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The fact that step training after stand training reduced patients' ability to stand may evidence the limited ability of the human spinal networks for motor learning. During stepping, spinal neuronal networks called pattern generators are thought to be fundamental to achieving coordinated movements. Pattern generators can actually generate coordinated movement without any descending input4input⁴. During standing, a variety of peripheral and central strategies are thought to play an important role in undisturbed and disturbed quiet stance. These include reflexes driven by peripheral feedback from proprioceptive and tactile sensory neurons, as well as feedforward strategies. The stretch reflex is thought to play an important role in disturbed quiet stance, particularly in the early response from zero to one seconds after a disturbance⁵. The sensory input for the stretch reflex comes from muscle spindle neurons, which wrap around specialized muscle fibers, but the response is thought to rely on central spinal networks⁶. It is possible that step training and stand training affect the ability of the descending, voluntary signals to influence these spinal networks, in particular the central pattern generator networks and the stretch reflex networks. 

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The results of this study indicate that stretch reflexes with a static gain tuned for undisturbed stance do not in and of themselves reduce the time to fall due to a disturbance. While these results may be surprising, they agree with prior research. Fitzpatrick et. al displaced subjects' limbs while collecting EMG muscle activation data to measure the gains associated with the stretch reflex. The authors concluded that the gain was too low for standing to be attributed to this feedback mechanism. This implies that muscle activations attributable to stretch reflexes do not make up the most significant component of muscle activations during quiet undisturbed stance. However, observation of the disturbance trials led to the conclusion that the stretch reflex controllers did help keep joint angles close to their initial angles throughout the fall, even at very low gain values (data not shown). It's possible that the baseline gains of the stretch reflex help maintain joint angles during the early response to the disturbance, which puts the body in a more favorable position for middle and late latency feedforward responses. Indeed, subjects who undergo anterior-posterior floor translations maintain relatively constant knee and hip angles while rotating about the ankle and rocking onto the heels and toes to maintain balance. With larger disturbances, subjects rotate at the waist to reposition their center of mass to be above their center of pressure⁵. Mansouri et. al used baseline activations with a stretch reflex controller that chose optimal gains at each timestep, and were able to achieve balance and return to resting position under a variety of disturbance forces. This demonstrates the viability of a continued role for the stretch reflexes even after feedforward mechanisms begin, as the gains of the stretch reflex can be tuned via supraspinal inputs. 

References

1.) Singh, Anoushka et al. “Global Prevalence and Incidence of Traumatic Spinal Cord Injury.” Clinical Epidemiology 6 (2014): 309–331. PMC. Web. 7 June 2017.

2.) Angeli, Claudia A., et al. "Altering spinal cord excitability enables voluntary movements after chronic complete paralysis in humans." Brain 137.5 (2014): 1394-1409.

3.) Rejc Enrico, Angeli Claudia A., Bryant Nicole, and Harkema Susan J.. Journal of Neurotrauma. May 2017, 34(9): 1787-1802. https://doi.org/10.1089/neu.2016.4516

4.) Guertin, Pierre A. “Central Pattern Generator for Locomotion: Anatomical, Physiological, and Pathophysiological Considerations.” Frontiers in Neurology 3 (2012): 183. PMC. Web. 7 June 2017.

5.) Gatev, Plamen et al. “Feedforward Ankle Strategy of Balance during Quiet Stance in Adults.” The Journal of Physiology 514.Pt 3 (1999): 915–928. PMC. Web. 7 June 2017.

6.) Musienko, P. E. et al. “Facilitation of Postural Limb Reflexes With Epidural Stimulation in Spinal Rabbits.” Journal of Neurophysiology 103.2 (2010): 1080–1092. PMC. Web. 7 June 2017.

7.) Mansouri, Misagh B., and Jeffrey A. Reinbolt. "A platform for dynamic simulation and control of human movement." (2011).

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