Muscle-specific endurance of the trapezius muscles using electrical twitch mechanomyography

Abstract

Background: Symptoms of fatigue and pain are often reported for the trapezius muscle in the shoulder. The present study evaluated endurance in the trapezius muscles of healthy participants using electric twitch mechanomyography (ETM).

Methods: Surface electrodes and a tri-axial accelerometer were placed over the left trapezius muscle. Muscles were stimulated for 3 min each at 2 Hz, 4 Hz and 6 Hz. Maintenance of acceleration during muscle twitches was used to calculate an endurance index (EI). Subjects (n = 9) were tested on two separate days to assess reproducibility of the trapezius EI measurements. The endurance measurements were made on the wrist flexor and vastus lateralis muscles for comparison. Near infrared spectroscopy was used to measure muscle oxygenation (HbO₂) during the stimulation protocol (n = 8).

Results: Mean (SD) EI was 84.9% (8.7%), 63.3% (19.1%) and 41.7% (20.0%) for 2 Hz, 4 Hz and 6 Hz, respectively. The coefficients of variation were 7.4%, 11.3% and 24.0% for 2 Hz, 4 Hz and 6 Hz, respectively. EI values were significantly lower in the trapezius compared to arm and leg muscles (p < 0.05). HbO₂ values were unchanged from resting values with electrical stimulation.

Conclusions: The EI as measured by ETM may provide a reproducible method of evaluating function in trapezius muscles that is not influenced by oxygen saturation.

Keywords: accelerometry; muscle fatigue; near infrared spectroscopy; shoulder muscles.

Figure 1.

Experimental set-up for the trapezius muscle endurance test. The stimulation electrodes are white and the accelerometer is black, located between the electrodes.

Figure 2.

Representative examples of muscle acceleration during electrical stimulation at 2 Hz, 4 Hz and 6 Hz. (a) trapezius muscle. (b) Forearm muscles. (c) leg muscles.

Figure 3.

Average endurance index (EI) values for the leg, arm and trapezius muscles. Significant differences between trapezius muscle EI and leg muscles are indicated by an asterisk (*) and significant differences between arm and trapezius muscles are indicated by a hash symbol (#). Values are the mean (SD).

Figure 4.

Endurance index values for the first and second day for each participant.

Figure 5.

Representative oxygen saturation values (HbO₂Diff) for the trapezius muscles at rest, during electrical stimulation and during a short duration of shoulder shrugs with arm weights. Values are given in optical density units.

Figure 6.

Average oxygen saturation values (HbO₂) for the trapezius muscles at rest and during electrical stimulation. Units are as a percentage of the signal change during a sustained isometric contraction. Values are the mean (SD).