Background: Aging leads to an alteration in the immune response, characterized by a chronic inflammatory state, and a progressive decrease in muscle quantity and quality, a situation that increases in women and in the presence of obesity. With respect to muscle quality, intramuscular infiltration of adipose tissue has been considered a relevant parameter, involved in the relationship between aging-obesity-inflammation. As a therapeutic strategy, physical training with resistance exercises (or also known as strength training) has been shown to be effective in increasing skeletal muscle mass in this age group. However, its role on muscle quality in normal-weight versus obese older women has not been fully addressed. Hypothesis: A 12-week resistance exercise training program is effective in improving muscle quality, immune response and physical performance in normal weight and obese older women. In addition to the above, the investigators hypothesize that women with obesity will present greater baseline alterations, so the percentage of change will be higher compared to older women with normal weight after the training program. Goals: The primary aim of this study is to evaluate the effects of a 12-week resistance exercise training on muscle quality (infiltration of intramuscular adipose tissue), immune response and physical performance in older women between 60 and 79 years of age with obesity compared to older women with normal weight of the same age range. Methodology: The present clinical trial will consider 2 groups of older women between 60 and 79 years old: normal weight (BMI=18.5 to 24.9 kg/m 2 and % fat \<25.9) and obese (BMI =30 to 39.9 Kg/m 2 and fat % \>32). Participants will perform 12 weeks of training with resistance exercises 3 times a week. Before and after training, intramuscular infiltration of adipose tissue (echogenicity) will be measured by ultrasound, followed by aspects of muscle architecture (muscle thickness, penile angle and fascicle length) and functional parameters of muscle quality (maximum strength determined by 1 repetition maximum-1RM, maximum voluntary isometric strength of knee extensors through a lower limb force and power transducer). Finally, fasting blood samples will be obtained (immune response) and physical performance, body composition, physical activity level, and quality of life will be evaluated.