Background <p>During resistance training, quercetin ingestion can enhance motor unit (MU) with a higher recruitment threshold in older adults.</p> Objective <p>We investigated the effects of daily quercetin glucoside ingestion on chronic adaptations in muscle strength and MU behavior following resistance training in healthy older adults.</p> Methods <p>Twenty-six older adults were randomly allocated to two groups that completed 6-week resistance training intervention with the ingestion of either placebo (PLA) or quercetin glycosides (QUE) at 200&#xa0;mg/day. Maximal voluntary force (MVF) during isometric knee extension, muscle mass, and MU firing behavior during ramp task at 70%MVF were measured before (PRE) and after (POST) intervention.</p> Results <p>In both groups, knee extensor MVF was significantly increased (both p &lt; 0.001), and the improvement in QUE (115.1 ± 11.0%) was greater than in PLA (105.3 ± 4.8%) (p &lt; 0.001) by the Mann–Whitney test. Muscle mass was not changed from PRE to POST in PLA or QUE (p &gt; 0.050). At POST, firing rates of Mus with relatively moderate (recruited between 20 and 40%MVF) or higher (recruited between 20 and 40%MVF) recruitment thresholds were higher in QUE than PLA (p &lt; 0.050). There was a significant correlation between %change in MVF and %change in firing rates of MUs with a relatively higher recruitment threshold from PRE to POST (p = 0.018, r = 0.642).</p> Conclusions <p>These results suggest that the adaptations of MUs with higher recruitment thresholds explain the greater improvement in muscle strength associated with QUE ingestion.</p> Clinical trial registry <p>UMIN000053019 (<a href="https://rctportal.niph.go.jp/detail/um?trial_id=UMIN000053019">https://rctportal.niph.go.jp/detail/um?trial_id=UMIN000053019</a>).</p>