[en] Context. Recent asteroseismic observations have led to the determination of rotational frequency splittings for ℓ = 1 mixed modes in red giants. <BR /> Aims: We investigate how these observed splittings can constrain the modelling of the physical processes transporting angular momentum in stellar interiors. <BR /> Methods: We first compare models including a comprehensive treatment of shellular rotation only, with the rotational splittings observed for the red giant <ASTROBJ>KIC 8366239</ASTROBJ>. We then study how these asteroseismic constraints can give us information about the efficiency of an additional mechanism for the internal transport of angular momentum. This is done by computing rotating models of <ASTROBJ>KIC 8366239</ASTROBJ> that include a constant viscosity corresponding to this physical process, in addition to the treatment of shellular rotation. <BR /> Results: We find that models of red giant stars including shellular rotation only predict steep rotation profiles, which are incompatible with the measurements of rotational splittings in the red giant <ASTROBJ>KIC 8366239</ASTROBJ>. Meridional circulation and shear mixing alone are found to produce an insufficient internal coupling so that an additional mechanism for the internal transport of angular momentum is needed during the post-main sequence evolution. We show that the viscosity ν[SUB]add[/SUB] corresponding to this mechanism is strongly constrained to be ν[SUB]add[/SUB] = 3 × 10[SUP]4[/SUP] cm[SUP]2[/SUP] s[SUP]-1[/SUP] thanks to the observed ratio of the splittings for modes in the wings to those at the centre of the dipole forests. Such a value of viscosity may suggest that the same unknown physical process is at work during the main sequence and the post-main sequence evolution.