*3.1.2. The maximum activation obtained during the task under investigation performed at maximum effort*

To reduce the possibility of obtaining normalized EMG levels during a task greater than 100%, investigators have used the EMG obtained during the task under investigation performed at maximum effort as the normalization value. For example, maximum EMG recorded during isometric shoulder abduction has been used to normalize the EMG during submaximal abduction [65], maximum crunch exercise for submaximal crunch exercise [66], maximum sprinting for normalizing the EMG during walking [44, 67] and maximum sprint cycling for normalizing the EMG during cycling [38].

This method of normalizing EMG data produces high reliability between trials [44, 67] and greatly reduces the possibility of obtaining EMG levels during the task of interest greater 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 between different tasks.

Normalization of EMG Signals: To Normalize or Not to Normalize and What to Normalize to? 185

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

**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

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

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

during the activity in each muscle for each individual separately.

between individuals over time [24, 25, 42, 90].

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

will lead to muscle fatigue.
