| Whole-body vibration training (WBVT) is intentional exposure to WBV with the purpose of improving health or performance such as leg muscle strength.;The purpose of the first study was to evaluate the role of Ia afferents in neuromuscular responses to (RV) and (VV). Ia sensitivity is greater during eccentric contractions than during concentric contractions because of increased intrafusal fiber tension. Ia stimulation during RV and VV is greater at larger knee angles (KA). Thus, Ia afferent stimulation and sensitivity were varied through a series of static and dynamic unloaded squats performed with and without RV and VV (30Hz; 4mmp-p). The neuromuscular activation of the right vastus lateralis (VL), biceps femoris (BF), gastrocnemius (GS), and tibialis anterior (TA) of 16 healthy adults was recorded using surface electromyography (EMG) while KA were obtained using a motion capture system. Increases in EMG during RV and VV relative to baseline were compared over a range of KA during concentric, eccentric, and isometric contractions (CT).;EMG was significantly increased during RV and VV across all conditions in TA (etap2: 0.84), GS (0.819), VL (0.588), and BF (0.287). Responses to WBVT varied with CT, KA, and M. However, the effects of CT (Ia sensitivity) and KA (Ia stimulation) were opposite to those hypothesized; greatest EMG enhancement occurred during concentric contractions and at small KA. Results suggest that enhancement of neuromuscular activation during WBVT may not be attributable to myotatic reflex contractions. Alternative possible mechanisms are discussed.;The purpose of the second study was to evaluate the likelihood of adverse health effects from RV and VV by quantifying biodynamic responses (Halpha rms and AMM) during WBVT and also by comparing the calculated vibration exposure (eVDV) with ISO guidelines on vibration exposure. KA, AMM, Halpha rms, and eVDV were measured as 16 healthy adults performed slow unloaded squats during RV and VV (30Hz; 4mmp-p).;Results supported the hypotheses that (1) AMM during RV and VV varies inversely with KA, (2) Halpharms is greater during VV than during RV, (3) Halpharms during RV and VV varies inversely with KA, (4) the effect of KA on Harms is different for RV and VV, and (5) eVDV during typical RV and VV training regimens (30Hz, 4mmp-p , 10min/day) exceeds the recommended daily limits as defined by ISO 2631-1. ISO standards indicate that daily exposure to WBVT is "likely" harmful to the human body; the risk of adverse health effects may be lower during RV than VV and at half-squats rather than full-squats or upright stance. |
