Phd Defense by Christopher Hovorka

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In partial fulfillment of the Requirements for the Degree of Doctor of Philosophy

 in the

 School of Applied Physiology

 Christopher F. Hovorka

 Will defend his thesis

 “Influencing Motor Behavior through Constraint of Lower Limb Motion”

 Friday, December 12, 2014

8:00 a.m.

School of Applied Physiology Auditorium (Room 1253)

Georgia Institute of Technology

 Thesis Advisor:

Robert Gregor, PhD

 Committee Members:

Young-Hui Chang, PhD

Geza Kogler, PhD

Mindy Millard-Stafford, PhD

Steven L. Wolf, PhD





Christopher Hovorka



Directed by Robert Gregor, PhD and Young-Hui Chang, PhD

 Limited knowledge of the neuromechanical response to use of an ankle foot orthosis-footwear combination (AFO-FC) has created a lack of consensus in understanding orthotic motion control as a therapeutic treatment. Lack of consensus may hinder the clinician’s ability to target the motion control needs of persons with movement impairment (e.g., peripheral nerve injury, stroke, etc.). Some evidence suggests a proportional relationship between joint motion and neuromuscular activity based on the notion that use of lower limb orthoses that constrain joint motion may invoke motor slacking and decreasing levels of muscle activity.

Use of AFO-FCs likely alters the biomechanical and neuromuscular output as the central control system gradually forms new movement patterns, but is there an equivalent proportional relationship between the magnitude of joint motion and muscle activity?  Considering principles of neuromechanical adjustment, my general hypothesis examines whether orthotic control of lower limb motion alters neuromuscular output in proportion to the biomechanical output as a representation of the limb’s dynamics updated by the neural control system. The rationale for this approach is that reference knowledge of the neuromechanical response is needed to inform clinicians about how a person responds to walking with motion controlling devices such as ankle foot orthoses combined with footwear.

In Aim 1a, I showed that a newly developed AFO maximizing leverage and stiffness provides effective control of talocrural motion. During treadmill walking, healthy subjects using the AFO in a STOP condition substantially limited the range of talocrural plantarflexion and dorsiflexion dynamics to 3.7° and in a FREE condition maintained ankle motion to 24.2° compared to 27.7° in the CONTROL (no AFO) condition. Similar constraint of talocrural motion in the STOP condition and free talocrural motion in the FREE condition was achieved in matched cadaveric and healthy limbs during a static loading study compared to similar healthy subjects in a gait study.

In Aim 1b, I showed that a newly developed curved-flexible footwear system provides effective rollover dynamics. The curved-flexible footwear system was integrated with an AFO in a STOP condition used by healthy subjects that elicited similar cadence, stance and swing duration and effective leg-ankle-foot roll over radius compared to walking in curved-flexible footwear integrated with the AFO in a FREE condition and a CONTROL (no AFO) condition. To validate rollover dynamics of the curved-flexible footwear, a follow up study of healthy subjects treadmill walking in newly developed flat-rigid footwear integrated with the AFO in a STOP condition elicited interrupted leg-ankle-foot rollover compared to walking in curved-flexible footwear in STOP, FREE and CONTROL conditions.

In Aim 2, I showed that the same subjects and ipsilateral AFO-footwear system presented in Aim 1 exhibited an immediate yet moderate 30% decline in EMG activity of ipsilateral Soleus, Medial and Lateral Gastrocnemii in the STOP condition compared to the CONTROL condition. The reduction in EMG activity was maintained for 15 minutes of treadmill walking. On the contralateral leg, there was an immediate yet small increase of 1% to 14% in EMG activity above baseline which after 10 minutes of walking declined to a similar level of EMG activity as in the CONTROL condition.

These findings provide strong evidence that the moderate 30% reduction in muscle activation is not an equivalent proportion compared to the 85% reduction in total range of talocrural motion. The clinical relevance of these findings suggests that short term use of orthotic constraint of motion in healthy subjects does not substantially reduce muscle activation. Additionally, these findings could be used to inform the development of orthoses and footwear as therapeutic motion control treatments for improved rehabilitation protocols. 



  • Workflow Status: Published
  • Created By: Danielle Ramirez
  • Created: 12/04/2014
  • Modified By: Fletcher Moore
  • Modified: 10/07/2016

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