Features of electromyography threshold of the respiratory muscles during incremental exercise test

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Highlights

Abstract

In this study, we aimed to examine the electromyography threshold (EMGT) of the respiratory muscle and accessory respiratory muscles. Sixteen healthy men underwent an incremental exercise test at 15 W/minute to the end point. Expired gases and electromyograms of the respiratory and lower limb muscles were measured. The breakpoints for the EMG and expired gas data were analyzed using a segmented regression model. EMGT of the sternocleidomastoid and diaphragm was significantly more delayed than the ventilation threshold (VT) (287.94 s, 288.15 s vs. 185.5 s, p = 0.028 and 0.044, respectively). The EMGT of respiratory muscles and VT were not related, though EMGT of rectus femoris and vastus lateralis correlated with VT (r = 0.854, p < 0.001; r = 0.657, p = 0.011, respectively). EMGT of respiratory muscles may be influenced by multiple factors, such as central command and afferent input of mechanical stimulation from muscles, in addition to VT-induced changes in metabolic dynamics.

Introduction

During the first few minutes of the incremental exercise test, ventilation increases in proportion to metabolism as measured by oxygen uptake (V˙O2) and carbon dioxide output (V˙CO2). At 40–60 % of the participant’s maximal oxygen uptake, there is a non-linear increase in V˙CO2 and ventilation, which is known as the “first ventilatory threshold (VT)” or “anaerobic threshold” (Mateika and Duffin, 1995; Wasserman et al., 1990; Wasserman and Koike, 1992). VT is often used as an indicator of training intensity in sports and rehabilitation settings. The increase in ventilation with exercise is considered to result from stimulation of respiratory neurons in the medulla oblongata via input from central command, peripheral chemoreceptors, and mechanoreceptors and chemoreceptors in skeletal muscles (Powers and Howley, 2020). It was reported that VT during the incremental exercise test was mainly due to peripheral receptor activity in response to lactic acidosis (Wasserman et al., 1990; Wasserman and Koike, 1992). However, since peripheral chemoreceptors alone are sufficiently significant to explain the large changes in ventilation response, multiple factors are assumed to influence the results.

Electromyography also shows a gradual increase in muscle activity during the incremental exercise test and occurs at a breakpoint. The breakpoint of electromyographic activity is called the electromyographic threshold (EMGT) and has been reported extensively in the lower limb muscles (Hug et al., 2003, 2006; Ertl et al., 2016). The mechanism of EMGT is considered to be affected by several factors, such as: a decrease in pH due to the accumulation of H+ (Ertl et al., 2016), accumulation of lactate (Ertl et al., 2016; Candotti et al., 2008), and the associated additional recruitment of motor units, mainly type 2 fibers (Candotti et al., 2008; Nagata et al., 1981). Furthermore, studies investigating the association between EMGT and VT have reported a high correlation between the two in the lower extremities (Candotti et al., 2008; Kang et al., 2014; Lucía et al., 1999; Nagata et al., 1981), and EMGT during incremental exercise testing likely depicts local metabolic changes in the muscle being recorded. However, most EMGT research has been focused on lower limb muscles, and respiratory muscles have not been investigated as much.

Since respiratory muscle activity increases with hyperventilation, EMGT of respiratory muscles is expected to be highly associated with VT causing hyperventilation. However, the time at which EMGT of respiratory muscle and accessory muscle occur is not known. From a functional point of view, we predict that VT and EMGT of the lower limb muscles are attained first, followed by EMGT of the diaphragm, which is the agonist muscle of respiration. EMGT of the accessory respiratory muscle is attained subsequently. Clarification of the relationship and time course of EMGT and VT in the lower extremities and respiratory muscles may help us understand breathing control and metabolic dynamics during exercise, and may provide useful data for countermeasures against respiratory muscle fatigue during exercise and for respiratory muscle training. Therefore, the main purpose of this study was to clarify the EMGT of respiratory muscles and accessory respiratory muscles. Additionally, we also aimed to compare the EMGT of the respiratory muscles with the VT and EMGT of the lower limbs muscles and identify the characteristics of the EMGT of the respiratory muscles.

Section snippets

Participants

Sixteen healthy men who were nonsmokers and did not engage in regular high-intensity exercise were recruited. Individuals with a history of respiratory or cardiovascular diseases or pneumothorax were excluded. The participants were recruited through word-of-mouth and advertisements posted on the university’s online message board. All participants underwent a pulmonary function test and an incremental exercise test. During the incremental exercise test, the electromyography for the respiratory

Participant characteristics and pulmonary function

Table 1 displays the participant characteristics and pulmonary function results. All participants were healthy young men, and their pulmonary function was %VC > 80 % and FEV 1.0 % >70 %.

Incremental exercise test

The incremental exercise test results are shown in Table 2. V˙O2 and HR at the endpoint of the exercise test were 31.63 ± 5.77 mL/min/kg and 166.12 ± 8.88 beats/min, respectively. Respiratory muscle parameters at the endpoint of the exercise test were as follows: EMGsc = 11.83 ± 8.01 %, EMGdi = 21.55 ± 9.74 %,

Discussion

In this study, we determined the EMGT of respiratory muscles during the incremental exercise test. The major findings of our study were as follows: 1) EMGT of some respiratory muscles was slower than the EMGT of VT, and 2) although the lower limb muscles were positively correlated with VT, the respiratory muscles were not. Thus, our findings suggest that EMGT in respiratory muscles occurs following VT and is caused by factors different from or including VT.

The EMGsc and EMGdi were significantly

Funding

This work was supported by JSPS KAKENHI Grant Number 21K17514.

Author contributions

K.K. and S.I. conceived and designed the research; K.K., S.I., M.K., Y.S., and K.T. performed the experiments; K.K. and S.I. analyzed data; K.K., S.I., and K.T. interpreted the results of experiments; K.K., S.I. prepared figures; K.K., Y.S., and K.T. drafted the manuscript; K.K., Y.S., and K.T. edited and revised the manuscript; K.K., S.I., M.K., Y.S., and K.T. approved the final version of the manuscript.

Declaration of Competing Interest

The authors report no competing interests.

Acknowledgments

We would like to thank Editage (www.editage.com) for English language editing.

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