Isotonic　exercise　is　an　effective　way　to　improve　the　ankle　joint　complex＇s　(AJC)　muscle　strength,　and　a　contracting　muscle　shortens　against　a　constant　load　during　the　process,　or　in　other　words,　the　muscle　maintains　equal　tone　while　shortening.　In　order　to　change　the　physical　human-robot　interaction　into　the　reciprocating　motion　of　the　parallel　ankle　rehabilitation　root　(PARR),　to　improve　its　compliance　and　enhance　muscle　strength　training,　we　implement　the　admittance-based　isotonic　exercise　strategy　on　our　PARR.　The　mechanical　design　of　the　PARR　and　the　admittance-based　isotonic　exercise　to　change　the　physical　human-robot　interaction　torque　into　the　angle　movement　were　described　at　first.　Then,　an　adaptive　law　of　the　admittance　parameters　was　further　proposed　to　adaptively　adjust　the　compliance.　Experiments　with　neurologically　intact　subjects　were　conducted,　and　the　experimental　results　showed　good　trajectory　tracking　performance　and　excellent　practicability,　which　can　improve　muscle　strength　training　safely.