The purpose of this study was to investigate whether quercetin ingestion increases the decline in muscle contractile capacity due to a single resistance exercise session at differing intensities. Thirty-eight older adults took part in this study. They were divided into three intensity groups: low- (LI), moderate- (MI), and high-intensity (HI) groups. Each participant completed two single resistance exercise sessions of resistance exercise at 40 (LI), 60 (MI), or 80 (HI) percent of maximal voluntary contraction. Before each session, they ingested either a placebo or quercetin glycoside (500 mg). Motor unit behavior and electrically elicited contraction torque were measured by high-density electromyography and neuromuscular electrical stimulation at pre-ingestion, post-ingestion, and post-exercise. Quercetin ingestion decreased the motor unit recruitment thresholds from pre- to post-ingestion (p < 0.001). The decline in electrically elicited contraction torque following exercise was greater with quercetin than placebo ingestion in LI and MI groups (low: p = 0.014; moderate: p = 0.025), but not in the HI group (p = 0.792). The greater the decline of electrically elicited contraction torque following exercise by quercetin ingestion, the greater the decrease in the recruitment threshold of motor units with a higher threshold by quercetin ingestion in LI and MI groups (low: p = 0.047, rs = 0.566; moderate: p = 0.011, rs = 0.692), but not in the HI group (p = 0.228). These results suggest that quercetin ingestion accelerates the decline in muscle contractile capacity following low- or moderate-intensity resistance exercise, possibly due to the additional recruitment of higher-threshold motor units.