Fixed shortening of a muscle, or contracture, often develops in individuals with an upper motor neuron disorder. A clinical measure of muscle length would therefore be useful for identifying the presence of muscle contracture, tracking changes over time and evaluating the effect of interventions. This study compared a novel ultrasound-tape length method with a previously validated freehand 3D ultrasound method for measuring muscle length. The ultrasound-tape method intra-session reliability was also assessed. Resting medial gastrocnemius muscle length was measured at three ankle joint angles in 15 typically developed (TD) adults and nine adults with cerebral palsy (CP) using the two methods. The ultrasound-tape method on average overestimated the muscle length in the TD group by < 0.1% (95% CI, 6%) and underestimated in the muscle length in the CP group by 0.1% (95% CI, 6%) compared with the 3D ultrasound method. Intra-session reliability of the ultrasound- tape method was high, with intra-class correlation coefficients > 0.99. The ultrasound-tape method has sufficient accuracy to detect clinically relevant differences and changes in medial gastrocnemius muscle length and may therefore be a useful clinical tool for assessing muscle length changes associated with contracture. Key words: acquired brain injury; cerebral palsy; muscle contracture; muscle length; spasticity; spinal injury; stroke; traumatic brain injury; ultrasound.