In vivo quantification of three dimensional volumetric strain in the human tibialis anterior

Intramuscular pressure (IMP), which is closely correlated with both active and passive muscle tension, may become a useful supplement to current clinical tools as a means to quantify individual muscle-generated force. A continuing challenge associated with this measure is its non-uniform distribution, which is not yet fully understood. Several studies have observed that pressure increases with muscle depth. Conservation of mass suggests that these regional pressure differences may result from non-uniformly distributed changes in local tissue volume. Therefore, the overarching goal of this work was to characterize volumetric strain distribution in skeletal muscle as a means to better understand the mechanism driving the non-uniform IMP distribution. Three-dimensional volumetric strain distribution had not been previously quantified in skeletal muscle; therefore the bulk of this thesis work revolved around developing and validating a method for this purpose using cine Phase Contrast (CPC) magnetic resonance imaging (MRI). CPC MRI has been previously used to quantify 2D strain distribution in skeletal muscle. Fortunately, the method lends itself to 3D measurements using multiple slice data collection, but this requires a lengthy data acquisition time. We chose to develop the method during passive tension of the human tibialis anterior (TA), because passive tension is closely correlated with IMP and the motion repeatability is more readily controlled and maintained for an extended duration than active tension. As hypothesized, volumetric strain was found to be non-uniformly distributed during passive tension of the human TA with a decreasing trend from the anterior (superficial) to the posterior (deep) muscle regions. These data align with previously observed trends of decreasing IMP near the muscle surface and may provide important insight into ideal sensor placement regions to maximize measurement repeatability. These results advance our understanding of the tension-IMP relationship in muscle by providing insight into the mechanism behind the non-uniform distribution of IMP. Furthermore, this work has strong potential to contribute to a computational model relating IMP to muscle tension by way of volumetric strain