Is the EMG fatigue threshold a valid tool to assess muscle function?

Quadriceps Muscles O2 Extraction and EMG Breakpoints during a Ramp Incremental Test

Muscle deoxygenated breakpoint ([HHb]BP) has been found to be associated with other indices of exercise tolerance in the vastus lateralis (VL) muscle but not in the vastus medialis (VM) and rectus femoris (RF). Purpose: To investigate whether the [HHb]BP occurs also in the VM and RF muscles and whether or not it is associated with other physiological indices of exercise tolerance, such as the EMG threshold (EMG_t) and the respiratory compensation point (RCP). Methods: Twelve young endurance trained participants performed maximal ramp incremental (RI) cycling tests (25-30 W·min^-1 increments). Muscle oxygen extraction and activity as well as ventilatory and gas exchange parameters were measured. After accounting for the mean response time, the oxygen uptake ([Formula: see text]O₂) corresponding to the RCP, [HHb]BP, and the EMG_t was determined. Results: Peak power output (PO_peak) was 359 ± 48 W. Maximal oxygen consumption ([Formula: see text]O_2max) was 3.87 ± 0.46 L·min^-1. The [Formula: see text]O₂ at the RCP was 3.39 ± 0.41 L·min^-1. The [Formula: see text]O₂ (L·min^-1) corresponding to the [HHb]BP and EMG_t were: 3.49 ± 0.46 and 3.40 ± 0.44; 3.44 ± 0.61 and 3.43 ± 0.49; 3.59 ± 0.52, and 3.48 ± 0.46 for VL, VM, and RF, respectively. Pearson's correlation between these thresholds ranged from 0.90 to 0.97 (P < 0.05). No difference was found for the absolute [Formula: see text]O₂ and the normalized PO (%) at which the thresholds occurred in all three muscles investigated (P > 0.05). Although in eight out of 12 participants, the [HHb]BP in the RF led to a steeper increase instead of leading to a plateau-like response as observed in the VL and VM, the [Formula: see text]O₂ at the breakpoints still coincided with that at the RCP. Conclusions: This study demonstrated that local indices of exercise tolerance derived from different portions of the quadriceps are not different to the systemic index of the RCP.

Utility of electromyographic fatigue threshold during treadmill running

INTRODUCTION

We investigated 2 different methods for determining muscle fatigue threshold by electromyography (EMG).

METHODS

Thirteen subjects completed an incremental treadmill running protocol for EMG fatigue threshold (EMGFT ) determination based on the critical power concept (EMGFT 1) and the breakpoint in the linear relationship between EMG amplitude and exercise intensity (EMGFT 2). Then, both the EMGFT 1 and EMGFT 2 were tested in a continuous treadmill running protocol. EMG was recorded from the rectus femoris (RF), vastus lateralis (VL), biceps femoris (BF), and lateral gastrocnemius (LG) muscles.

RESULTS

For BF, EMGFT 2 was higher than EMGFT 1, and EMGFT 1 for BF was lower than EMGFT 1 for LG. EMG of RF was higher at EMGFT 2 than at EMGFT 1, and LG EMG was lower at EMGFT 2.

CONCLUSIONS

EMGFT can be determined during a single treadmill running test, and EMGFT 1 may be the most appropriate method to estimate the muscle fatigue threshold during running.

The electromyographic fatigue threshold is not a valid tool to assess muscle function

The present study aimed at determining the electromyographic fatigue threshold (EMG(FT)) from the EMG activity level and the EMG frequency content of the First Dorsal Interosseous. Thirty-seven healthy subjects performed seven isometric index abductions at randomly ordered percentages of maximal voluntary contraction (i.e., 20%, 25%, 30%, 35%, 40%, 50% and 60%). During these bouts, surface EMG was measured using a linear electrodes array (i.e., seven EMG channels) in the First Dorsal Interosseous. For each subject the EMG(FT) was determined from both Root Mean Square (RMS) and Mean Power Frequency (MPF) values, only if the following criteria were met: (i) significant positive linear regression (P<0.05) between force and slope coefficient, (ii) an adjusted coefficient of determination for force versus slope coefficient relationship greater than 0.85, and (iii) a standard error for the EMG(FT) below 5% of MVC. The results showed the inability to determine an EMG(FT) in all of the 37 subjects from both RMS (9 out of 37 subjects) and MPF (27 out of 37 subjects). In addition, for the 12 subjects tested twice, the reproducibility of the EMG(FT) determination was weak (ICC=-0.029 and SEM=7.5% of MVC for EMG(FT) determined from MPF). The present results suggest that the EMG(FT) is not a valid tool to assess muscle function.