Muscular fatigue occurs when motor units are no longer able to maintain an established force level. From an electromyographic point of view, fatigue is associated with a decrement of median frequency of contracting fibers. When the muscle is exposed to mechanical vibration, it undergoes a superimposition of voluntary contraction and stretch reflex, latter being synchronized to the mechanical frequency. The present paper is aimed to study the changes of muscular synchronization in long-lasting muscular tasks using a specific index change (ΔSL). Different frequencies of mechanical vibration (20, 30, 33, 40 Hz) and different levels of muscular task (20, 30, 40, 60% of the maximum) are studied by surface electromyography (sEMG) in prolonged exposures (t = 45 s). Results show a prominence of muscular synchronization (i.e. ΔSL) for a combination of force-frequency values at 30% MVC - 33 Hz. This is consistent but not identical with the behavior of other sEMG indicators of muscular fatigue, such as median frequency decay (MDF_d), as reported in literature, and suggesting that present results could better describe different aspects of the vibration stretch reflex. Using this knowledge may promote more effective vibrating tools for industry, oriented to reduce occupational fatigue. Since some force-frequency combination can stress the hand-arm system more than others, these results may help avoid work-related occupational injury.