Multijoint exercises are believed to be superior to isolation exercises. But what if the exercise requires one muscle to span two joints? And what if those joints require muscles to both lengthen and contract?


Researchers studied the activity of the rectus femoris during leg extensions and leg presses at various weight loads. Activation of the rectus femoris, vastus lateralis, and medialis were measured and compared. 


Rectus femoris activation was higher during leg extensions than during leg presses. No differences were seen between exercises for the vastus lateralis or medialis. Rectus femoris activation was not significantly different, either.


Rectus femoris activation during compound exercises is low and does not increase with fatigue. The action of hip extension during compound movements for the quads reduces and surpasses rectus femoris activation. 


Isolation exercise for the quads is not limited to just leg extensions. Movements such as lunges and sissy squats also restrict hip extension. During lunges, focus on the back leg, allowing a full stretch of the quads, and then use the back leg to lift rather than just support the front leg. To perform sissy squats, use a stationary object that you can hold on to as you lean back bending only at the knees.