The gut microbiome is an important regulator of metabolism and has recently emerged as a relevant contributor to musculoskeletal health and disease, specifically to changes in skeletal muscle mass and function through a communication axis known as the gut-muscle axis. This posited gut-muscle communication pathway raises the attractive possibility that by manipulating the gut microbiota composition and function, scientists could mediate changes in skeletal muscle health and prevent sarcopenia adverse health outcomes. Thus, therapeutic strategies developed to modulate gut microbiome composition may have clinically meaningful implications in age-related muscle loss and physical function decline. Exercise has been shown to modulate the gut microbiome in older adults, but the evidence from well-designed randomized clinical trials is scarce. On the one hand, current guidelines recommend resistance training to reduce age-related skeletal muscle loss and physical function impairment. Still, on the other hand, aerobic exercise training is associated with increased microbiota diversity and an increase in SCFA-producing taxa, which has been linked to skeletal muscle health benefits and improved function. Given that these two exercise training regimens have different bioenergetic pathways, the study team hypothesized that each regimen promotes distinct alterations in gut microbiome composition. In contrast, combining both forms of exercise (resistance + aerobic) may lead to an added effect. Therefore, unraveling the impact of these exercise regimens on the gut microbiome is crucial to fill this relevant knowledge gap and to help clarify the optimal exercise regimen to manage sarcopenia. The research team long-term goal is to develop a consecutive line of research to explore the impact of these different exercise training regimens (i.e., resistance, aerobic, and concurrent exercise training) on gut microbiome composition and gut-derived metabolites and investigate its effects on skeletal muscle mass and function in older adults with sarcopenia. Methodology: Study design: This project will use a pilot 3-arm open-label randomized controlled trial (RCT), designed to evaluate the impact of distinct exercise training regimens on gut microbiome composition in older adults (age ≥ 60 years old) with sarcopenia. Methods: This study will use the recruitment, logistic and material resources from a community exercise program. Those who met our pre-screening criteria will be contacted by a study member, exposing the study's purposes, procedures, risk-discomforts, and benefits. Participants who agree will be invited to an in-person medical screening visit to assess the final inclusion/exclusion criteria and to provide their written informed consent (before enrollment). In this visit, volunteers will be evaluated through clinical examination, medication history, and a handgrip dynamometry test (an objective indicator of sarcopenia) to assess participants eligibility. Those who meet all inclusion/exclusion criteria will be randomly assigned (N= 69; 23/group) to three distinct exercise training programs: i) aerobic exercise (AER) training, ii) resistance exercise (RES), or iii) concurrent training (RES+AER). This project aims to randomize up to n=69 participants (n=23 / group) based on evidence from a recent systematic review with meta-analysis (11). Participants will engage in supervised center-based exercise interventions (12 weeks, 3d/w, 60min/d) consisting of either moderate aerobic training \[60-70% HRmax\], or resistance exercise \[8 exercises/major muscle groups; 6-12 reps; 60-75% 1RM\] or a combination of both (RES+AER). Assessments: All participants will be assessed at: i) baseline; ii) end of intervention (14-weeks); and iii) at close-out (26-weeks). Total length of the study is 26-weeks. At each study visit, participants will complete a skeletal muscle strength test, physical performance tests, assessment of body composition and provide a fasted blood, urine and fecal sample. Other important confounding variables will also be recorded including nutritional intake (using a 3-day diary), daily physical activity (using a physical activity tracker- wrist Fitbit), medication and supplementation use and associated comorbidities (assessed through medical and medication history questionnaire). Per the study design, the study team will also evaluate: a) retention and attrition rates; b) adherence to exercise intervention; c) safety; d) intervention effect on dependent outcomes. The success of recruitment and retention rates will be measured by the number of participants recruited, the number of drop-outs and losses to follow-up throughout the trial. Adherence to the exercise intervention will be carefully documented by study staff and will be measured by the number of sessions attended. Safety will be measured by the number and/or seriousness of adverse events attributable to the intervention and monitored and recorded by study staff throughout the trial. Objectives: Primary aim: The primary outcome will be the change in the relative abundance of Faecalibacterium prausnitzii and other SCFA producing bacteria after the intervention (14-weeks). Secondary outcomes include: 1) change of Faecalibacterium prausnitzii relative abundance at close-out (26-weeks); 2) change of relative abundance of Lactobacillus and Bifidobacterium genera after the intervention and at the end of the follow-up. Secondary aim: A set of complementary outcomes will also be assessed to broadly characterize the impact of each exercise intervention, including body composition, skeletal muscle function, functional performance and general gut microbiome composition. This broad set of outcomes will allow a comprehensive evaluation of the impact of these distinct exercise regimens on gut microbiota and on several health outcomes that are crucial for modulating gut microbiome and sarcopenia and may point to other mechanistic links that are currently undetermined. Expected Outcomes: The investigative team hypothesized that combining aerobic and resistance training will have a greater impact on the gut microbiome, specifically in increasing the relative abundance of Faecalibacterium prausnitzii and other SCFA-producing bacterial strains, an increase in skeletal muscle mass and strength, as well as enhanced physical performance measures in the concurrent exercise training group.