Abstract: A structure testing machine for carrying out tests on a test structure (25) for an assessment of the response of the structure to test loads applied to the structure in the direction of a predetermined axis of the structure under the control of a monitored control system (21) which forms part of the machine and which has a forward path which includes an actuator (24) which is supported by a machine frame and which upon actuation applies to the test structure a test load in the direction of the predetermined axis, and a controller (22) responsive to a controller input signal and to controller parameters to apply to the actuator an actuating signal to cause the actuator to subject the test structure to a test load which produces dimensional changes in the test structure in the direction of the predetermined axis. The machine is provided with an adaptive control loop which comprises a stiffness estimator (27) which generates from the dimensional changes in the test structure adaptive control signals (E.sub.