Neuromuscular spindles relative to joint movement complexities

Small muscles, such as those producing movements of the digits, are known to contain high densities of muscle spindles compared to larger muscles of the limb. These high densities have been associated with an increased need for proprioceptive feedback during precise manipulative movements. A recent study has indicated that spindle densities are related to muscle size rather than to functional capability. By contrast, the current study examined whether spindle numbers, actual spindle densities and relative spindle densities correlated with the complexity of joint movements in two species with different muscle sizes, human and guinea pig. Published data were used for the histological parameters of each species and measurements were made of movement range in each kinematic degree of freedom for each joint of the human and guinea pig pectoral and pelvic limbs. Muscle weights of newborn humans were 315 times greater than those of guinea pigs but joint movement complexities were almost the same in the two species and actual spindle numbers of human were only 7.3 times those of guinea pig. Joint movement complexity tended to be greatest most proximally on each limb and progressively decreased more distally. Regardless of muscle weight differences, actual spindle numbers and relative spindle densities correlated positively and significantly with joint movement complexity and actual spindle densities correlated negatively in each species. Hence muscle spindle numbers and densities around joints appear to reflect joint functional capability for appropriate proprioceptive feedback to the central nervous system in the control of joint movement.