acoustic radiation force impulse imaging of in vivo vastus medialis muscle under varying isometric load acoustic radiation force impulse arfi imaging is a method for characterizing local variations in tissue mechanical properties in this method a single ultrasonic transducer array is used to both apply temporally short localized radiation forces within tissue and to track the resulting displacements through time images of tissue displacement immediately after force cessation maximum tissue displacement the time it takes for the tissue to reach its maximum displacement and the recovery time constant of the tissue are generated from the arfi data sets the information in each of these images demonstrates good agreement with matched bmode images the study presented here was designed to evaluate the relationship between changes in these arfi parameters with known tissue mechanical properties in vivo utilizing a modified siemens elegra scanner with a 75l40 transducer array arfi images of vastus medialis muscle were generated in three of the authors under four levels of activation 0 57 145 and 233 nm four arfi datasets were acquired for each loading condition the observed trends were that displacement magnitude the time it took for the tissue to reach its maximum displacement and recovery time constant decreased with increasing load ie increasing muscle stiffness significant differences were observed between load levels and subjects for all parameters p001 the results indicate that arfi imaging may be capable of quantifying tissue stiffness in realtime measurements although further investigation is required