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THE INFLUENCE OF ATTENTION TO THE INACTIVE LIMB ON MUSCLE AFTER CONTRACTION IN THE DELTOID AND TIBIALIS ANTERIORPurpose: The purpose of this study was to investigate the origins of differing tonic muscle activity in three populations with known differences in neuromuscular processing. Methods: We manipulated healthy young adults, healthy older adults and individuals with Parkinson's disease (PD), both on and off dopamine therapy medication, with a novel technique related to muscle after contraction (MAC). We also investigated the transfer of tonic activity to the contralateral (unmanipulated) limbs to determine whether tonic activity is modulated through inhibitory and facilitatory interneuronal processes at a peripheral, spinal, or supraspinal level. In independent trials, we examined MAC in both proximal (medial deltoid) and distal (tibialis anterior) postural musculature by having subjects perform a voluntary, isometric contraction of the upper arm or ankle, respectively. Using surface electromyography (sEMG) to record the activity of the motor units before, during and after contraction, we were able to examine the differences in motor unit recruitment across the different populations from both an amplitude and frequency analyses. To test the transfer of tonic muscle activity to contralateral limbs, we manipulated conscious attention by having individuals focus on the unmanipulated limb during the post-contraction phase. Results: Our result shows significant changes in the prevalence of MAC with age and disease state. Moreover, some subjects with PDs showed variable evidence of tonic motor irradiation across the body, which was not seen in healthy individuals. Subjects with PD also showed unintended involuntary movement in the contralateral limb when treating the affected side, but were able to inhibit this behavior when explicit attention was paid to the unmanipulated side. Healthy young adult subjects consistently showed a MAC in their medial deltoids. Healthy older adults returned to baseline muscle activity following a contraction. During the attention conditions, there was increased muscle activity in the contralateral limb across subject groups. Conclusions: The findings from this series of studies may deepen our understanding of how aging and neural disease affect the unconscious tonic neuromuscular processes that serve as the foundation for all motor activity. This may help shape future rehabilitation techniques, particularly in decreasing fall risk, for the PD population as well as healthy agers.