Sit on seat and position forefeet on pedals. Grasp handles to sides and extend knees but maintain small bend in knees.
Push left pedal by extending ankle knee Simultaneously allow right pedal to return back by relaxing ankle and bending knee more. Reverse motion by pushing right pedal and allowing left pedal to return back by same means. Continue to alternate motion.
Position seat to allow full range of motion. Gastrocnemius act as dynamic stabilizers because when they 'contract' at ankle they are simultaneously 'stretched' at knee. It could be argued they also act as synergist since bend at knee could be considered relatively smaller range of motion as compared to ankle articulation resulting in net contraction of Gastrocnemius. See Calf Exercise Analyses. Also see Lever Alternating Seated Calf Extension with straight knee.