Continuous measurement of contractile length has been traditionally achieved using animal preparations in which the muscle and tendon are exposed. More modern methods, e.g. sonomicroscopy, are still invasive. There is a widely perceived need for a non-invasive, in vivo method of measuring continuous changes of human muscle contractile length. Ultrasonography has been used for several years to measure relatively static, discrete changes in tendon, aponeurosis and muscle fascicle length. We have recently developed this technique to continuously track changes in muscle contractile length during quiet standing. Here we present the tracking algorithm and use externally applied perturbations to establish the spatial and temporal resolution of the technique. Subjects maintained a low level of ankle torque while a pneumatic actuator applied rapid, square pulse, ankle rotations of defined magnitude and 0.2 s duration. Tracked changes in gastrocnemius and soleus contractile length follow the temporal profile of the perturbations and scale progressively (5 to 400 µm) with the size of the ankle rotation (0.03 to 0.7 degs). In a second experiment we tracked a wire oscillating in water with known peak to peak amplitudes of 1.5 µm to 8mm. The ultrasound tracking procedure had near 100% accuracy at all amplitudes for frequencies up to 3 Hz and showed attenuation at higher frequencies consistent with an effective sampling frequency of 12 Hz and sampling time of 80 ms. This non-invasive technique is sensitive, without averaging, to changes as small as 1 µm, and is suitable for observing neuro-motor activity in posture and locomotion.