Mirror Electromyografic Activity in the Upper and Lower Extremity: A Comparison between Endurance Athletes and Non-Athletes

Abstract

During unimanual motor tasks, muscle activity may not be restricted to the contracting muscle, but rather occurs involuntarily in the contralateral resting limb, even in healthy individuals. This phenomenon has been referred to as mirror electromyographic activity (MEMG). To date, the physiological (non-pathological) form of MEMG has been observed predominately in upper extremities (UE), while remaining sparsely described in lower extremities (LE). Accordingly, evidence regarding the underlying mechanisms and modulation capability of MEMG, i.e., the extent of MEMG in dependency of exerted force during unilateral isometric contractions are insufficiently investigated in terms of LE. Furthermore, it still remains elusive if and how MEMG is affected by long-term exercise training. Here, we provide novel quantitative evidence for physiological MEMG in homologous muscles of LE (tibialis anterior (TA), rectus femoris (RF)) during submaximal unilateral dorsiflexion in healthy young adults. Furthermore, endurance athletes (EA, n = 11) show a higher extent of MEMG in LE compared to non-athletes (NA, n = 11) at high force demands (80% MVC, maximum voluntary contraction). While the underlying neurophysiological mechanisms of MEMG still remain elusive, our study indicates, at least indirectly, that sport-related long-term training might affect the amount of MEMG during strong isometric contractions specifically in trained limbs. To support this assumption of exercise-induced limb-specific MEMG modulation, future studies including different sports disciplines with contrasting movement patterns and parameters should additionally be performed.

Voluntary Inhibition of Physiological Mirror Activity: An EEG-EMG Study

“…Investigations of right-handed adults, using similar isometric contractions to our implemented task, reported no side differences in pMA between right and left-hand contractions ( Addamo et al, 2009 ; Koerte et al, 2010 ; Sehm et al, 2016 ; Maudrich et al, 2017 , 2018 ). Therefore, it seems reasonable to assume that the choice of the contracting hand in the current study (left), most likely did not influence the obtained result.…”

“…A consistent and robust observation is that the amplitude of pMA is modulated by force demands of unilateral movements, i.e., increases with rising force requirements ( Todor and Lazarus, 1986 ; Aranyi and Rosler, 2002 ; Shinohara et al, 2003 ; van Duinen et al, 2008 ; Sehm et al, 2010 , 2016 ; Maudrich et al, 2017 ). To avoid any influence of this relationship, the force level in this study was kept constant (40 ± 5% MVC).…”

Structural Neural Correlates of Physiological Mirror Activity During Isometric Contractions of Non-Dominant Hand Muscles

“…All voluntary EMG amplitudes were normalized with respect to individual MVC values. In accordance to previous investigations of pMA 12 , 19 , 20 , pMA amplitudes were expressed as multitudes of the 1000 ms pre-burst baseline EMG signal. This means, that for example, a pMA value of 1 states there was no change compared to the 1000 ms pre-burst baseline signal and a value of 2 states that the involuntarily occurring EMG activity increased by 100% compared to the 1000 ms pre-burst baseline signal.…”

“…The following descriptions of the experimental setup, as well as methodological and statistical approaches for the analysis of all acquired behavioral data are based on a previous study we conducted 20 . For details, please refer to the respective manuscript.…”

“…Therefore, we classified the participants into three different groups in accordance to their amount of regular physical exercise. This was motivated by the fact that in our previous work we hypothesized that regular practice of different sports disciplines might affect pMA in specific ways due to long-term neuroplasticity and neural reorganization induced by the respective training regimen 20 . To correct for this possible confound we divided our sample into participants with less than 2 hours per week of combined sports practice (n = 11), endurance type activities of more than 2 hours per week (n = 9) and strength/power type activities of more than 2 hours per week (n = 6).…”