# Effect of Whole Body Vibration Conditions on Lower Limb Muscles during Sling Exercise
## 1 Introduction  
In recent years, for enhancing the training effect in sports and rehabilitation exercises, various intervention methods have been implemented, such as providing an unstable environment, whole body vibration, progressive resistance exercise, and task-oriented training [1]. Among the intervention methods, exercising in an unstable environment stimulates the neuromuscular system to induce joint contraction of muscles, and improves mobility and stability through co-contraction of agonistic and antagonistic muscles, thus maximizing the exercise effect. In addition, through the exercise, neuromuscular control training and sensorimotor integration for a single joint can be obtained; muscle activation also increases [2, 3]. Therefore, exercising in an unstable environment improves neuromuscular adaptation by providing effective training stimuli. There are various devices that provide an unstable environment, such as the Togu, Bosu ball, elastic band, and the recently developed sling and total resistance exercise system (TRX), which can control the intensity of exercise using the mechanism of a suspension point [4]. The sling exercise is an active neuromuscular control technique performed using a dangling rope and appropriate auxiliary devices. It reduces the pressure on the joints by pulling the weight, and simultaneously stimulates the muscles suppressed by pain to facilitate reactivation of the muscles [5]. While performing the sling exercise, the position of the suspension point can be changed for active involvement in the exercise,while minimizing the patient's pain. The intensity and form of the exercise is controlled using the helping hand principle. Compared to exercises on a fixed surface, it has the advantage of simultaneously strengthening multiple muscles involved in balance and stability [2]. Therapeutic exercise using a sling includes relaxation, sensorimotor integration training, stabilization, muscle-strengthening, and muscular endurance. Recently, the sling exercise has been adopted in various fields such as preventive medicine, industrial accident prevention, and for addressing lifestyle problems, as well as for fitness improvement in hospitals, home training, fitness centers, and rehabilitation [6]. According to previous studies comparing lumge exercises with and without a sling, activation of the rectus femoris, hamstring, and gluteus medius increased in the latter [7]. The whole body vibration (WBV) exercise was first conducted by the Russian Aeronautics and Space Administration in the 1970s for training astronauts. Since then, active research has been conducted on the WBV exercise, mainly in Europe. It is used in various fields to restore the physical function of patients with nervous system and musculoskeletal disorders [8,9]. WBV stimulates the muscle spindles of sensory receptors through short and fast mechanical vibrations by controlling the length of the muscles and tendons, causing the tonic vibration reflex (TVR) to increase muscle tone [10]. When muscle tone increases, sensitivity in the neurotransmitter system of the spinal cord increases, and the firing rate responses of the motor units increases. As a result, the overall functions of the neuromuscular system, such as muscle strength, flexibility, and balance, are improved [11]. The WBV machine is driven in various ways, such as the vertical method, seesaw method, and sonic vibration method. In the vertical vibration method, the entire platform moves in a vertical direction, providing vibration of the same intensity to bilateral lower limbs, allowing for balanced vibration stimulation for the body. In the seesaw vibration method, the platform moves in a vertical direction, alternating between left and right with the center of the vibration platform as the axis. In this method, only frequency control is possible; control of the vibration intensity is impossible. In addition, the method has disadvantages in that it is considerably noisy and can cause strain on joints [12].