Muscle electromyographic activity normalized to maximal muscle activity, not to Mmax, better represents voluntary activation

Abstract

In human applied physiology studies, the amplitude of recorded muscle electromyographic activity (EMG) is often normalized to maximal EMG recorded during a maximal voluntary contraction. When maximal contractions cannot be reliably obtained (e.g. in people with muscle paralysis, anterior cruciate ligament injury, or arthritis), EMG is sometimes normalized to the maximal compound muscle action potiential evoked by stimulation, the Mmax. However, it is not known how these two methods of normalization affect the conclusions and comparability of studies. To address this limitation, we investigated the relationship between voluntary muscle activation and EMG normalized either to maximal EMG or to Mmax. Twenty-five able-bodied adults performed voluntary isometric ankle plantarflexion contractions to a range of percentages of maximal voluntary torque. Ankle torque, plantarflexor muscle EMG, and voluntary muscle activation measured by twitch interpolation were recorded. EMG recorded at each contraction intensity was normalized to maximal EMG or to Mmax for each plantarflexor muscle, and the relationship between the two normalization approaches quantified. A slope >1 indicated EMG amplitude normalized to maximal EMG (vertical axis) was greater than EMG normalized to Mmax (horizontal axis). Mean estimates of the slopes were large and had moderate precision: soleus 8.7 (95% CI 6.9 to 11.0), medial gastrocnemius 13.4 (10.5 to 17.0), lateral gastrocnemius 11.4 (9.4 to 14.0). This indicates EMG normalized to Mmax is approximately eleven times smaller than EMG normalized to maximal EMG. Normalization to maximal EMG gave closer approximations to the level of voluntary muscle activation assessed by twitch interpolation.

Force, angle, and velocity parameters of finger movements are reflected in corticospinal excitability

“…Mmax was elicited by electric stimulation of the median nerve proximal to the elbow (pulse width: 200 μs, amplitude: 400 V, variable electrical current up to 100 mA) and recorded from the FDP muscle (stimulator model DS7A, Digitimer Ltd, Welwyn garden city, England). Although we recorded Mmax, we chose to normalize EMG data to EMG during MVC, since this better represents voluntary activation (Diong et al, 2022), and because the Mmax measurement was not reliable in some participants.…”

Evidence of optimal control theory over active inference in corticospinal excitability modulations

“…Although Mmax was recorded, we opted to normalize EMG data relative to the MVC. This approach was chosen because it has been argued that it more accurately reflects voluntary muscle activation 21 , and because Mmax measurements in some participants were not reliable.…”