P 57 The influence of sports expertise on the extent of physiological mirror-EMG-activity in the upper and lower extremity

Abstract

Introduction During unimanual motor tasks, muscle activity may not be restricted to the contracting muscle, but has also been reported to occur involuntarily in the contralateral resting limb in healthy subjects, referred to as physiological mirror electromyographic (MEMG) activity ( Sehm et al., 2015 ). To date, however, it is unknown if the physiological form of MEMG can also be observed in lower extremities during the performance of unilateral isometric leg contractions. Furthermore it still remains elusive if and how MEMG is affected by long-term exercise training. Here we investigated the influence of sports expertise on the extent of MEMG in healthy young adults. We hypothesized that MEMG isn’t restricted to both upper extremities but is also reliably observable in both lower extremities and that the extent of MEMG is related to force requirements during motor tasks. In addition we expected that endurance athletes show more MEMG than non-athletes during strong unilateral contractions of hand and foot muscles. Methods Twenty-two healthy young adults participated in this study and were split into two equally sized experimental groups in accordance to their athletic background (Endurance Group (EG): n = 11, age: 23.0 (6.0) years; Control Group (CG): n = 11, age: 25.0 (7.0) years; median (IQR)). Subjects performed one isometric force generation task for the upper (UEX) and lower extremity (LEX) respectively on two separate days. The test procedure corresponds to a block design with varying force levels (20%, 50% & 80%) which were individually adjusted in accordance to the appropriate maximum voluntary contraction (MVC). EMG surface electrodes were applied bilaterally on a distal and proximal muscle. The bilateral set-up allowed us to measure EMG activity over the primary moving muscles as well as subliminal MEMG over the homologous muscles of the resting limb during behavioral tests. Repeated isometric contractions of left and right extremities were tested in a randomized order. Results The extent of MEMG significantly increased in distal and proximal muscles of LEX and UEX respectively as a function of force demands. Group comparison between athletes and controls revealed that athletes demonstrated a higher extent of MEMG specifically in the trained LEX but only during very strong contractions (80% MVC). There was no difference in the extent of MEMG in UEX between groups. Discussion Physiological MEMG is measurable in UEX as well as LEX during strong unilateral contractions and significantly increases as a function of force demands. Long-term sports participation induces modulations of interhemispheric connectivity which could in accordance to the concept of Motor Overflow lead to increased reversal of IHI to IHF connecting motor relevant brain regions during strong unilateral contractions ( Perez and Cohen, 2008 ) and may therefore be responsible for the higher extent of MEMG in endurance athletes compared to controls under high force demands specifically in trained limbs.