August 23, 2018

Power, Kevin E. 1,2, Lockyer, Evan J. 2, Forman, Davis A. 3, and Button, Duane C. 1,2

1 School of Human Kinetics and Recreation and 2 Faculty of Medicine, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada
3 Faculty of Sciences, University of Ontario Institute of Technology, Oshawa, Ontario, Canada

Walking is a rhythmic motor output that can be performed with little to no conscious effort. Despite seeming like a fairly simple task, walking is actually quite complex with numerous muscles, limbs and joints that must be controlled with precise timing and coordination in order to produce this activity. A significant portion of this highly-coordinated movement is controlled by networks of neurones within the spinal cord known as central pattern generators (CPGs). Ultimately, however, it is the activation of spinal motoneurones that project out to, and activate skeletal muscles. Thus, understanding the factors regulating activity of spinal motoneurones is of the utmost importance if we are to truly understand how movement is produced and influenced by exercise.

Much of the current understanding regarding the regulation of motoneurone activation during CPG-mediated motor outputs, such as locomotion, comes from work done using non-human animal preparations. This research has shown that the properties of spinal motoneurones undergo transient, reversible changes that alter their ability to activate muscles during walking. In the literature review by Power et al. (2018), we summarize many of the findings from this research and discuss how those findings have led to the design of our current research program aimed at translating this information to human-based studies with the addition of measures of supraspinal excitability given its importance in human motor control.

The human version of a CPG-mediated motor output that we use to examine neural control is arm cycling. To do so, we rely on non-invasive stimulation techniques at the level of the motor cortex and spinal cord to asses supraspinal and spinal motoneurone excitability, respectively. In our review, we discuss how supraspinal and spinal motoneurone excitability are modulated differently during arm cycling depending on several factors, including the state (pre-cycling versus steady-state cycling), task (cycling versus tonic contraction), phase (flexion versus extension), intensity (cadence versus load), and muscle being examined. We highlight potential mechanisms that may be underlying the findings and speculate ways in which we can investigate them in future studies. In essence, we show that spinal motoneurone excitability is modulated in a task-dependent manner, much like that of the non-human animal studies. Of importance, however, is that supraspinal input to the motoneurone pool appears to play a more crucial role in the control of this CPG-mediated motor output (i.e. arm cycling) in humans than in our non-human animal counterparts.

The information we are gathering on the neural control of arm cycling has the potential to be directly impactful in the ‘real world’ mainly because arm cycling is frequently used as a rehabilitation tool for individuals with motor impairment following stroke or spinal cord injury. By developing a better understanding of supraspinal and spinal motoneurone excitability during arm cycling, we allow for the possibility of improving current rehabilitative and training strategies for these populations.

Take away points:

  1. The neural control of rhythmic motor outputs in humans shares similarities with non-human animals, but there also appears to be many important differences.
  2. Spinal motoneurones activate muscle. Thus, understanding the factors that influence spinal motoneurone activity is crucial to our understanding of how movement is produced.
  3. Spinal motoneurone activity depends on many factors such as the task being performed, the intensity of the task and even the muscle being activated. These findings align with the training specificity principle.

Original Article:

Power, Kevin E., Lockyer, Evan J., Forman, Davis A. and Button, Duane C. Modulation of motoneurone excitability during rhythmic motor outputs. Appl Physiol Nutr Metab. 2018 March 9. doi: 10.1139/apnm-2018-0077

This article is a summary of an article published in Applied Physiology, Nutrition & Metabolism. If you intend to cite any information in this article, please consult the original article and cite that source. This summary was written for the Canadian Society for Exercise Physiology and it has been reviewed by the CSEP Knowledge Translation Committee.