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![muscle spindle muscle spindle](https://www.unm.edu/~lkravitz/Extras3/spindle.gif)
It is now widely established that the relative abundance of muscle spindles is best described using residual values of the linear regression of the log-transforms of spindle number against muscle mass. This negates much of the previously hypothesized underpinning of muscle spindle composition and leaves a substantial gap in our understanding of the functional determinants of muscle spindle composition. Second, the use of muscle spindle density has since been statistically discredited as a meaningful measure to compare muscle spindle composition. There are however two significant issues: first, these hypotheses were proposed without the support of quantitative functional data, and with subjective descriptions of ‘fine motor control’. The higher densities in smaller muscles were often seen as a functional correlate of those muscles being particularly important for fine motor control or functioning as kinesiological monitors. Early studies suggested that, in general, smaller muscles contain greater densities of muscle spindles than larger muscles. By contrast, corresponding data for GTOs have been less well catalogued. Measures of the number and distribution of these sensory organs have almost always been derived from muscle histological preparations, with comprehensive libraries of muscle spindle densities (number of spindles per gram of muscle) in humans having been compiled. Muscles differ considerably in their number of muscle spindles and GTOs, however, it is unknown what physiological or functional signal determines this variation. These proprioceptive signals are detected through two peripherally located sensory apparatuses: muscle spindles and Golgi tendon organs (GTOs).
MUSCLE SPINDLE DRIVERS
The coordination of activation and fibre length change across multiple muscle groups is a complex process, where the successful execution of a motor command relies on constant proprioceptive feedback to modulate central drivers of movement. Activation of extrafusal muscle fibres and changes in fascial length enable muscles to produce power. Skeletal muscle is functionally diverse, simultaneously operating as a motor to drive locomotion while also as a sensory organ to detect limb positions and modulate posture. These insights may be combined with neuromechanics and robotic studies of motor control to help further tease apart the functional drivers of muscle spindle composition. These data demonstrate that muscle fibre length, lengthening velocity and fibre force are key physiological signals to the central nervous system and its modulation of locomotion, and that muscle spindle abundance may be tightly correlated to how a muscle generates work. Novel correlations between functional indices and spindle abundance are also recovered, where muscles with a high abundance predominantly function as springs, compared to those with a lower abundance mostly functioning as brakes during walking.
![muscle spindle muscle spindle](https://yogauonline.com/sites/default/files/muscle_spindle_2__0.jpg)
These analyses indicate that muscle spindle number is tightly correlated with muscle fascicle length, absolute fascicle length change, velocity of fibre lengthening and active muscle forces during walking. Herein, we use integrated medical imaging and subject-specific musculoskeletal models to investigate the relationship between spindle abundance, muscle architecture and in vivo muscle behaviour in the human locomotor system. Previous use of spindle abundance as a correlate for muscle function implies a mechanical underpinning to this variation, but these ideas have not been tested. Muscle spindle abundance is highly variable within and across species, but we currently lack any clear picture of the mechanistic causes or consequences of this variation.
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