Soy Sauce Odor Improves Upper Limb Motor Performance with Preliminary Evidence of Increased Alpha-Band Intermuscular...

Background/Objectives: While odor stimuli can modify motor function, the underlying mechanisms remain unclear. This study examined the impact of olfactory stimuli on muscle activities involved in upper limb motor performance and balance in nine healthy young Japanese males. Methods: In this exploratory, within-subject crossover study, odors of soy sauce, β-phenylethyl alcohol (PEA, a component of rose odor), and water were used to assess their effects on motor function through the modified functional reach test (mFRT), which measures sitting balance during upper limb extension. Surface electromyography from four muscles (upper trapezius, anterior deltoid, serratus anterior, and lumbar multifidus) was recorded to analyze intermuscular coherence (IMC) before and after odor exposure. Results: Only soy sauce odor increased mFRT reach distance. Within-condition analysis revealed that soy sauce odor was associated with increased α-band IMC in the serratus anterior-lumbar multifidus pair at the overall reach and second half of reach (both Benjamini-Hochberg-corrected p < 0.05; Cohen's d > 1.0), while no significant between-condition difference (Group × Time interaction) was observed. No significant within-condition changes were observed for PEA or water. Conclusions: These preliminary findings suggest that soy sauce odor improved mFRT performance. While an exploratory within-condition analysis showed a concomitant increase in α-band IMC in a postural muscle pair (serratus anterior-lumbar multifidus), the absence of a significant Group × Time interaction prevents attributing the behavioral improvement to this specific neuromuscular change. Given the small, homogeneous sample (nine healthy young Japanese males) and the exploratory design, these results are hypothesis-generating and require replication in larger, adequately powered studies before any conclusions about underlying mechanisms or clinical application can be drawn.