It could be that the position of full extension provides a larger range for the Gastrocnemius to move in; thus requiring a more powerful contraction due to additional weight being place through the lower extremities, when compared to slight flexion of the knee. Furthermore, this could link in with Gastrocnemius being more of a phasic muscle (Fast twitch) which requires power and speed in place of repetition.

Full extension also ensures that the maximum amount of fibres within the Gastrocnemius are targeted; the same way that full extension + dorsiflexion is used to stretch and isolate the Gastrocnemius and its fibres from the Soleus.

The Logical is there; if its the reason why full extention is used; who knows, but I know it works.


Hope this helps,
Aaron.