This study examined the effects of pulsed shortwave diathermy on intramuscular temperature, surface electromyography (EMG), and mechanomyography (MMG) of the vastus lateralis. Thirty-five men were assigned to diathermy (n = 13), sham-diathermy (n = 12), or control (n = 10) groups. Each subject performed isometric maximal voluntary contractions (MVCs) and incremental ramp contractions (10%-90% MVC) before and after treatment. Torque, intramuscular temperature, EMG, and MMG were recorded. Temperature for the diathermy group increased (P <or= 0.05). MMG amplitude and instantaneous mean frequency (IMF) increased (P <or= 0.05) during the MVCs with the greatest increases observed for the diathermy group. During ramp contractions, MMG amplitude and IMF increased at all percentages of MVC (10%-90%) for the diathermy group only (P <or= 0.05). There were no changes in MVC torque, EMG amplitude, or IMF. Diathermy treatments may decrease musculotendinous stiffness, but not absolute strength or motor control strategies that influence force production.