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Purpose: The combined treatment effects of nutritional supplementation and exercise on muscle quality (MQ) remain unexamined in individuals at risk for sarcopenia. This secondary analysis aimed to test whether β-hydroxy-β-methylbutyrate (HMB) supplementation enhances the effects of resistance training (RT) on MQ in older women with low muscle mass. Methods: This randomized, double-blind, placebo-controlled, two-by-two factorial design trial was conducted at the Tokyo Metropolitan Institute of Gerontology. Women age 65–79 yr with a skeletal muscle mass index of <5.7 kg·m−2 were randomly allocated to one of four arms (RT + HMB, RT + placebo, education + HMB, and education + placebo). Participants attended supervised RT sessions twice weekly or education programs every 2 wk and were provided calcium-HMB (1500 mg) or placebo supplements once daily for 12 wk. The main outcomes were changes in functional MQ, assessed by knee extension strength per quadriceps thickness, and compositional MQ, assessed by the echo intensity of the rectus femoris muscle. Results: Overall, 156 and 149 participants completed baseline and follow-up assessments, respectively. Two-factorial analysis of variance showed no significant interaction of HMB supplementation with RT in terms of functional or compositional MQ. Main-effects analyses revealed that the decline in functional MQ of the group performing RT was smaller by 0.71 N·mm−1 (95% confidence interval, 0.11–1.31) compared with that in the education group. No significant main effects of RT or HMB supplementation on compositional MQ were observed. Conclusions: HMB supplementation did not enhance the treatment effects of RT for either MQ parameter; however, RT reduced the decline in functional MQ. Thus, adding HMB supplementation to RT is not significantly useful for improving MQ in this population. [ABSTRACT FROM AUTHOR]