If glycogen degradation  and glycogen synthesis were allowed to occur concurrently,  the  net  effect  would  be  hydrolysis  of  UTP,  a  so-called  futile  cycle. To stop this, both pathways need to be strongly controlled.  This control is  carried  out  by allosteric  regulation  and  covalent  modification  of  both  the glycogen phosphorylase and synthase.  Additionally, the covalent modification is under close hormonal control.

Figure: Simultaneous operation of glycogen synthesis and degradation would result in net hydrolysis of UTP.