Effects of Vibration Frequency and Knee Flexion on Lower Extremity Muscle Loads

Occupational exposure to vibration has been identified as a major health risk. However, vibration has been commonly used in physiotherapy and muscle strength training. Despite long-term interest in the effects of vibration on muscles, the literature presents conflicting results regarding muscle activation/ performance following vibration stimulation. A commercially available electric vibrating machine was used and 20 subjects were recruited to statically stand with knee flexion at different angles (0°, 60°, 90°) and dynamically stand (stand up/squat down) on vibrating platform, under exposure to different vibration frequencies (0Hz, 20Hz, 35Hz, 50Hz). Surface electromyography (sEMG) was used to assess the effects of posture and vibration frequency on the activation of gastrocnemius, rectus femoris, and vastus lateralis muscles. The results showed that knee flexion angle had a significant effect on muscles of the lower extremity, especially thigh muscles, which support body weight. The most obvious impact was on the calf muscles when vibrations were transmitted from the foot. The frequency of vibration had a certain influence on muscle activation, but was not as significant as the influence of posture. The higher the vibration frequency, the greater the muscle activation. However, when muscles were activated to a certain extent, vibration had little additional effect on muscle activation. Moreover, sEMG signals detected during dynamic posture were generally higher than those detected during static posture. Vibration at higher frequency activated muscles more easily. However, excessive muscle fatigue can cause injuries.