A real-time high-frequency ultrasound acquisition system was used to describe coordination of individual muscle fibers during an isometric contraction. Effects of contraction size and muscle fatigue on axial strain and strain rate were examined in isolated leg muscles from a rat and mouse. Strain was estimated with a phase-sensitive speckle-tracking algorithm, capable of detecting displacements less than 100 nm. The spatial distribution of axial strain was not uniform and ranged from 0.5% - 5% during twitch stimulation. Also, strain and strain rate paralleled contraction size and degree of muscle fatigue. This potentially noninvasive technique may be appropriate to assess severity of movement disorders, such as cerebral palsy, and track coordination during therapy and rehabilitation after muscle injury.