Duane A. Morrow

Transversely isotropic tensile material properties of skeletal muscle tissue.

Of the plethora of work performed analyzing skeletal muscle tissue, relatively little has been done in the examination of its passive material properties. Previous studies of the passive properties of skeletal muscle have been primarily performed along the longitudinal material direction. In order to ensure the accuracy of the predictions of computational models of skeletal muscles, a better understanding of the tensile three-dimensional material properties of muscle tissue is necessary. To that end, the purpose of this study was to collect a comprehensive set of tensile stress-strain data from skeletal muscle tissue. Load-deformation data was collected from eighteen extensor digitorum longus muscles, dissected free of aponeuroses, from nine New Zealand White rabbits tested under longitudinal extension (LE), transverse extension (TE), or longitudinal shear (LS). The linear modulus, ultimate stress, and failure strain were calculated from stress-strain results. Results indicate that the linear modulus under LE is significantly higher than the modulus of either TE or LS. Additionally, the ultimate stress of muscle was seen to be significantly higher under LE than TE. Conversely, the failure strain was significantly higher under TE than under LE.

Performance characteristics of a pressure microsensor

A new fiber optic microsensor has been developed for pressure measurement. The pressure microsensor is extremely small (360 micro m). Performance characteristics of the microsensor were evaluated over a pressure range from 0 to 250mmHg. Five sensors were compared to a National Institute of Standards and Technology traceable reference pressure. The microsensor had an accuracy, repeatability, and linearity better than 2% full-scale output (FSO) and a hysteresis of 4.5% FSO.

Constitutive Modeling of Skeletal Muscle Tissue with an Explicit Strain-Energy Function

While much work has previously been done in the modeling of skeletal muscle, no model has, to date, been developed that describes the mechanical behavior with an explicit strain-energy function associated with the active response of skeletal muscle tissue. A model is presented herein that has been developed to accommodate this design consideration using a robust dynamical approach. The model shows excellent agreement with a previously published model of both the active and passive length-tension properties of skeletal muscle.

Intraneural ganglia: a clinical problem deserving a mechanistic explanation and model

Intraneural ganglion cysts have been considered a curiosity for 2 centuries. Based on a unifying articular (synovial) theory, recent evidence has provided a logical explanation for their formation and propagation. The fundamental principle is that of a joint origin and a capsular defect through which synovial fluid escapes following the articular branch, typically into the parent nerve. A stereotypical, reproducible appearance has been characterized that suggests a shared pathogenesis. In the present report the authors will provide a mechanistic explanation that can then be mathematically tested using a preliminary model created by finite element analysis.

A method for assessing the fit of a constitutive material model to experimental stress-strain data

Higher-order polynomial functions can be used as a constitutive model to represent the mechanical behaviour of biological materials. The goal of this study was to present a method for assessing the fit of a given constitutive three-dimensional material model. Goodness of fit was assessed using multiple parameters including the root mean square error and Hotellings T 2-test. Specifically, a polynomial model was used to characterise the stress–strain data, varying the number of model terms used (45 combinations of between 3 and 11 terms) and the manner of optimisation used to establish model coefficients (i.e. determining coefficients either by parameterisation of all data simultaneously or averaging coefficients obtained by parameterising individual data trials). This framework for model fitting helps to ensure that a given constitutive formulation provides the best characterisation of biological material mechanics.

Skeletal muscle tensile strain dependence: Hyperviscoelastic nonlinearity

INTRODUCTION Computational modeling of skeletal muscle requires characterization at the tissue level. While most skeletal muscle studies focus on hyperelasticity, the goal of this study was to examine and model the nonlinear behavior of both time-independent and time-dependent properties of skeletal muscle as a function of strain. MATERIALS AND METHODS Nine tibialis anterior muscles from New Zealand White rabbits were subject to five consecutive stress relaxation cycles of roughly 3% strain. Individual relaxation steps were fit with a three-term linear Prony series. Prony series coefficients and relaxation ratio were assessed for strain dependence using a general linear statistical model. A fully nonlinear constitutive model was employed to capture the strain dependence of both the viscoelastic and instantaneous components. RESULTS Instantaneous modulus (p<0.0005) and mid-range relaxation (p<0.0005) increased significantly with strain level, while relaxation at longer time periods decreased with strain (p<0.0005). Time constants and overall relaxation ratio did not change with strain level (p>0.1). Additionally, the fully nonlinear hyperviscoelastic constitutive model provided an excellent fit to experimental data, while other models which included linear components failed to capture muscle function as accurately. CONCLUSIONS Material properties of skeletal muscle are strain-dependent at the tissue level. This strain dependence can be included in computational models of skeletal muscle performance with a fully nonlinear hyperviscoelastic model.

Error analysis of cine phase contrast MRI velocity measurements used for strain calculation

Cine Phase Contrast (CPC) MRI offers unique insight into localized skeletal muscle behavior by providing the ability to quantify muscle strain distribution during cyclic motion. Muscle strain is obtained by temporally integrating and spatially differentiating CPC-encoded velocity. The aim of this study was to quantify CPC measurement accuracy and precision and to describe error propagation into displacement and strain. Using an MRI-compatible jig to move a B-gel phantom within a 1.5 T MRI bore, CPC-encoded velocities were collected. The three orthogonal encoding gradients (through plane, frequency, and phase) were evaluated independently in post-processing. Two systematic error types were corrected: eddy current-induced bias and calibration-type error. Measurement accuracy and precision were quantified before and after removal of systematic error. Through plane- and frequency-encoded data accuracy were within 0.4 mm/s after removal of systematic error - a 70% improvement over the raw data. Corrected phase-encoded data accuracy was within 1.3 mm/s. Measured random error was between 1 to 1.4 mm/s, which followed the theoretical prediction. Propagation of random measurement error into displacement and strain was found to depend on the number of tracked time segments, time segment duration, mesh size, and dimensional order. To verify this, theoretical predictions were compared to experimentally calculated displacement and strain error. For the parameters tested, experimental and theoretical results aligned well. Random strain error approximately halved with a two-fold mesh size increase, as predicted. Displacement and strain accuracy were within 2.6 mm and 3.3%, respectively. These results can be used to predict the accuracy and precision of displacement and strain in user-specific applications.

Design Considerations of a Fiber Optic Pressure Sensor Protective Housing for Intramuscular Pressure Measurements

Intramuscular pressure (IMP), defined as skeletal muscle interstitial fluid pressure, reflects changes in individual muscle tension and may provide crucial insight into musculoskeletal biomechanics and pathologies. IMP may be measured using fiber-optic fluid pressure sensors, provided the sensor is adequately anchored to and shielded from surrounding muscle tissue. Ineffective anchoring enables sensor motion and inadequate shielding facilitates direct sensor-tissue interaction, which result in measurement artifacts and force-IMP dissociation. The purpose of this study was to compare the effectiveness of polyimide and nitinol protective housing designs to anchor pressure sensors to muscle tissue, prevent IMP measurement artifacts, and optimize the force-IMP correlation. Anchoring capacity was quantified as force required to dislodge sensors from muscle tissue. Force-IMP correlations and non-physiological measurement artifacts were quantified during isometric muscle activations of the rabbit tibialis anterior. Housing structural integrity was assessed after both anchoring and activation testing. Although there was no statistically significant difference in anchoring capacity, nitinol housings demonstrated greater structural integrity and superior force-IMP correlations. Further design improvements are needed to prevent tissue accumulation in the housing recess associated with artificially high IMP measurements. These findings emphasize fundamental protective housing design elements crucial for achieving reliable IMP measurements.

Evaluating the dynamic performance of a fibre optic pressure microsensor

The dynamic performance of a new fibre optic sensor intended for measuring physiological fluid pressures is assessed in water. The sensors sensitivity is evaluated at 23 degrees, 35 degrees and 37 degrees C against a Millar pressure catheter for sinusoidal pressure inputs with frequency ranging from 0.5 to 10 Hz. We found that sensitivity versus frequency is flat to 6 Hz and decreases slightly between 6 and 10 Hz. The sensitivity is slightly lower at 23 degrees C than at 37 degrees C. The reproducibility of measurements is excellent (two separate calibration tests in two consecutive days). The output of the fibre optic system used shows a constant time delay (0.13 s) for all frequencies tested. Experiments suggest that, with current sensor design, its immersion in degassed water prior to use ensures a reliable performance.

Analysis of fluid movement in skeletal muscle using fluorescent microspheres

Regional variability in interstitial fluid pressure confounds use of intramuscular pressure measurement to assess muscle force. It is hypothesized that interstitial flow is dependent on intramuscular pressure. The goal of this study was to assess the feasibility of using fluorescent microspheres to evaluate movement of interstitial fluid in skeletal muscle.