Estimating Neural Control from Concentric vs. Eccentric Surface Electromyographic Representations during Fatiguing, Cyclic Submaximal Back Extension Exercises.

Gerold R Ebenbichler,Paolo Bonato,Richard Habenicht,Lena Unterlerchner,Sara Riegler,Patrick Mair,Josef Kollmitzer,Thomas Kienbacher

doi:10.3389/fphys.2017.00299

Abstract

Purpose: To investigate the differences in neural control of back muscles activated during the eccentric vs. the concentric portions of a cyclic, submaximal, fatiguing trunk extension exercise via the analysis of amplitude and time-frequency parameters derived from surface electromyographic (SEMG) data.Methods: Using back dynamometers, 87 healthy volunteers performed three maximum voluntary isometric trunk extensions (MVC's), an isometric trunk extension at 80% MVC, and 25 cyclic, dynamic trunk extensions at 50% MVC. Dynamic testing was performed with the trunk angular displacement ranging from 0° to 40° and the trunk angular velocity set at 20°/s. SEMG data was recorded bilaterally from the iliocostalis lumborum at L1, the longissimus dorsi at L2, and the multifidus muscles at L5. The initial value and slope of the root mean square (RMS-SEMG) and the instantaneous median frequency (IMDF-SEMG) estimates derived from the SEMG recorded during each exercise cycle were used to investigate the differences in MU control marking the eccentric vs. the concentric portions of the exercise.Results: During the concentric portions of the exercise, the initial RMS-SEMG values were almost twice those observed during the eccentric portions of the exercise. The RMS-SEMG values generally increased during the concentric portions of the exercise while they mostly remained unchanged during the eccentric portions of the exercise with significant differences between contraction types. Neither the initial IMDF-SEMG values nor the time-course of the IMDF-SEMG values significantly differed between the eccentric and the concentric portions of the exercise.Conclusions: The comparison of the investigated SEMG parameters revealed distinct neural control strategies during the eccentric vs. the concentric portions of the cyclic exercise. We explain these differences by relying upon the principles of orderly recruitment and common drive governing motor unit behavior.

Highlights

Progressive trunk muscle resistance training is widely recommended to increase back muscle strength, endurance and power, which in turn allows one to maintain pain-free functional performance of trunk movements and to build injury resilience (Scharrer et al, 2012; Watson et al, 2015; Steffens et al, 2016)
As muscle fiber structures of the back extensors are known to differ between males and females (Mannion, 1999), and the composition of the back extensor muscles as well as the size of the motor units likely depends on age (Kienbacher et al, 2014a; Hunter et al, 2016), we further examined whether sex and age would affect the characteristics of the surface electromyographic (SEMG) data collected during concentric vs. eccentric portions of the task
The RMS-SEMG initial values for the concentric portions of the cyclic exercise were found to be different from the initial RMSSEMG values for the eccentric portions of the exercise

Summary

Introduction

Progressive trunk muscle resistance training is widely recommended to increase back muscle strength, endurance and power, which in turn allows one to maintain pain-free functional performance of trunk movements and to build injury resilience (Scharrer et al, 2012; Watson et al, 2015; Steffens et al, 2016). Recruitment and de-recruitment of MUs during non-isometric, cyclic contractions have been shown to follow a hierarchical order according to the MU size and the principle of common drive of MUs (De Luca et al, 2015), which is the synchronous modulation of the firing rate of all active MUs of a given muscle (De Luca and Erim, 1994). Such modulation corresponds to a simultaneous increase/decrease in the excitation of all MU pools. This observation suggests that some similarities in MU behavior are likely to mark the eccentric and concentric muscle contractions during slow cyclic exercises

Methods

Results

Conclusion