Master of Science
Dr. Charles L. Rice
Exploring muscle architecture in vivo and estimating the number of MUs in the human anconeus muscle have important implications related to the neuromuscular function of this muscle as a model for study in health and disease. The two studies presented in this thesis investigate the functional anatomy of the anconeus in 10 healthy young men (25±3y).
Ultrasound imaging has facilitated the measure of the architectural variables, fascicle length (LF) and pennation angle (PA), in many human skeletal muscles in vivo. However, the functional anatomy of the anconeus has been investigated mainly from cadavers exclusively. Thus, the purpose of Chapter 2 was to evaluate, using ultrasonography, the degree of change in architectural features, LF and PA, of the anconeus at rest, across the full range of motion for the elbow joint. The protocol involved imaging the anconeus at 135°, 120°, 90°, 45°, and 0° of elbow flexion. The results indicate that anconeus muscle architecture is dynamic, with LF and PA decreasing and increasing, respectively, with extension of the elbow. The values obtained here are more representative of architectural changes at various elbow joint positions than those reported in cadaveric studies.
Motor unit number estimates (MUNE) can be determined electrophysiologically using decomposition-enhanced spike-triggered averaging. To provide the most representative MUNE, muscle activation should equal or exceed the upper limit of MU recruitment to activate the majority of the MU pool. A limitation of muscles studied to date, using DE-STA, is an inability to obtain reliable MUNEs at forces higher than ~30% of a maximum voluntary contraction. Unique features of the anconeus muscle may permit MUNEs at higher muscle activation levels. Thus, the purpose of Chapter 3 was to estimate the number of functional MUs in the anconeus, using DE-STA, at low (10%), moderate (30%), and higher (50%) relative muscle activation levels (root-mean-square of maximum voluntary contraction (RMSMVC)), to determine the effect of muscle activation on MUNEs in this muscle. Low average MUNEs of 58, 38, and 25 were found for the low, moderate, and higher muscle activations, respectively. A histogram of the distribution of surface-detected MU potentials and elbow extensor force-EMG relationship suggest the most representative MUNE was obtained at 50%RMSMVC.
The main findings of this thesis are that; 1) anconeus muscle architecture is dynamic, 2) anconeus allows for a more representative MUNE derived at higher muscle activation levels, and 3) the high signal-to-noise ratio that has made the anconeus a choice model in the study of MU properties, is more likely attributed to a relatively low number of MUs than minimal absolute change in its muscle architecture with elbow excursion.
Stevens, Daniel E., "Functional Anatomy of the Anconeus: Muscle Architecture and Motor Unit Number Estimation" (2014). Electronic Thesis and Dissertation Repository. 1945.