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The effects of 72 h of dynamic ankle immobilization on neural excitability and lower extremity kinematics
Jasmine J Cash1, Herman van Werkhoven2, Kelly J Cole2
1Department of Health Sciences and Research, Medical University of South Carolina, Charleston, SC, USA.
Background:
Ankle injuries can foster maladaptive changes in nervous system function that predisposes patients to subsequent injury. Patients are often placed in a dynamic boot immobilizer (BI) following injury; however, little is known about the effects of this treatment on neuromechanical function.
Research Question:
We aimed to determine the effect of 72 h of BI-use on neural excitability and lower extremity joint motion in a healthy cohort.
Methods:
Twelve uninjured individuals (20.8 ± 1.4 yrs, 1.7 ± 0.1 m, 75.2 ± 9.9 kg) participated in this crossover study. Neural excitability and lower extremity kinematics were assessed before and after 72 h of BI or compression sock (CS) use. Neural excitability was assessed via the Hoffmann (H) reflex and transcranial magnetic stimulation of the motor cortex by measuring muscle activation at the tibialis anterior, peroneus longus, and soleus of the immobilized extremity. Three-dimensional lower extremity joint angles were assessed while participants walked on a treadmill. Repeated-measures analyses of variance detected changes in neural excitability and peak joint angles across time-points and testing conditions, while statistical parametric mapping (SPM) was implemented to determine continuous joint angle changes (α = 0.05).
Results:
Pre-BI to post-BI, HMax:MMax ratio (F = 6.496; p = 0.031) significantly decreased. The BI did not alter resting motor threshold (F = 0.601; p = 0.468), or motor evoked potential amplitudes (F > 2.82; p > 0.608). Significant changes in peak knee and hip angles in the frontal and transverse planes were observed (p < 0.05), with no changes at the ankle. SPM analyses revealed significant hip and knee changes in range of motion (p < 0.05).
Significance:
Decreased measures of reflex but not corticospinal excitability suggest that BI-use for 72 h unloaded the joint enough to generate peripheral changes, but not the CNS, as has been described in casting models. Further, kinematic changes were observed in proximal lower extremity joints, likely due to swing-phase adaptations while wearing the BI.
