A main shaft assembly of a wind turbine and method for manufacturing the same are provided. Accordingly, the main shaft assembly includes a structural/shaft body defining a cavity therein. The shaft body is configured to transmit a load of the wind turbine developed in response to the wind. An inner body is located within the cavity. The inner body is non-loadbearing with respect to the load. At least one sensor is coupled to the inner body and positioned within the cavity for detecting a deflection of the shaft body in response to the load.