Background: Dynapenia and sarcopenia are age-related conditions characterized by reduced muscle mass, impacting daily activities such as sit-to-stand movements. Elastic bands, which offer progressive resistance, are commonly used in strength training to improve muscle mass and strength. Mechanical energy flow analysis is essential for understanding dynamics of movements. This study investigates the mechanical energy flow pattern in elderly individuals with dynapenia and sarcopenia following high-load resistance band training. Methods: Fifty participants, including twenty-seven with dynapenia and twenty-three with sarcopenia, underwent an 8-week high-load elastic resistance band training program (three 60-min sessions per week). Mechanical energy flow during sit-to-stand tasks was assessed before and after the intervention, with a focus on five key events: (1) start of seat unloading, (2) seat-off, (3) end of momentum transfer, (4) peak knee power, and (5) start of stabilization. Results: In the dynapenia group, the pelvis transferred energy to the left hip joint after training (0.04 J/kg) at the start of seat unloading. The sarcopenia group showed a significant shift in energy flow to the pelvis after the training (0.03 J/kg), accompanied by reduced trunk energy transmission. Both groups primarily generated energy at the knee joint, particularly during the seat-off phase. Conclusion: Resistance band training improved pelvic energy utilization during the sit-to-stand task in elderly with dynapenia and sarcopenia. Future interventions are warranted to integrate multi-joint training to enhance mechanical energy efficiency, movement patterns, so as to the functional capacity.