Fatigue in Neck Muscles & Trapezius Activity During Rest

The velocity recovery function in sternocleidomastoid muscle fibers and its dependence on fatigue- 179.22

My poster presentation was during Sunday morning's session. I don't typically stray very far from my own poser during the time it is up but I was able to review the work of a few posters in my area. Immediately next to me was a poster by Dr. Deborah Falla. Dr. Falla sought to investigate how muscle fatigue would alter the velocity recovery function of sternocleidomastoid muscle fibers. A linear array 8 eight electrodes was used to provide subjects with real-time visual feedback of a single motor unit. After subjects practiced the controlling of the motor unit, they performed 30-s contractions modulating the motor unit from it's minimal to it's maximal discharge rate. Subjects then performed a fatiguing contraction with the sternocleidomastoid. Immediately following the fatiguing contraction the minimal and maximal rates were assessed again. Among other things, the conduction velocity of the target motor unit action potential was assessed at the minimum and maximum discharge rate pre- and post-fatigue and during the recovery contraction. Dr. Falla found that the minimum (7.9 pps) and the maximum (28.0 pps) discharge rates did not differ across contractions. Action potential conduction velocity was found to be lowest at minimum compared to maximum discharge rates in all contractions. I learned from Dr. Falla's poster that the reduction in action potential conduction velocity with fatigue depends on the discharge rate of motor unit activation. This means that the velocity recovery function of sternocleidomastoid muscle fibers is altered by fatigue.

Spontaneous discharge of trapezius motor units during periods of instructed rest exhibits modulation at respiratory frequencies- 179.20

I also managed to take a look at the poster of fellow PhD candidate Jennifer Stephenson. The aim of Jenni's study was "to examine the modulation of trapezius motor units that were spontaneously active during periods of instructed muscle rest under conditions of low and high acute psychosocial stress." Subjects performed brief contractions of the trapezius muscle that were separated by 60 s of instructed rest. During the rest the subjects performed a verbal math task in two separate conditions: 1) a low-stress condition required an easy math task performed without punishment for time or accuracy, whereas the high-stress condition combined a difficult math task with social evaluative threat (yikes!). Motor unit activity of the trapezius was quantified using the demodulated power spectra and the coherence between concurrently active motor units. Jenni looked at the frequencies corresponding to respiratory (0.1-0.5 Hz) and cortical (16-32 Hz) inputs to the motor units. Peaks were identified at the frequency corresponding respiratory, but not cortical inputs. The coherence at the respiratory frequencies was similar across stress conditions. Whereas motor unit coherence at respiratory frequencies was similar during periods of instructed rest and voluntary contraction, coherence at cortical frequencies was greater during voluntary contraction than during instructed rest. Jenni's data indicate that cortical input is not a major source of excitation underlying trapezius motor unit activity during periods of instructed rest.

- Mike