Sullivan, W, Gardin, FA, Bellon, CR, and Leigh, S. Effect of traditional vs. modified bent-knee sit-up on abdominal and hip flexor muscle electromyographic activity. J Strength Cond Res 29(12): 3472–3479, 2015—The traditional sit-up may be a poor choice for core strength training due to its focus on hip flexion. The purpose of this study was to determine differences in abdominal and hip flexor muscle activation and trunk and hip kinematics between the traditional U.S. Army sit-up and a modified sit-up focusing on trunk flexion. Eighteen trained males performed 30 seconds of repetitions of each sit-up style, while muscle activation of the rectus abdominis (RA), external oblique (EO), and rectus femoris (RF) was recorded using electromyography (EMG). Trunk and hip kinematics were measured using 2-D videography. Maximum and mean muscle activation, integrated EMG (iEMG), and trunk and hip flexion were compared using a repeated-measures design. Maximum EMG of the RF and EO and mean EMG and iEMG of the RF were greater during the traditional sit-up. In contrast, mean EMG and iEMG of the RA and EO were greater during the modified sit-up. Peak trunk flexion was greater during the modified sit-up, and peak hip flexion was greater during the traditional sit-up. The greater RF EMG activity and peak hip flexion during the traditional sit-up suggest a greater emphasis on hip flexion during this sit-up style, which may result in lumbar hyperextension. The greater RA and EO activity and peak trunk flexion during the modified sit-up suggest a greater emphasis on trunk flexion during this exercise, which may decrease the lumbar spine load. Therefore, the modified sit-up may be a better exercise selection to train the abdominal muscles.