Timing of Back Muscles during Whole-Body Vibration with Transients – Its Significance for the Internal Spinal Load

To examine the response of back muscles to whole-body vibration (WBV) consisting of a sinusoidal “background” (4 Hz, r.m.s. acceleration 0.52 ms⁻²) and 10 interspersed periods with peak-to-peak amplitudes (“transients”) that varied in three steps (3.5, 7.2, and 11.1 ms⁻²), six surface electromyograms (EMGs) were obtained from different back muscles of 8 subjects at 3 different sitting postures (relaxed = R, bent forward = B and straight erect = E). The rectified undistorted EMGs and the force at the interface vibrator/subject of all subjects were averaged. The myoelectric activity at R was minimal, B and E were accompanied by a high basic EMG-activity and pronounced responses to the transients. The timing of the maximum EMG-responses varied. A nonlinear increase of the internal load was predicted with rising amplitudes of transients. The threshold for this prominent unfavourable biological effect might be located between 3.5 and 5 ms⁻².