go with the current intensity, and the stronger the contraction.
The school of thought for dissimilar sizes recommends placing the smaller pad on the belly of the muscle. The rationale is that with a smaller electrode the current will concentrate on the motor nerve plate, which is right on the center of the muscle. It believes further that the concentration will also help the current flow reach more deeply into the muscle before traversing it to the other pad placed proximally. However, simulations run using the laws of electricity show that this concentration is marginal.
Therefore, my suggestion is try both and see which works best for you.
At the beginning of this white paper on pad position, I explained that my belief is that pad position is not important, but that there are people who disagree with this. This has been a bone of contention between people who have espoused the different ideas.
In a nutshell, the current generated by an EMS device like the Globus Premium Sport and Fitness is alternate current by definition. Granted, it is a special type of alternate current, because its waveform is rectangular and not sinusoidal like the current in the outlet at home. Nevertheless each lead keeps alternating between + and – polarity. Therefore there isn't a properly named + lead to connect to one pad or the other. You can convince yourself of this fact, by pausing your EMS session in the middle of it, switching the leads, and then resuming. Did you feel any difference? I do not. Well, respected researchers say that in the end pad placement and connections should go with the arrangement that causes the strongest contraction. So if you didn't experience any change of strength of contraction, it must be because the two leads are indifferent to either of the two pads.
Many researchers in the field of EMS for sport training agree that the two leads can be connected indifferently. However, in the field of therapeutic uses of electrostimulation, there are several applications with asymmetrical currents (i.e. currents that do not repeat exactly on the - side what they do on the + side). Most notable are certain types of TENS (pain therapy). For these applications therefore it does make a difference the way the + and - leads are connected. Since EMS is intimately connected to the use of elecrostimulation for therapy, most scholars in their research papers mention the way the leads were connected in their labs. And this tends to stick in the minds of some in the EMS community. Some authors refer to the pad over the motor point as the active electrode, and the other pad as the inactive electrode. But because of the intermixing between the medical field and the training field, the word active and + tend to be used interchangeably, and so are the words inactive and -.
Also, most of the companies producing EMS devices also produce devices for the medical market. This causes the leads and many electrode position pictures to be labeled with + and -. But that's about the only reason why you see the polarity in manuals, pictures and articles, not for any other reason. Relax and connect your pads with whichever lead you prefer.
The ideas expressed in this white paper reflect the opinions of the author, and should not be construed as prescription of medical or therapeutic nature.