(to work) at maximum level
to (at) one's (its) full capacity
to capacity
at one's full potential.
«На воле голова редко бывает занята одной работой... Значит, на воле инженер не может работать в полную силу» (Копелев 1). "On the outside (i.e., when not in prison), your head is rarely concerned only with work... .So, that means that on the outside an engineer can't work to his full capacity..." (1a).
Большой русско-английский фразеологический словарь. — М.: ACT-ПРЕСС КНИГА. С.И. Лубенская. 2004.