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J Appl Physiol 102: 144-148, 2007. First published September 7, 2006; doi:10.1152/japplphysiol.00362.2006
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Ia-afferent input to motoneurons during shortening and lengthening muscle contractions in humans

¹Institute of Exercise and Sport Sciences and Department of Medical Physiology, University of Copenhagen, Copenhagen N, Denmark; ²Prince of Wales Medical Research Institute, Randwick, New South Wales, Australia; ³School of Human Kinetics, University of British Columbia, Vancouver, Canada; ⁴School of Human Movement Studies and Division of Physiotherapy, The University of Queensland, St. Lucia, Queensland, Australia; and ⁵Department of Neuroscience, Karolinska Institutet, Stockholm, Sweden

Submitted 23 March 2006 ; accepted in final form 31 August 2006

The central nervous system employs different strategies to execute specific motor tasks. Because afferent feedback during shortening and lengthening muscle contractions differs, the neural strategy underlying these tasks may be quite distinct. Cortical drive may be adjusted or afferent input regulated. The exact mechanisms are not clear. Here, we examine the control of synaptic transmission across the Ia synapse during shortening and lengthening muscle contractions. Subjects were instructed to maintain isolated activity in a single tibialis anterior (TA) motor unit while muscle length was varied from flexion to extension and back. At a fixed interval after a firing of the active motor unit, a single electrical stimulus was applied to the common peroneal nerve to activate Ia afferents from the TA muscle. We investigated the stimulus-induced change in firing probability of 19 individual low-threshold TA motor units during shortening and lengthening contractions. Any change in firing probability depends on both pre- and postsynaptic mechanisms. In this experiment, motoneuron firing rate was similar during both contraction types. There was no difference in the firing probability between shortening and lengthening contractions (0.23 ± 0.03 and 0.20 ± 0.02, respectively). We suggest that there is no contraction type-specific control of Ia input to the motoneurons during shortening and lengthening muscle contractions. Cortical adjustments may have occurred.

motor unit; poststimulus time histogram; concentric; eccentric

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				J. Duchateau and R. M. Enoka Neural control of shortening and lengthening contractions: influence of task constraints J. Physiol., December 15, 2008; 586(24): 5853 - 5864. [Abstract] [Full Text] [PDF]