A previously proposed model to simulate the behaviour of the chelonian muscle spindle during mechanical stretch has been extended to include the properties of the spindle during activation of the intrafusal muscle fibres. It is assumed that the overall transfer function of the non-activated spindle can be entirely ascribed to the visco-elastic properties of its intrafusal fibres. It is found that the activated spindle can then be simulated by incorporating a force generator into the visco-elastic model and by accepting stepwise changes in its parameter values at the onset and at the end of fusimotor stimulation. The influence of extrafusal fibre contraction has been accounted for by inserting the Voigt muscle model in parallel with the spindle model.