*3.1.3. The maximum activation obtained at a range of joint angles under maximum effort during dynamic contraction*

There is a debate about whether isometric contraction can be used to obtain reference EMG levels for use during dynamic tasks [25]. Some research has found that the EMG levels change with muscle length [68-71], while other studies indicate that joint angle has little effect on maximum EMG levels [72-74] or that there is no consistent pattern of change in the EMG levels with joint angle [74-76]. To address this potential problem, it has been recommended that maximum dynamic (usually isokinetic) contractions be used to obtain reference EMG levels in order to normalize EMG data obtained during movement [77]. In this method, the individual performs a maximum isokinetic contraction at a speed similar to the dynamic task under investigation. The activation levels vs joint angle curve generated from the maximum dynamic contraction is then used to normalize the EMG data [77].

This normalization method has been shown to have low within subject reliability [78] and, because EMG is depended on the velocity of movement for a given force level [79], normalization curves need to be generated for different speeds of movement.

The use of supramaximal stimulation to determine if voluntary contractions are being performed at maximum levels

Maximal voluntary activation can be assessed by interpolation of an electrical stimulus to all or part of the nerve supply to a muscle during maximum voluntary effort. Single electrical stimuli are delivered to the nerve that innervates the muscle during maximum voluntary contraction with increasing intensity until no additional increment in force can be seen. Then 2-4 electric stimuli trains (20 ms between stimuli) are delivered at that intensity as they produce substantially larger evoked responses [80-82]. If the stimulus fails to evoke an increment in force it can be deduced that all motoneurones innervating the muscle are recruited i.e. that the muscle is being maximally activated [83-85].

One criticism of this method of generating maximal activation in a given muscle is that the force output of a muscle during a synchronous activation of the motor neurons, due to the stimulation of a nerve, does not necessarily produce the same force as when the motor neurons are being asynchronously activated by the central nervous system [4].In addition, its use for some muscles will be problematic due to difficulty accessing the nerve/s supplying these muscles e.g. branches of the brachial plexus supplying shoulder muscles. It also has the disadvantage that strong contractions maintained for more than a few seconds will lead to muscle fatigue.

Computational Intelligence in Electromyography Analysis – 184 A Perspective on Current Applications and Future Challenges

between different tasks.

*during dynamic contraction* 

performed at maximum levels

than the reference value. However, the maximum activation levels of muscles are unknown since maximum force production during the task under investigation does not necessarily produce a maximum activation level in any of the muscles under investigation [8]. In addition, different individuals may use different muscle control strategies to produce the same movement, resulting in different activation levels during the reference contraction in a given muscle between individuals. Therefore, although highly reliable, the use of this method to normalize EMG data to compare muscle activation levels between individuals and between muscles in the task being investigated is not valid. In addition, because this reference value is task dependent, it cannot be used to compare muscle activation levels

*3.1.3. The maximum activation obtained at a range of joint angles under maximum effort* 

There is a debate about whether isometric contraction can be used to obtain reference EMG levels for use during dynamic tasks [25]. Some research has found that the EMG levels change with muscle length [68-71], while other studies indicate that joint angle has little effect on maximum EMG levels [72-74] or that there is no consistent pattern of change in the EMG levels with joint angle [74-76]. To address this potential problem, it has been recommended that maximum dynamic (usually isokinetic) contractions be used to obtain reference EMG levels in order to normalize EMG data obtained during movement [77]. In this method, the individual performs a maximum isokinetic contraction at a speed similar to the dynamic task under investigation. The activation levels vs joint angle curve generated from the maximum dynamic contraction is then used to normalize the EMG data [77].

This normalization method has been shown to have low within subject reliability [78] and, because EMG is depended on the velocity of movement for a given force level [79],

The use of supramaximal stimulation to determine if voluntary contractions are being

Maximal voluntary activation can be assessed by interpolation of an electrical stimulus to all or part of the nerve supply to a muscle during maximum voluntary effort. Single electrical stimuli are delivered to the nerve that innervates the muscle during maximum voluntary contraction with increasing intensity until no additional increment in force can be seen. Then 2-4 electric stimuli trains (20 ms between stimuli) are delivered at that intensity as they produce substantially larger evoked responses [80-82]. If the stimulus fails to evoke an increment in force it can be deduced that all motoneurones innervating the muscle are

One criticism of this method of generating maximal activation in a given muscle is that the force output of a muscle during a synchronous activation of the motor neurons, due to the stimulation of a nerve, does not necessarily produce the same force as when the motor neurons are being asynchronously activated by the central nervous system [4].In addition,

normalization curves need to be generated for different speeds of movement.

recruited i.e. that the muscle is being maximally activated [83-85].

#### **3.2. Peak or mean activation levels obtained during the task under investigation**

The first report of normalized EMG signals [9] presented quadriceps EMG signals during walking as a percentage of the peak muscle activity that occurred during the gait cycle [8]. Since then, this method has been used to investigate muscle activation patterns during various activities e.g. walking [25, 86], cycling [87], biceps curl exercise [24] and kayaking [88]. In this method, the EMG data is normalized to the peak or mean activity obtained during the activity in each muscle for each individual separately.

Normalising to the peak or mean amplitude during the activity of interest has been shown to decrease the variability between individuals compared to using raw EMG data or when normalising to MVICs [24, 25, 86, 87]. Normalizing to the mean amplitude during the activity of interest has been reported to be either comparable to [34], or better than [24, 42, 89, 90], normalizing to the peak amplitude during the activity in reducing the variability between subjects. Although the within subject and within day reliability have been shown to be high for both peak and mean amplitude during an activity [42], it has also been shown that they may be less reliable between days in the same individuals compared to normalizing to MVICs [90].

However, the reduction in the variability between individuals by normalising to the peak or mean amplitude recorded during an activity is achieved by removing some real biological variation (e.g. strength difference) between individuals [24, 90]. The amount of muscle activity required to lift a given load, would vary according to each individual's strength. As the reference value used in this method is relative to the task and not to the maximum capacity of the muscle, muscle activity levels cannot be compared between muscles, tasks or individuals. This method, however, can be used to compare patterns of muscle activation between individuals over time [24, 25, 42, 90].

#### **3.3. Activation levels during submaximal isometric contractions**

The use of maximal contractions to obtain reference EMG levels has been questioned because of difficulty in getting subjects to mobilize their maximal potential especially in symptomatic subjects who cannot perform a maximum contraction because of pain, muscle inhibition [42, 91] or risk of injury [91]. As a result, the use of tests at submaximal contraction levels have been used to produce reference EMG levels for the purposes of normalizing the EMG signals. De Luca [4] encouraged the use of EMGs from contractions < 80% of MVIC. However, there is no consensus as to whether submaximal contractions have higher within-day reliability than [23], or similar reliability to [92], maximal contractions. Commonly used submaximal isometric contractions include holding a limb against gravity [24, 26, 48, 87, 92] or holding a given load, either an absolute load [24, 93-95] or a relative load determined as a percentage of each individual's maximum load [25]. The muscle activity recorded during the submaximal isometric contraction is then used to normalize the EMG in the same muscle while performing the task under investigation.

Normalization of EMG Signals: To Normalize or Not to Normalize and What to Normalize to? 187

between individuals over time with high reliability but does not allow comparisons of activity levels between muscles, tasks or individuals. The normalization methods of submaximal isometric contractions or maximum activation during the task under investigation performed at maximum effort also do not allow valid comparisons of muscle activity levels between muscles or individuals, and in addition, muscle activation patterns between individuals are potentially more variable because different individual motor control strategies may be used. Finally, the use of maximum activation levels obtained under maximum effort during dynamic contraction and the M-max methods to normalize EMG signals are associated with

Studies use EMG to identify differences in the activation levels and patterns between normal subjects and those with neuro-musculo-skeletal dysfunction with the aim of understanding the cause of the dysfunction and developing improved rehabilitation programs to treat the dysfunction. Since the use of MVICs is the most valid method to normalize EMG data allowing comparison of activity levels between muscles in different individuals, it should be the normalization method of choice when evaluating muscle function in clinical populations provided symptomatic individuals can produce MVICs. Indeed recent studies have shown that individuals from some clinical populations (moderate knee osteoarthritis [58], following knee surgery [103], back pain [104, 105], cerebral palsy [106], stroke [45, 107]), are able to produce maximum activation levels using the same MVIC tests as healthy individuals [8]. If symptomatic individuals are unable to elicit maximal contractions, e.g. as a result of pain due to illness or injury, then comparisons between these clinical populations and normal subjects can only be made using normalization to peak or mean activation levels obtained during the task under investigation. Under these circumstances comparisons of activity levels between muscles, between tasks and between individuals are not valid. Only comparison of muscle

activation patterns between normal and symptomatic individuals can be made.

*Discipline of Exercise and Sport Science, Faculty of Health Science, The University of Sydney,* 

*Discipline of Biomedical Sciences, Sydney Medical School, The University of Sydney, Sydney,* 

[1] Cram JR, Kasman GS. (2011) The basics of surface electromyography. In: Criswell E, Cram JR, editors. Cram's introduction to surface electromyography. 2nd ed. Sudbury,

**Author details** 

*Sydney, Australia* 

**6. References** 

MA: Jones and Bartlett. p. 1-170.

Mark Halaki

Karen Ginn

*Australia* 

low within subject reliability and cannot be recommended.

**5. EMG Normalization in clinical populations** 

The main limitation of using submaximal isometric contractions is that comparisons of activity levels between muscles and individuals are not valid because, once again, the reference value used in this method is not relative to the maximum capacity of the muscle. Lifting an absolute load of say 1 kg mass might require 10% of the maximum muscle capacity in a strong individual compared to say 40% of the maximum muscle capacity in another person who is not as strong. It is not possible to estimate maximum muscle activity from a relative submaximal contraction by linear extrapolation because the torque/EMG relationship is nonlinear [96]. Additionally, the lengths of muscle moment arms in individuals vary and since the EMG signal is related to the force produced by the muscle and not the torque produced by the limb, the force required by the muscle to produce a given torque would be different between individuals. Another limitation is that the motor strategy may not be the same between individuals or between sides within the same individual [95] during the reference submaximal contraction. This is not a problem during maximal contractions as heightened central drive engages all possible muscle resources to achieve the maximum force possible. Therefore, using submaximal isometric contractions as the reference for normalizing EMG data is reliable but doesn't allow valid comparisons between muscles or individuals.

## **3.4. Peak to peak amplitude of the maximum M-wave (M-max)**

This method of normalizing EMG signals involves external stimulation of α-motor neurons. When a peripheral motor nerve is stimulated at a point proximal to a muscle it activates the muscle to contract. This signal is called the M-wave and can be recorded using EMG electrodes placed on/in that muscle. To obtain maximum activation in the muscle and produce a maximum M-wave (M-max), the amplitude of stimulation is increased until the peak to peak amplitude of the M-wave does not increase further. To ensure maximum simulation, the amplitude of the stimulation is increased by an additional 30%. The amplitude of the M-max is then used to normalize EMG signals from the same muscle during the tasks of interest [97]. Currently, this normalization method is problematic as the repeatability of the M-max is questionable. It seems to be less reliable as the background contraction level increases [98], decreases with time [99], and is dependent on muscle length [100-102] and the task performed [98, 102]. If these factors that affect the M-max values could be controlled resulting in more reliable measurements, this method to normalize EMG data has the potential to facilitate comparisons between muscle, between tasks and between individuals.

#### **4. Summary**

In summary, only the normalization method that uses MVICs as the reference level can be validly used to compare muscle activity levels and activation patterns between muscles, tasks and individuals, provided that maximum neural activation is achieved in all muscles and individuals tested. The use of peak or mean activation levels obtained during the task under investigation as the reference EMG level can be used to compare patterns of muscle activation between individuals over time with high reliability but does not allow comparisons of activity levels between muscles, tasks or individuals. The normalization methods of submaximal isometric contractions or maximum activation during the task under investigation performed at maximum effort also do not allow valid comparisons of muscle activity levels between muscles or individuals, and in addition, muscle activation patterns between individuals are potentially more variable because different individual motor control strategies may be used. Finally, the use of maximum activation levels obtained under maximum effort during dynamic contraction and the M-max methods to normalize EMG signals are associated with low within subject reliability and cannot be recommended.
