Orthotic Ankle Constraint Changes Muscle Activation During Walking
Hovorka, Christopher F.
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Ankle foot orthoses combined with footwear (AFO-FC) are commonly prescribed as a treatment to control ankle motion during walking. Despite their widespread use in rehabilitation, the influence of AFO-FC on the neuromuscular system is unclear. The lack of knowledge regarding AFO-FC on users' neuromuscular responses hinders clinicians, and designers of orthosis-footwear technology in providing evidence-based healthcare. To address the lack of knowledge, our aim was to examine the neuromuscular responses of subjects to short term use of orthotic ankle constraint while walking. We hypothesize that subjects walking with orthotic ankle constraint for 15 minutes will elicit no difference in Soleus and Tibialis Anterior muscle activation during stance phase compared to walking with no ankle constraint. To test this hypothesis, we recruited healthy subjects to walk at their self-selected speed on a dual belt treadmill while using three conditions a.) footshell-footwear system (control) and two randomized conditions, b.) using AFO-FC with free ankle plantarflexion and dorsiflexion (FREE) and c.) using AFO-FC with stopped ankle plantarflexion and dorsiflexion (CONSTRAIN). While subjects walked at self-selected speed, surface EMG of 8 lower limb muscles (Soleus, Medial and Lateral Gastrocnemius, Tibialis Anterior, Vastus Medialis, Rectus Femoris, Biceps Femoris, Gluteus Maximus) for each leg was collected (via wireless EMG system) in addition to kinematics (via high speed infrared cameras) and kinetics (via instrumented force plates under each treadmill). EMG, kinematics and kinetics data was synchronized and processed, then analyzed using MatJab to quantify bursts of each muscle's activation (iEMG) during stance phase of each gait cycle. Muscle fatigue was analyzed using Fast Fourier transformation of raw EMG data followed by analysis of median frequencies during each gait cycle. Seven subjects' data was processed and three subjects (2 male, 1 female) data was analyzed. Results reveal subjects elicited no trend of decrease in slope for median frequency of Tibialis Anterior or Soleus muscles during each condition (control, FREE, CONSTRAIN). Mean±SD iEMG (normalized to control) of ipsilateral soleus muscle during CONSTRAIN decreased 22.11 ±1.58%, ipsilateral soleus during FREE decreased 0.39±1.97%, contralateral Soleus during CONSTRAIN increased 0.78±1.25% and contralateral Soleus during FREE decreased 1.02 ± 1.34. Mean±SD iEMG of Tibialis Anterior during ipsilateral TA CONSTRAIN decreased 25.50±3.42%, ipsilateral TA during FREE decreased 2.18±3.71%, contralateral TA during CONSTRAIN decreased 2.18±4.19%, and contralateral TA during FREE increased 10.14±2.77%. Based on these results, the null hypothesis was rejected. No reduction in slope of median frequency for Soleus and Tibialis Anterior muscles suggest the changes in muscle activation are not due to fatigue, but instead are due to use of AFO-FC during CONSTRAIN. The rapid decrease in subjects' ipsilateral Soleus and ipsilateral Tibialis Anterior muscle activation to AFO in CONSTRAIN condition and no change in activation during AFO-FC in FREE condition suggests muscle spindle length dependent responses. Continued research is needed to confirm these findings.