Exp Physiol 93.3 pp 391–398 391 Experimental Physiology Evidence from proprioception of fusimotor coactivation during voluntary contractions in humans Trevor J. Allen, Gabrielle E. Ansems and Uwe Proske Department of Physiology, Monash University, Clayton, Victoria 3800, Australia In experiments on position sense at the elbow joint in the horizontal plane, blindfolded subjects were required to match the position of one forearm (reference) by placement of their other arm (indicator). Position errors were measured after conditioning elbow muscles of the reference arm with an isometric contraction while the arm was held either flexed or extended. The difference in errors after the two forms of conditioning was large when the conditioned muscles remained relaxed during the matching process and it became less when elbow muscles were required to lift a load during the match (10 and 25% of maximal voluntary contraction, respectively). Errors from muscle conditioning were attributed to signals arising in muscle spindles and were hypothesized to result from the thixotropic property of passive intrafusal fibres. Active muscle does not exhibit thixotropy. It is proposed that during a voluntary contraction the errors after conditioning are less, because the spindles become coactivated through the fusimotor system. The distribution of errors is therefore seen to be a reflection of fusimotor recruitment thresholds. For elbow flexors most, but not all, fusimotor fibres appear to be recruited by 10% of a maximal contraction. (Received 17 September 2007; accepted after revision 19 November 2007; first published online 26 November 2007) Corresponding author U. Proske: Department of Physiology, PO Box 13F, Monash University, Clayton, Victoria, 3800, Australia. Email: uwe.proske@med.monash.edu.au One of the ways in which an understanding has been sought of the role of muscle spindles in motor control is by study of the motor supply to the spindles, the fusimotor system. It was first shown by A.B. Vallbo that during a voluntary contraction there is coactivation of skeletomotor and fusimotor neurones (Vallbo, 1971, 1974). There is evidence that both static and dynamic fusimotor neurones are coactivated (Kakuda & Nagaoka, 1998). Static fusimotor activation is presumably associated with intrafusal shortening, to offset whole muscle shortening, while activation of dynamic fusimotor neurones allows maintenance of spindle dynamic sensitivity. This would be important for the capacity of the spindle to promptly signal small disturbances during tasks such as maintenance of postural stability (Matthews, 1981). In recent years, the observations of Vallbo have been confirmed and extended. Edin & Vallbo (1990) found that about three-quarters of their sample of primary and secondary endings of finger extensor muscle spindles had increased their discharge rate with a graded isometric contraction of up to 10% maximal voluntary contraction (MVC). The remainder decreased their discharge rate, presumably because their fusimotor recruitment threshold was higher and they were being mechanically unloaded by the contraction. Wilson et al. (1997) reported that in response to ramp forces of up to 20% MVC applied to tibialis anterior, most spindles (90%) increased their discharge rates at forces less than 3.2% MVC. It suggested that for tibialis the majority of fusimotor neurones had been coactivated at close to muscle contraction threshold. Similar findings were made by Kakuda & Nagaoka (1998) on finger extensor muscles. Interestingly, they too encountered some spindles which slowed their discharge rate for force levels up to 20% MVC. Taken together, the data suggest that the majority of spindles are coactivated at close to motor threshold. However, thresholds are distributed, and for some spindles fusimotor coactivation requires contractions of more than 20% MVC. All of the studies that have provided these data have used the technique of microneurography to study spindle discharge directly. One of the problems with the technique is that movement associated with muscle contraction can dislodge the recording electrode. It has meant that maintaining recordings during voluntary contractions of more than 20% MVC is difficult. C  2008 The Authors. Journal compilation C  2008 The Physiological Society DOI: 10.1113/expphysiol.2007.040741 ) at CAPES - Usage on March 10, 2010 ep.physoc.org Downloaded from Exp Physiol (