Antagonist muscle activity during reactive balance responses is elevated in Parkinson’s disease and in balance impairment

Abstract

BACKGROUND: Abnormal antagonist leg muscle activity could indicate increased muscle co-contraction and clarify mechanisms of balance impairments in Parkinsons disease (PD). Prior studies in carefully selected patients showed PD patients demonstrate earlier, longer, and larger antagonist muscle activation during reactive balance responses to perturbations. RESEARCH QUESTION: Here, we tested whether antagonist leg muscle activity was abnormal in a group of PD patients who were not selected for phenotype, and most of whom had volunteered for exercise-based rehabilitation. METHODS: We compared antagonist activation during reactive balance responses to multidirectional support-surface translation perturbations in 31 patients with mild-moderate PD (age 68{+/-}9; H&Y 1-3; UPDRS -III 32{+/-}10) and 13 matched individuals (age 65{+/-}9). We quantified modulation of muscle activity (i.e., the ability to activate and inhibit muscles appropriately according to the perturbation direction) using modulation indices (MI) derived from minimum and maximum EMG activation levels observed across perturbation directions. RESULTS: Antagonist leg muscle activity was abnormal in unselected PD patients compared to controls. Linear mixed models identified significant associations between impaired modulation and PD (P<0.05), PD severity (P<0.01), and balance ability (P<0.05), but not age (P=0.10). SIGNIFICANCE: Antagonist activity is increased during reactive balance responses in PD patients of varying phenotypes who are candidates for rehabilitation. Abnormal antagonist activity may contribute to balance impairments in PD and be a potential rehabilitation target or outcome measure.\n\nFunding SourcesThis work was supported by the National Institutes of Health (NIH) KL2 TR000455, K25 HD086276, R01 HD46922, R21 HD075612, TL1TR000456, and UL1 TR000424. The study sponsors had no role in study design; in the collection, analysis and interpretation of data; in the writing of the report; or in the decision to submit the article for publication.\n\nDeclaration of InterestConflicts of interest: None.\n\nAuthor ContributionsResearch project: Conception, JLM, LHT, MEH; Organization, JLM, KCL, LHT, MEH; Execution, JLM, KCL, LHT, MEH.\n\nStatistical Analysis: Design and Execution, JLM, KCL; Review and Critique, LHT, MEH. Manuscript Preparation: Writing of the first draft, KCL; Review and Critique: JLM, LHT, MEH.\n\nHighlights O_LIWe quantified abnormal antagonist muscle activation during balance in PD.\nC_LIO_LIAbnormalities were related to PD severity as well as to overall balance ability.\nC_LIO_LIAbnormalities were similar across PD phenotypes with and without tremor.\nC_LIO_LIBalance antagonist activation may be a useful rehabilitative outcome measure.\nC_LI