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Huang M.1, Fu S.1, Kwok L.1, Pang M.Y.C.1
1Hong Kong Polytechnic University, Department of Rehabilitation Science, Hong Kong, Hong Kong
Background: Motor impairment and disuse may potentially lead to material changes, and hence musculature mechanical properties alternation in the paretic limbs after stroke. Weakness of the ankle dorsiflexors is common among individuals with stroke and may give rise to unsuccessful foot clearance during gait. Tibialis anterior is a major ankle dorsiflexor but its mechanical properties remain understudied among people with stroke. Shear wave ultrasound elastography is an advanced technology that allows the evaluation of muscle elasticity in vivo non-invasively.
Purpose: This study aimed to compare the mechanical properties of tibialis anterior muscle between the paretic and non-paretic sides among people with chronic stroke, using shear wave ultrasound elastography.
Methods: Fourteen individuals with chronic stroke [6 women and 8 men; mean age: 63.5 years (SD: 6.0); median Fugl-Meyerlower-limb motor score: 25 (IQR:4); median Modified Ashworth Scale: 1.5 (IQR=1)] participated in this study. Both the paretic and non-paretic sides were assessed in random order. Shear wave ultrasound elastography was used to measure shear modulus (i.e. an elastic moduli) of the tibialis anterior muscle during passive movement of the ankle from dorsiflexion (median: 15°) to plantarflexion (median: 45°) (i.e., stretching the tibialis anterior muscle). The outcome variables were:
(1) slack angle: joint angle beyond which the muscle begins to develop passive elastic force upon further stretch,
(2) slack elasticity: shear modulus value measured at slack angle, and
(3) rate of increase in shear modulus as the muscle is being stretched beyond the slack angle.
Elasticity-angle data was curve-fitted by optimizing the slack angle, slack elasticity, and rate of increase in shear modulus within a piecewise exponential model. Paired-t tests were used to compare the slack angle, slack elasticity, and rate of increase in elasticity between paretic and non-paretic sides.
Results: Elasticity-angle data for both sides obtained from all participants were well fitted with the piecewise exponential model (coefficients of determination=0.957 to 0.978). On the paretic side, the mean (SD) value of the slack angle, slack elasticity, and rate of increase in shear modulus were 16.2° (7.2°), 5.7 (1.6) kPa, and 0.044 (0.012) respectively. And in the non-paretic side, the corresponding values were 11.3° (7.7°), 5.6 (1.5) kPa, and 0.036 (0.012). The rate of increase in shear modulus of the tibial anterior muscle on the paretic side was significantly greater than that on the non-paretic side (p=0.017). No significant side-to-side differences were found in the slack angle and slack elasticity (p>0.05).
Conclusion(s): Our results demonstrate no significant side-to-side difference in elasticity when the tibialis anterior muscle is put in a slack position. As the muscle is being stretched beyond the slack angle, the rate of increase in shear modulus is greater on the paretic side than the non-paretic side, indicating decreased passive elastic performance of the muscle on the paretic side.
Implications: Shear wave ultrasound elastography would be a useful tool to assess the response to passive stretch of individual muscles in people after stroke. The mechanical properties of the tibialis anterior muscle on the paretic side are altered, which may compromise other motor functions and should warrant further investigations.
Funding acknowledgements: Meizhen HUANG is supported by the Hong Kong Polytechnic University Research Studentship as a full-time Ph.D.
Topic: Neurology: stroke
Ethics approval: This study was approved by the Human Subject Ethics Subcommittee of the Hong Kong Polytechnic University.
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