by Kevin Neeld
As strength and conditioning coaches, much of what we do involves training the neuromuscular system. Consequently, the more we can learn about the way the neuromuscular system works to produce force and coordinated movement the better we can train our athletes. One of the tools used to monitor the neural input to muscles is electromyography (EMG). EMG can be a confusing area because the signal is affected by so many different things, including well-known things like muscle fiber size and the number of muscle fibers, and less well-known things like body fat and blood flow. While much of the EMG research may be of interest to the strength and conditioning community, it’s important to understand what exactly it is and what it measures so that research isn’t misinterpreted. Hopefully this article will shed some light on EMG misnomers.
What is EMG?
EMG is a measurement of the electrical activity of one or more muscle fibers. In general, there are two types of EMG: surface EMG and indwelling EMG. Surface EMG is a more global measure of the electrical activity of the entire muscle. The electrical activity is a summation of the electrical activity of all of the individual muscle fibers within the measurable range of that particular electrode. Indwelling electrodes include both needle and fine-wire electrodes. These electrodes measure the electrical activity of a limited sample of muscle fibers most proximal to the electrode inside of the muscle, as opposed to on the surface. These electrodes are more effective at measuring changes in the quality and quantity of individual motor unit (MU) firing. For example, indwelling electrodes can assess things like: Individual motor unit firing rate changes. An individual motor unit will increase its firing rate in order to produce more force, as discussed below.
Incidence of doublet firing. Doublet firing involves an individual motor unit discharging twice at a very short latency. For instance, if a motor unit is discharging every 30 milliseconds (ms), and then discharges twice within 10 ms, this would be considered a ‘doublet’.
Common Drive. This word describes two motor units simultaneously changing their firing rates together. For instance, if MU1 increases its firing rate, MU2 will increase its firing rate at the same time. If MU1 decreases its firing rate, MU2 will simultaneously decrease its firing rate.
Synchronization. If two motor units discharge at almost exactly the same time, this is referred to as synchronization. Typically, if MU1 and MU2 discharge within 5 ms of each other, they are said to have fired synchronously.
All of the above examples are strategies that the nervous system uses to increase muscular force that can be most accurately measured using indwelling electrodes. While these electrodes are capable of providing extremely useful and interesting information, much of the EMG confusion is associated with surface EMG, so the rest of the article will focus on that.
What surface EMG is…
In general, surface EMG measures total muscle activation. It is important not to confuse activation with recruitment. Although the two words are sometimes used interchangeably, they are NOT the same thing! Recruitment involves additional motor units producing force. Activation includes both recruitment AND rate coding. As a refresher, rate coding involves an individual motor unit changing its firing rate. For any given motor unit, a faster firing rate will result in a higher force production than a lower firing rate. If you read that there was an increase in EMG activity, this could be due to the already active motor units increasing their firing rates, the recruitment of additional motor units, or, and probably most likely at lower force levels, both.
Also, surface EMG probably doesn’t measure activity from the whole muscle. Keep in mind that the same size electrode is used for muscles of all shapes and sizes. On a very small muscle, the electrode may be able to pick up activity from the whole muscle, and probably surrounding muscles (This is called cross-talk and is discussed later). That same electrode on a very large muscle will pick up activity from the most proximal muscle fibers, but probably not the deepest ones, which are typically more slow-twitch.
What surface EMG isn’t…
This is probably one of the biggest misunderstandings: Surface EMG is NOT indicative of the mechanical force production of the muscle. The electrical signal is the nervous system INPUT signal to the muscle. So when EMG activity increases, mechanical force production increases at a delay (typically around 50ms) that is dependent on the characteristics of the muscle fibers and the type of contraction. This means that when surface EMG activity is completely non-existent, the muscle can still be producing force. Remember, the muscle will continue to produce force as long as calcium is available. The decline in force production (both the shape and time course) is coincident with the uptake of remaining calcium back into the sarcoplasmic reticulum. The delay between electrical activity and measurable increases in muscular tension also makes it possible, during periods of very brief activation, for electrical activity to start and stop before any increase in muscular tension can be detected. Also, most contractions involve some amount of antagonistic co-contraction, especially at higher forces. This will have a significant effect on the amount of observed force.
To be overly simplistic, if the biceps are pulling with 20 pounds of force and the triceps are resisting through co-contraction with 5 pounds of force, the biceps will only produce 15 pounds of observed force. Consequently, it is possible that biceps activity will increase, but observed force production will actually decrease. This is also possible during fatiguing contractions. As slower twitch motor units are operating at maximal firing rates and still unable to maintain a given force level, more impulses are sent to fast twitch (more superficial) muscle fibers causing the surface EMG activity to increase, although force production may actually begin to decrease. As you can see, it is dangerous to conclude that an increase in EMG activity necessarily results in an increase in force production.
What influences surface EMG activity?
1. Number of muscle fibers: More muscle fibers = more activity
2. Muscle fiber diameter: Larger fiber diameter = more activity
3. Distance between electrode and muscle fiber: The closer the muscle fiber is to the electrode, the clearer the signal. Think of it like someone yelling something to you. If you were 10 yards away, you’d probably hear them loud and clear. If you were 50 yards away, slightly less clear, 150 yards, less clear, etc.
4. Preferential recruitment of fast-twitch fibers: Fast-twitch are typically more superficial than slow-twitch muscle fibers. Because surface electrodes record the most clear signal from the most proximal muscle fibers, it is possible that surface EMG measurements overrepresent fast-twitch muscle fiber activity.
5. Blood flow: EMG activity can be affected by the overlying blood vessels in the muscle. If there’s a blood vessel between the electrode and the active muscle fibers (there frequently is), this will affect the quality of the signal. Sticking with the yelling example, just imagine how different things sound with your head above and below water. Overly simple…maybe, but it’ll do for our purposes here.
6. Muscle fiber direction: Placing the electrodes perpendicular to the direction of the underlying muscle fibers results in a significantly decreased EMG amplitude compared to placing the electrodes parallel to the muscle fibers. You can imagine how difficult it is to consistently account for this, especially while measuring the activity of multi-pennate muscles.
7. Cross talk: In many instances, the observed EMG activity includes electrical activity from other nearby muscles. This is almost unavoidable in some areas of the body as many different muscles are packed into a relatively small area (the inner thigh region is a great example).
8. Motion artifact: Movement of the electrode on the skin surface will actually produce measurable activity.
This is by no means an exhaustive list. The EMG signal is also affected by a host of other things including: muscle fiber conduction velocity, amount of subcutaneous fat, interelectrode spacing, number of active motor units, motor unit firing rate, etc.
What we can learn from surface EMG
While there are clearly several limitations to surface EMG, it does have a few extremely beneficial uses.
1. Time of activation: Surface EMG is a great way to measure the latency between any sort of stimulus (response to a visual, auditory, or tactile stimulus, response to a stretch, etc.) and the initiation of a muscular response. This can be used to track different timing characteristics among stimulus modalities.
2. Duration of activation: A reasonably accurate measure of the duration of activation can be made from surface EMG. Although, as we discussed previously, the cessation of EMG activity does not mean the cessation of force production.
3. Crude estimate of amount of activation: Holding all other conditions equal, more EMG activity is indicative of an increased effort of the nervous system to produce more force.
4. Patterns of activation: During complex and ballistic movements, surface EMG can give us an idea of when certain synergistic and antagonistic muscles fire relative to one another as well as when certain postural muscles may fire before extremity movement (e.g. delayed transverse abdominis firing with low back pain).
Surface EMG continues to be a useful tool for research and professional purposes. Information about the neural strategies of producing force and coordinated movements can be of particular interest to strength coaches. Hopefully this article has been helpful in describing some of the factors that warrant consideration when reading and interpreting EMG research. If not, there’s always wikipedia!
This article was originally published on www.StrengthCoach.com, an athletic development website where some of the world’s experts in strength and conditioning print their articles and discuss current issues.
Kevin Neeld, BSc, MS, CSCS is the Director of Athletic Development at Endeavor Fitness in Sewell, NJ and the author of Hockey Training University’s “Off-Ice Performance Training Course,” a must-have resource for every hockey program. Through the application of functional anatomy, biomechanics, and neural control, Kevin specializes in guiding hockey players to optimal health and performance. Kevin developed an incredible ice hockey training membership site packed full of training programs, exercise videos, and articles specific to hockey. For a FREE copy of “Strong Hockey Core Training”, one of the sessions from his course, go to his hockey training website.
Kevin has rapidly established himself as a leader in the field of physical preparation and sports science for ice hockey. He’s currently the Director of Performance at Endeavor Sports Performance in Pitman, NJ, the Strength and Conditioning Coach and Manual Therapist for the Philadelphia Flyers Junior Team, and a Strength and Conditioning Coach with USA Hockey’s Women’s National Team.