Why normalize emg data

In addition, because this reference value is task dependent, it cannot be used to compare muscle activation levels between different tasks. 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 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. 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 ].

It also has the disadvantage that strong contractions maintained for more than a few seconds will lead to muscle fatigue. 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. 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. 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 ]. 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. However, there is no consensus as to whether submaximal contractions have higher within-day reliability than [ 23 ], or similar reliability to [ 92 ], maximal contractions. 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.

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. 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. This method of normalizing EMG signals involves external stimulation of -motor neurons. When a peripheral motor nerve is stimulated at a point proximal to a muscle it activates the muscle to contract.

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. 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 [ - ] and the task performed [ 98 , ]. 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. 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. 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 [ ], back pain [ , ], cerebral palsy [ ], stroke [ 45 , ] , 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. 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.

Licensee IntechOpen. Highlight the run s you wish to normalize in the Data window, located on the left side of the program. In the parameter window, check off Normalize Data and select your normalizing series as the MVC trial. Select OK and the calculation will run in the background, appearing in the Data window when finished. Plot the MVC normalization by selecting and right-clicking the series you wish to view.

You have the options to plot these separately or as subplots. Go to Top. Results: Interpretation of the unnormalized EMG signal suggests that a large difference in neural activation of the upper and lower sections of the rectus abdominis is occurring. In this condition the average activity in the lower rectus is This interpretation is incorrect.