The purpose of this study was to assess the effect of direct vibration during two sub-maximal isometric loading conditions on electromyographic output. Sixteen participants performed isometric knee extensions, at an angle of 150°, under four conditions in random order: 1) a load of 10% one repetition maximum [1 RM] with no vibration (10%+NV); 2) a load of 30% 1RM with no vibration (30%+NV); 3) a load of 10% 1RM with vibration (10%+V); and 4) a load of 30% 1RM with vibration (30%+V). Direct vibration (amplitude = 1.2mm, frequency = 65 Hz) was applied to the distal end of the quadriceps using a purpose built portable vibrator. Electromyographic root-mean-squared (EMGrms) output of the rectus femoris (RF), vastus lateralis (VL) and vastus medialis (VM) was measured and assessed. Vibration significantly enhanced RF, VL and VM EMGrms output in comparison to no vibration in both the 10% 1RM and 30% 1RM loading conditions, but the degree of vibration induced enhancement was not affected by the magnitude of loading (Vibration vs no vibration, RF: 58 ìV vs 47 ìV, VL: 59 ìV vs 47 ìV, VM: 87 ìV vs 57 ìV, p < 0.05).
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