Changes in electromyographic activity, mechanical power, and relaxation rates following inspiratory ribcage muscle fatigue

Antonio Sarmento,Andrea Aliverti,Vanessa Resqueti,Layana Marques,Guilherme Fregonezi,Maria Lira,Francesca Pennati

doi:10.1038/s41598-021-92060-y

Antonio Sarmento, Andrea Aliverti + Show 5 more

Open Access

https://doi.org/10.1038/s41598-021-92060-y

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Abstract

Muscle fatigue is a complex phenomenon enclosing various mechanisms. Despite technological advances, these mechanisms are still not fully understood in vivo. Here, simultaneous measurements of pressure, volume, and ribcage inspiratory muscle activity were performed non-invasively during fatigue (inspiratory threshold valve set at 70% of maximal inspiratory pressure) and recovery to verify if inspiratory ribcage muscle fatigue (1) leads to slowing of contraction and relaxation properties of ribcage muscles and (2) alters median frequency and high-to-low frequency ratio (H/L). During the fatigue protocol, sternocleidomastoid showed the fastest decrease in median frequency and slowest decrease in H/L. Fatigue was also characterized by a reduction in the relative power of the high-frequency and increase of the low-frequency. During recovery, changes in mechanical power were due to changes in shortening velocity with long-lasting reduction in pressure generation, and slowing of relaxation [i.e., tau (τ), half-relaxation time (½RT), and maximum relaxation rate (MRR)] was observed with no significant changes in contractile properties. Recovery of median frequency was faster than H/L, and relaxation rates correlated with shortening velocity and mechanical power of inspiratory ribcage muscles; however, with different time courses. Time constant of the inspiratory ribcage muscles during fatigue and recovery is not uniform (i.e., different inspiratory muscles may have different underlying mechanisms of fatigue), and MRR, ½RT, and τ are not only useful predictors of inspiratory ribcage muscle recovery but may also share common underlying mechanisms with shortening velocity.

Highlights

Muscle fatigue is a complex phenomenon enclosing various mechanisms
As fatigue is related to muscle fiber constituency[19], and this feature is measured in terms of how rapidly EMG spectral variables are reduced as a function of contraction d uration[20], median frequency and high-to-low frequency ratio (H/L) of the ribcage muscles would change with different time constants during fatigue and recovery
Experiments were performed in healthy adults, and associations between changes in relaxation rates and shortening velocity and mechanical power were investigated to observe if these parameters share a common underlying mechanism during recovery from fatigue

Summary

Introduction

Muscle fatigue is a complex phenomenon enclosing various mechanisms. Despite technological advances, these mechanisms are still not fully understood in vivo. Recovery of median frequency was faster than H/L, and relaxation rates correlated with shortening velocity and mechanical power of inspiratory ribcage muscles; with different time courses. As pressure and shortening velocity are known, mechanical power of respiratory muscles can be estimated non-invasively[13,15] In this context, the use of only non-invasive devices and simultaneous assessment of power spectrum sEMG changes during dynamically loaded contractions may help understand respiratory muscles fatigue development and recovery in vivo. We aimed to investigate contractile and relaxation properties, shortening velocity indexes, mechanical power, and sEMG changes of three inspiratory ribcage muscles before (pre-fatigue) and after (recovery) a fatigue protocol using 70% of individuals’ maximal inspiratory pressure. Relationships were calculated with power spectrum parameters to understand whether relaxation rates were useful predictors of recovery

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