Human motor unit characteristics of the superior trapezius muscle with age-related comparisons

Abstract

Current understanding of human motor unit (MU) control and aging is mostly derived from hand and limb muscles that have spinal motor neuron innervations. The aim here was to characterize and test whether a muscle with a shared innervation supply from brainstem and spinal MU populations would demonstrate similar age-related adaptations as those reported for other muscles. In humans, the superior trapezius (ST) muscle acts to elevate and stabilize the scapula and has primary efferent supply from the spinal accessory nerve (cranial nerve XI) located in the brainstem. We compared electrophysiological properties obtained from intramuscular and surface recordings between 10 young (22-33 yr) and 10 old (77-88 yr) men at a range of voluntary isometric contraction intensities (from 15 to 100% of maximal efforts). The old group was 41% weaker with 43% lower MU discharge frequencies compared with the young (47.2 ± 9.6 Hz young and 26.7 ± 5.8 Hz old, P < 0.05) during maximal efforts. There was no difference in MU number estimation between age groups (228 ± 105 young and 209 ± 89 old, P = 0.33). Furthermore, there were no differences in needle detected near fiber (NF) stability parameters of jitter or jiggle. The old group had lower amplitude and smaller area of the stimulated compound muscle action potential and smaller NF MU potential area with higher NF counts. Thus, despite age-related ST weakness and lower MU discharge rates, there was minimal evidence of MU loss or compensatory reinnervation.NEW & NOTEWORTHY The human superior trapezius (ST) has shared spinal and brainstem motor neuron innervation providing a unique model to explore the impact of aging on motor unit (MU) properties. Although the ST showed higher MU discharge rates compared with most spinally innervated muscles, voluntary strength and mean MU rates were lower in old compared with young at all contraction intensities. There was no age-related difference in MU number estimates with minimal electrophysiological evidence of collateral reinnervation.

Keywords: aging; firing rate; neck; upper trapezius; voluntary force.

Fig. 1.

Representations of the experimental set-up and recorded electromyography (EMG). A: schematic of the dynamometer. B: motor unit (MU) potential overlay of the monopolar needle EMG and force tracing during an isometric maximal voluntary contraction (MVC) of an old participant (82 yr). C: surface EMG and concentric needle EMG during 15% of MVC. LC, load cell. Scale bars are associated with the EMG and force tracings.

Fig. 2.

Motor unit (MU) discharge frequencies of the superior trapezius (ST) muscle. A, D, and G: scatterplots are of mean MU frequencies and SD (error bars) for each participant dependent on age for 25% of maximal voluntary contraction (MVC; A), 50% of MVC (D), and MVC intensities (G) (n = 20). B, C, E, F, H, and I: kernel density plots representing the distribution of MU discharge frequencies dependent on age group and contraction intensity, with the smoothing bandwidth being scaled to the SD of the smoothing kernel.

Fig. 3.

Negative peak motor unit (MU) number estimates (MUNE) of the superior trapezius muscle. A: scatterplot representing the MUNE for each participant dependent on age (n = 20). B and C: near fiber (NF) MU potential overlay from the concentric needle and measurement from the decomposition-based quantitative electromyography algorithm between a young (B) and old (C) participant, respectively. the y-axis is 10 kV·s⁻²·division⁻¹, and the x-axis is 1 ms/division.