Whole-body vibration: Is gravitational force a valid measurement of exercise intensity?

The primary aim of the whole-body vibration (WBV) investigation was to determine if g-force is a valid measurement of vibration exercise intensity (VEI). For this purpose, twelve healthy lean adults (male, 7; female 5; age, 29.4 ± 6.4 years; height 171.4 ± 4.9 cm; weight, 67.4 ± 10.4 kg; BMI, 22.9 ± 2.6 kg/m2) voluntarily participated for the study. To examine the relationship between g-force and VEI, a 3-way (4-muscle group x 4-frequency x 2-amplitude) repeated measures design was employed, which analyzed electromyography (EMG) muscle activity in the vastus lateralis (V.L.), vastus inedialis (V.M.), gastrocnemius lateralis (G.L.) and gastrocnemius medialis (G.M.) muscles during WBV exercise. Participants, performed a standard unloaded isometric high-squat position (knee angle 30 degrees, hip angle 30 degrees, feet [middle toe to middle toe] 30cm apart) while at eight different levels (k = 8) of g-force. The levels of g-force were derived from four different preset frequency settings (30, 35, 40, and 50 Hz) measured at both the low and high amplitude settings. Each participant underwent four trials at each of the amplitudes. In each trial a participant experienced all four preset frequencies and a control condition. The VEI was operationally defined as the WBV-induced percent increase in EMGrms compared with the control condition (no vibration). Control data was expressed as a percent of maximum voluntary contraction (MVC), thus allowing for inter-subject comparisons. The results of the study clearly indicated that the highest g-forces were not associated with the highest VEI responses. The highest VEI responses were found at the lower frequencies (30 and 35 Hz) with exception of the G.M. (40 Hz). Furthermore the higher amplitude produced an approximately 35% higher VEI response than the low amplitude. Based on the findings it was concluded that g-force is not a valid measurement of VET. Additionally, the study revealed that both a large degree of inter-subject variability in VET responses exists and that WBV only produces a small amount of muscle activity when expressed relative to MVC data. Thus, it was concluded that WBV should only be used as a supplement to traditional exercise prescription, not as a replacement.