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Team Members
- Rob Carrera
Narration of presentation
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The goal of this research was to further investigate how the stretch reflex affects the ability to maintain quiet stance under disturbance. How might a reduced stretch reflex response affect the ability of a patient to withstand disturbance forces during quiet stance? The answer to this question is relevant to a patient who has a reduced ability to drive the stretch reflex response due to the nature of their specific injury, or who may have a reduced ability to drive the stretch reflex after step training. To investigate the role of the stretch reflex on maintaining quiet stance under disturbance, I applied horizontal disturbance forces in the anterior-posterior direction to the pelvis of a nine degree of freedom (DOF), eighteen muscle model confined to motions within the sagittal plane.
Methods
I used the 9 DOF, eighteen muscle model (below) developed by Ajay Seth, Darryl Thelen, Frank C. Anderson, and Scott L. Delp, which was subsequently modified by Carmichael Ong. The ankle and hip are modeled as pin joints, and the knee as a planar joint with prescribed x-y motions as a function of knee angle. Insertion of the quadriceps is handled by moving points in the tibia frame. Two contact spheres on the feet produce the ground reaction forces needed for standing support. The Hunt-Crossley contact model is used to calculate the contact force.
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To disturb the model, I applied a sinusoidal positive half cycle disturbance force to the model. The disturbance force had a duration of 0.2 seconds and a peak magnitude of either 2 Newtons or 10 Newtons. The force was directed in the anterior-posterior direction and was applied to the center of mass of the model's pelvis. The model had either the pure position or pure velocity stretch reflex controllers applied to the biarticular muscles. I also ran two additional trials with pure velocity stretch reflex controllers applied to the soleus and tibialis anterior as well as to the biarticular muscles. The performance metric of the disturbance trials was fall time. I approximated fall time as the time until one of the foot contact spheres lost contact with the ground, i.e. until the vertical component of the ground reaction force on that sphere dropped to zero.
Results
The results of the disturbance trials are shown below. Neither the pure velocity nor the pure position stretch reflex controllers improved time to fall relative to just the baseline activations without stretch reflex controllers. Additionally, applying stretch reflex controllers to the soleus and tibialis anterior either did not improve time to fall or reduced time to fall relative to the trials where stretch reflexes were applied only to biarticular muscles.
Discussion
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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