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Muscle rigidity was quantified in the labial muscles of parkinsonian and normal subjects by applying known forces and observing the resultant displacements to determine labial stiffness coefficients. In parallel, labial speech movements were observed from these same subjects to evaluate hypokinesia. Labial rigidity was consistently correlated with decrements in the range of lip movement. EMG from orbicularis oris and mentalis muscles indicated a causal relationship between rigidity and hypokinesia.
Hunker et al. (Thu,) studied this question.