The purpose of this study was to further validate the scattering mechanism of ultrasound in the skeletal muscle tissue. It was hypothesized that the endomysial collagen fibers are a major determinant of ultrasonic scattering in the skeletal muscle. Previous studies have found that the ultrasonic backscattering from avian skeletal muscle changed as it was passively stretched from 0 to 40%. In this study, ultrasonic backscattering was measured from chicken breast muscles as they were stretched passively in increments of 10% of original length up to 60%. The integrated backscatter was found to reach a peak as the specimen was stretched to 40% and leveled off as it was further stretched from 40 to 60%. This finding was supported by results from scanning electron microscopy (SEM) of the specimens. SEM images showed that the orientation of the endomysial collagen fibers surrounding muscle fibers became approximately parallel to the axis of the muscle fiber when the muscle was stretched up to 40% of its original length, yielding maximal ultrasonic backscatter and as the muscle was further stretched, no apparent alteration of fiber orientation could be observed.