Muscle power is a better indicator of musculoskeletal health and functional capacity than muscle strength. The Load-Velocity (L-V) relationship provides a method for assessing muscle function during dynamic multi-joint movements, making it valuable for identifying age-related neuromuscular decline. This study aimed to examine the relationship between variables derived from the L-V relationship (L-0, V-0, and A(line)) obtained during the loaded sit-to-stand test and various muscle performance indices, including knee extension peak force (KEPF) and various muscle quality indices (MQI). A total of 113 participants (49 middle-aged adults and 64 older adults; age = 61 +/- 9.92 years) performed the loaded 5-Sit-To-Stand using Functional Electromechanical Dynamometry. L-V variables were calculated, and their relationships with indicators of muscle performance and quality were analyzed. The impact of age on these variables was also evaluated. The results showed a high positive correlation between A(line) and KEPF, MQI(Equation1), MQI(Equation2) and MQI(Equation3) (rs = 0.56-0.59, p <= 0.001-0.01), and between V-0 and KEFP (rs = 0.51, p < 0.001). Older adults exhibited significantly lower L-0 compared to middle-aged adults (76.86 +/- 29.74 kg vs. 94.62 +/- 35.69 kg, p = 0.007), as well as lower V-0 (0.92 +/- 0.28 m<middle dot>s(-1) vs. 1.22 +/- 0.19 m<middle dot>s(-1), p < 0.001) and A(line) values (35.39 +/- 18.95 kg<middle dot>m<middle dot>s(-1) vs. 57.75 +/- 23.84 kg<middle dot>m<middle dot>s(-1), p < 0.001). These findings suggest that L-V variables are valuable indicators for assessing muscle function deterioration and guiding training interventions, providing a more comprehensive and sensitive assessment of muscle quality and functional status in aging populations.