Measurement of the EMG-force relationship in a human upper airway muscle

Abstract

The upper airway muscles play an important role in breathing, swallowing, and speaking, but little is known about the electromyogram (EMG)-force relationship of these muscles. We have measured the peak integrated EMG activity (iEMG) and force of human nasal dilator muscles (NDM) with a custom-designed headpiece that was attached via the forehead and upper lip. The headpiece contains a micromanipulator that holds a rod with a load cell mounted on its tip. The reproducibility of the force measurements was examined by measuring the lateral or “flaring” force of the NDM in multiple trials on two separate occasions in 13 subjects. For these studies the subjects were instructed to perform maximal voluntary contractions (MVCs). Test-retest reproducibility averaged 8.3% (coefficient of variation) for within-day comparisons and 13.7% between days. We also measured iEMG and NDM force during an incremental exercise test in nine of the subjects; they were instructed to breathe nasally throughout one 30-s epoch at rest and at each workload. The iEMG and force during peak exercise (175–275 W) averaged 81 +/- 26% (SD) MVC and 235 +/- 127 mN (approximately 75% MVC), respectively. The iEMG during incremental exercise was linearly related to the peak force (r = 0.90, P < 0.001). Contractile properties were measured in seven of the subjects by application of single supramaximal shocks (0.1-ms pulse) to the facial nerve. Twitch force averaged 9 +/- 6% MVC, and the time to peak force was 62 +/- 13 ms, which is considerably faster than that in human diaphragm or elbow flexors.