Animals can move robustly through unpredictable and complex environments. This work seeks to determine how the muscular system of the lamprey, Ichthyomyzon unicuspis, responds to deviations from a steady rhythm. In particular, we aim to understand how the muscle itself, separately from the nervous system, can react to and compensate for perturbations during periodic motion. We dissected a small section of axial musculature, approximately 5-10 mm in length, and performed standard work loop methods. We imposed a baseline sinusoidal length change to the muscle while measuring the force produced. To mimic muscle activation by motor neurons we electrically stimulated the muscle at one of four phases in the cycle. Afterwards we performed a similar test but added small sinusoidal length perturbations. We analyzed the data using a new system identification technique based on harmonic transfer functions that addresses the system identification problem in the frequency domain. This technique allows us to determine how the muscle responds during its cyclic motion to external perturbations at a range of different frequencies and at different phases during the periodic motion. Preliminary results indicate that both the effective stiffness and damping properties of the muscle change within a cycle. We will also present results that show how these properties, and in fact the entire harmonic transfer functions, differ for muscle when it is activated during lengthening compared to when it is activated during shortening.