Acute effects of a vibration-like stimulus during knee extension exercise

Purpose: This study was conducted to test whether a low-frequency vibration-like stimulus (rapid variable resistance) applied during a single session of knee extension exercise would alter muscle performance. Methods: Torque, knee joint angle, EMG activity of rectus femoris (RF) and vastus lateralis (VL) muscles, and VL muscle oxygenation status (near-infrared spectroscopy) were recorded during metronome-guided knee extension exercise. Nine healthy adults completed four trials exercising at contraction intensities of 35% (L) or 70% (H) of one-repetition maximum (IRM) in control (no vibration, Vb-) or vibrated condition (superimposed 10-Hz vibration-like stimulus, Vb+). Maximum voluntary contraction and IRM were tested pre- and postexercise. Results: During IRM tests, muscle dynamic strength (P = 0.02) and power (P = 0.05) were significantly higher during vibrated rather than nonvibrated trials, and strength was significantly higher post- than preexercise (P = 0.002), except during LVb- trial. Median spectral frequency of VL and RF EMG activity was significantly higher during postexercise than preexercise IRM test in the vibration trials but unchanged in the control trials (P < 0.02). The rate of muscle deoxygenation was 58% faster during H than L exercise (P = 0.001), and vibration superimposition tended to speed muscle deoxygenation rate (P = 0.065, 36% effect size) particularly during L trials. Conclusion: Vibration superimposition during knee extension exercise at low contraction intensity enhanced muscle performance. This effect appears to result from adaptation of neural factors such as motor unit excitability (recruitment and firing frequency, conduction velocity of excitation) in response to sensory receptor stimulation. Muscle vibration may increase the training effects derived from light-to-moderate exercise.