During movement, muscles undergo combinations of active and passive shortening and lengthening. We measured muscle-tendon unit (MTU) length-change by proxy of ankle kinematics across nine gait-slope combinations for two monoarticular muscles in the mouse, Soleus, and Tibialis anterior. Markers on the shin, ankle, and foot were digitized to calculate the ankle angle. Tendon-travel experiments were used to measure moment arms (r) for both muscles. Soleus r was 0.033 mm/degree and Tibialis anterior r only -0.01 mm/degree. Based on these values, we calculated muscle length-change trajectory as L_MTUmin + (&theta_ankle x r), to obtain realistic MTU excursions for forthcoming work-loop experiments. Mean length-change varied across gait-slope combinations. Regardless of gait (walk, trot, or gallop), mean shortening was greatest during uphill locomotion (Soleus: -0.98 ± 0.18 mm, Tibialis Anterior: -0.39 ±0.019mm: mean ± S.D.). The combinations resulted in net muscle shortening during stance with most gait-slope combinations involving around approx. 1 mm (for Soleus) and 0.3mm (for Tibialis Anterior) muscle shortening (approx. 10% strain for Soleus and 3% for Tibialis anterior), whereas level walking only involved 0.13 ± 0.02 mm MTU shortening for Soleus. The work-loop technique traditionally involves sinusoidal or saw-tooth length-trajectories, which differ significantly from the non-monotonic length-trajectories that MTU’s undergo in-vivo. With in-vivo length-trajectories for the two MTUs calculated, we can now determine how a muscle produces power and work in-vivo, and how these power and work outputs differ from those estimated using the simpler length-trajectories traditionally used for work-loops.