The purpose of this study was to estimate nine major muscle forces of lower extremity, which was conducted by optimization method, during counter-movement jump. For this purpose, five subjects who have not had any leg-related injury were selected and video motion picture method was used to obtain three joint moments which were ankle, knee and hip joint moments on sagittal, frontal and coronal plane, respectively. Calculated joint moments on sagittal plane then were distributed to each nine muscles. Object function used for calculation was to minimize muscle fatigue that was sum of squares of muscle stress. In order to validate the predicted muscle forces, measured IEMG of four muscles(TA, SO, ST, and RF) were utilized. Through the above procedures, following conclusions were made. The dominant muscles that affect the performance through total phase were RF, VA, GLM and So. The predicted muscle forces(Newton/Body Mass) were: mean 67.02, max. 79.42, min. 55.11 for RF, mean 46.39, max 63.75, min. 31.24 for VA, mean 39.26, max. 49.05, min. 27.60 for GLM, and mean 39.12, max. 46.23, min. 34.43 for SO. Based on these findings, specific analyses on functions of nine lower extremity muscles would be made.