Movie S2.

Video showing the responses of two physical models of muscle morphology to squeeze. A model with helical fibers oriented at roughly 25^° relative to the model long axis shortens and lifts a 200 g mass when squeezed via a pair of pneumatic pressure cuffs. A model with fibers oriented at roughly 75^° lengthens in response to the same perturbation. Text animations indicate periods of squeezing. During these periods, cuffs were manually inflated with air via an in-series syringe.