The purpose of this study was to examine muscle activities during seven traditional different free dynamic exercises designed to strengthen the abdominal muscles. Eleven adult men with experience in weight training were asked to perform three repetitions of LSU, BSU, TSU, TC, RSU, LR and SLR. Activities of the upper rectus abdominis (URA), lower rectus abdominis (LRA), external abdominal oblique (EAO), and rectus femoris (RF) during the hip flexion and hip extension phases of each exercise were examined by electromyography (EMG) and analyzed using root mean square (RMS) values. The following results were obtained: (1) The mean RMS values for the URA were larger during the RSU and SLR than during the other five exercises. The value for the same muscle was larger during the TC exercise than during the LSU, BSU, TSU, and LR exercises. The mean RMS value for the LRA was largest during the RSU exercise, while that during the SLR exercise was larger than those during the LSU, BSU, TSU, TC, and LR exercises. (2) The mean RMS value for the EAO was largest during the SLR exercise, while that during the RSU exercise was second-largest and that during the TSU exercise was third-largest. The mean RMS values for the RF were larger during the RSU and SLR than during the other five exercises. The smallest value for this muscle was recorded during the TC exercise. (3) In most of the exercises, RMS in the hip flexion phase was larger than in the hip extension phase, and each muscle exhibited a knee flexion phase/knee extension phase ratio of between 0.5 and 0.8. However, the ratio for EAO and RF exceeded this range. (4) The muscle activity for the RF muscle in the RSU and SLR exercises was large, implying excessive stress for the lumbar vertebrae. Nevertheless, these exercises induced active muscle activity, and appeared desirable to perform with sufficient attention to safety.
These results suggest that RSU and SLR exercises are the most effective movements for stimulating the URA, LRA and EAO. Moreover, TC is an effective movement for training the URA, and TSU is an effective movement for training.