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Muscular strength can be estimated by quantification of muscle area. For this purpose, we developed a flexible measuring system for muscle area using ultrasonography. This method is completely safe and is particularly suitable for elderly people because the subjects are not required to perform any muscular contraction during measurement. The ultrasound probe is installed on a mechanical arm, and continuously scans fragmental images along the body surface. A wide-area cross-sectional image is then constructed using the measured images. The link mechanism is very flexible, enabling the operator to measure images for any body posture and body site. Use of the spatial compounding method reduces speckle and artifact noise in the resultant cross-sectional images. The operator can observe individual muscles (extensor, flexor muscle, etc.) in detail. We conducted experiments to evaluate the performance of the system. In the experiments, the position of the ultrasound probe was calculated with high accuracy according to the link posture. In addition, a high degree of correlation was verified between MR images and those of the developed system. We observed a reduction in noise due to use of the spatial compounding method, and propose a new calibration method for correcting the measured muscle area, which were slightly deformed by the contact pressure of the ultrasound probe. Finally, we examined the relation between muscular area and muscular strength in young and middle-aged subjects. The results of these experiments confirm that the developed system can estimate muscular strength based on muscular area.