Comparison of the muscular electrical activity and hip–knee joint amplitude during bent-knee sit-up movement and abdominal exercises using a five-minute shaper device: a case study on an unconditioned subject

Problem statement: Different fitness devices that stimulate the abdominal muscles to improve sports performance or body image have been reported in the literature. Many of these devices are used by subjects without physical conditioning which, together with bad use of the devices, can generate serious musculoskeletal injuries. Purpose: This study compares the Five-minute Shaper (FMS) device and traditional bent-knee sit-up (BKS) exercise through hip–knee joint amplitudes and muscle electrical activity parameters. Materials and methods: A healthy volunteer participated in this study. This subject performed ten repetitions using the two exercise modalities. FMS is analyzed through three different levels (i.e., easy, medium, and extreme), and the traditional exercise is performed on a mat. We analyzed the knee and hip joint movement and performed surface electromyography (sEMG) of external abdominal oblique, internal abdominal oblique, upper rectus abdominis, erector spinae, rectus femoris, and biceps femoris muscles. The movements were performed in the sagittal plane in a 2D capture, and EMG was recorded using an eight-channel electrophysiographic measurement with bipolar surface electrodes. The joint amplitude was computed using Kinovea and processed by MATLAB. The muscle activation was processed by Root Mean Square (RMS) and normalized by Dynamic Peak Activity (PDA). Results: The obtained results show that the BKS exercise activates fewer muscles or at a lower intensity than FMS. Consequently, the FMS device presented greater muscle activation and better biomechanical posture than the traditional BKS exercise. The device has greater ergonomics than the traditional BKS exercise and is associated with a decrease in back injury. Nevertheless, there were significant differences (P < 0.05) in muscle activation for each level of FMS and traditional exercise. Conclusion: This study allows to conclude that the use of the FMS device may increase muscle strength in the abdominal and lower limb areas. Additionally, FMS presents a lower risk of suffering spine injuries in people who are not accustomed to regular exercise because flexion–extension angles are not exceeded.