As previously discussed, the mammalian tongue consists of an intricate array of variably aligned and extensively interwoven muscle fibers, meaning the DSI method used to analyze the excised bovine tongue ( Gilbert et al., 2006) gave superior results in regions of crossing fibers. The streamline DTI method, used in the human tongue study ( Gaige et al., 2007), substituted this angular resolution for the reduction in scan time necessary to perform an in vivo study of lingual myoarchitecture, highlighting an apparent trade-off between tracking precision and experimental feasibility. Tractography provided a good evaluation of muscle shape and of fiber orientation in human thigh muscles ( Budzik et al., 2007), and the 3D myoarchitecture of human ( Gaige et al., 2007) and bovine ( Gilbert et al., 2006) tongue have also been investigated. A 3D reconstruction of the tibialis anterior has shown the physiological cross-sectional area, fiber length, and pennation angle (the angle between the longitudinal axes of the whole muscle and its fibers) to be in agreement with the values obtained previously using invasive methods ( Heemskerk et al., 2005). Voxel-scale estimates of fiber orientations have been shown to correspond to the anatomical reality ( Van Doorn et al., 1996 Van Donkelaar et al., 1999), and a series of publications by Damon and Heemsherk have gone some way to validating fiber-tracking methods in skeletal muscle ( Damon et al., 2002 Heemskerk et al., 2005 Heemskerk and Damon, 2007 ). Parker, in Diffusion MRI (Second Edition), 2014 MuscleĪs described in Section 20.2.2, muscle is a useful model with which to test the validity of the tractography process. ![]() The differences between tonic (or slow) and twitch (or fast) fiber types are reflected in rise and fall time constants of the excitation dynamics that model the sagging or yielding properties, the activation frequency that represents the calcium dynamics, and the muscle force-length and muscle force-velocity properties. The level of effective activation of each fiber results from a linear combination of multiple motor unit activations weighted by their respective fractional physiological cross-sectional area. The effective recruitment signal is characterized by a rise and fall time constant that is determined by the first-order dynamics of the exchange of calcium between the nerve cells and muscle fiber within the motor unit. Upon recruitment, the lumped motor unit modulates its frequency according to an effective recruitment signal that is proportional to the amount of muscle recruited. The sudden recruitment of a motor unit at its initial firing rate causes a step increase of muscle force the size of that step is determined by the fractional physiological cross-sectional area of that unit, which is a function of the number of motor units. ![]() ![]() To model the forces within a muscle, it is essential to consider several recruitment schemes of multiple motor units for different fibers.
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