# Effect of Whole Body Vibration Conditions on Lower Limb Muscles during Sling Exercise
## 2 Materials and Methods
### Experimental Method  
The experiment was performed in the position of BSS, a type of lunge exercise. While the subject was performing BSS, a sling device (Newton 3D Sling, Easy Step Co., Gyeonggi-do, Korea) was connected to the hindlimb and sonic vibration was applied to the forelimb. At this time, activation of the lower limb muscles was measured. In the experiment, repetitions of BSS with a sling and sonic vibration (BSS-sling with vibration frequency: 0, 4, 8, 12, 20, and 30 Hz) were performed under six conditions in total. A single motion of BSS-sling with vibration refers to when the forelimb is placed on a vibration platform, the hindlimb is proposed against the sling device support, the rectus femoris is bent down to a point parallel to the ground, and then the knee is extended to return to the starting position (Figure 1). The selection criterion for the forelimb was the leg used when performing kicking. The sequence of each condition was randomly assigned to exclude the effect that may occur due to the order of sequence. Before the experiment, the subjects performed sufficient stretching for at least 15 min to prevent injury and were asked to practice the movements performed in the experiment. In addition, to ensure the consistency of motions during the experiment, the rear step length was measured in consideration of the length of the lower limb of each subject, and the experiment was conducted by controlling the motion for each subject at a constant level based on the measured step length. Subjects who completed maximum voluntary isometric contraction (MVC) measurement participated in the main experiment after sufficient rest. BSS motions were performed at a set timing according to the beat of the metronome (60 BPM, four beats), and each condition was repeated five times in total. To minimize muscle fatigue, a rest time of 60 s was given for each condition. A block diagram of the experimental procedure is shown in Figure 2. Surface electromyography (sEMG) was used to measure the activation of the lower limb muscles during the experiment, and electrodes were attached to gluteus medius, biceps femoris, rectus femoris, vastus medialis, and vastus lateralis, respectively.  
Figure 1: Bulgarian split squat using a sling according to the vibration frequency condition.  
A telemetry system for EMG (Telemyo DTS, Noraxon Co., Arizona, USA) was used for EMG measurement, and the attachment positions of the electrode were gluteus medius, biceps femoris, rectus femoris, vastus medialis, and vastus lateralis of the forefoot, respectively, with reference to the guidelines of Surface Electromyography for the Non-Invasive Assessment of Muscles (SENIAM). To minimize the range of errors due to skin resistance during measurement, the surface for the attachment of electrodes was wiped with alcohol, and the electrodes were attached to the center of the muscle, along the direction of the muscle fibers, observed when maximum voluntary isometric contraction (MVIC) was induced using the manual muscle test (MMT) method [1].  
To prevent errors in data analysis due to differences in the muscle strength level between subjects, and to increase the objectivity of the measured values, the data were quantified using the MVIC value. Each muscle was measured thrice for about 5 s, and the average value was used as the MVIC value. The MVIC of gluteus medius was measured by hip abduction, applying maximum force in the lateral position. For biceps femoris, MVIC was measured by fixing the upper body and pelvis in the prone position and applying resistance to the upper ankle joint. The MVIC values of rectus femoris, vastus medialis, and vastus lateralis were measured by extending the knee joint while maintaining 90\({}^{\circ}\) flexion of the knee joint while sitting on a table with the feet off the ground.