Originally Posted by bhupaes
I think Serena uses a combination of ISR and biceps, pretty late in the stroke (close to contact), hence one might say it's abrupt. But that doesn't really matter. In answer to your question on how to create a linear stroke, notice how the path of the racquet head is pretty much linear for most of the stroke. It becomes circular close to contact due to her ISR and biceps action. Now ask yourself, if the ideal stroke is totally linear, why should Serena change a good thing (linear motion) into a bad thing (motion in an arc), and so close to contact at that? Could it be because the act of pulling in the racquet increases RHS both forwards and upwards?!
I think that Serena doesn’t use right arm bicep significantly, because she doesn’t bend additionally her elbow around and after impact. See picture below.
About straight linear motion: She applies bend elbow FH technique, because she can use ISR, the fastest motion of the arm, to build translational motion of the ball. In case of straight arm ISR can create spin and practically zero translational speed. That’s why almost all WTA players use bend elbow FH.