Transmissibility and waveform purity of whole-body vibrations in older adults

Background: This study examined the transmission power and waveform purity of vertical (synchronous) whole-body vibrations upon its propagation in the human body among older adults.

Methods: Forty community-dwelling older adults participated in the study (33 women; mean age: 60.3 (SD 5.7) years). Four vibration frequencies (25, 30, 35, 40Hz), two amplitudes (0.6 and 0.9mm), and six different postures were tested. Skin-mounted tri-axial accelerometers were placed at the medial malleolus, tibial tuberosity, greater trochanter, third lumbar vertebra, and forehead. The transmissibility of vibration was computed as the ratio of the root-mean-square-acceleration at different body sites to that of the platform. Signal purity was expressed by the percentage of total transmitted power within 1Hz of the nominal frequency delivered by the platform.

Findings: Vibration frequency and amplitude were inversely associated with transmissibility in all anatomical landmarks except the medial malleolus. Amplification of signals was noted at the medial malleolus in most testing conditions. The effect of posture on whole-body vibration transmission depends on its frequency and amplitude. In general, toe-standing led to the lowest transmissibility. Single-leg standing had the highest vibration transmission to the hip, while erect standing had the highest transmissibility to the head. The purity of waveform of the vibration signals was well conserved as the vibrations were transmitted from the feet to the upper body.

Interpretation: Whole-body vibration transmissibility was highly influenced by signal frequency, amplitude and posture. These parameters should be carefully considered when prescribing whole-body vibration to older adults.