The inverse dynamics of fully actuated mechanical systems with unknown physical parameters can be represented by linear combination of unknown constants and known time varying functions. This property, which allows the design of adaptive control for fully actuated mechanical systems, does not hold in general for underactuated mechanical systems, such as the acrobot. A control strategy which overcomes this difficulty is presented in recent results, for the case of collocated adaptive control of underactuated mechanical systems. In the present work a new control strategy for generalizing the aforementioned results is introduced and discussed in two examples: non-collocated adaptive control and adaptive virtual constraint of the acrobot.