The enthalpy relaxation in amorphous and semi-crystalline poly(L-lactic acid) (PLLA) has been investigated at low and high undercooling to identify the different scale motions that contribute to physical ageing, and the temperature ranges in which they are active. The temperature limit below which exclusively small-scale localized motions produce structural relaxation in PLLA has been identified. In addition, motions different from pure cooperative large-scale motions and pure small-scale local rearrangements, localized in the mobile amorphous fraction, are found to mainly contribute to structural relaxation at intermediate undercoolings. These motions are suggested to be related with the Johari-Goldstein process.