Muscle contractile elements operate in series with elastic structures that influence performance. A crucial tool in characterizing these elastic structures and their behavior has been the quick-release experiment. When an isolated muscle experiences a very rapid change in length, the associated change in force characterizes elastic behavior, as long as the motion occurs at time scales shorter than required for cross-bridge cycling. Series elasticity can influence muscle shortening, but muscles also undergo three-dimensional shape changes during contraction. To determine whether elastic elements contribute to these shape changes we designed a quick-release experiment to quantify the relationship between force and length in the transverse (orthogonal to the line of action) direction. A pennate muscle, bullfrog flexor digitorum superficialis brevis, was attached to a servomotor and implanted with markers to track fiber length, muscle length, fiber pennation angle and thickness. A control signal was used to quickly drop the regulated force of the servomotor mid-way through a contraction. Dimensional changes were tracked by video at 500 fps and force was measured by the servomotor. Our results show that a quick drop in force is associated with a bulging of muscle thickness and a reorientation of fibers to a higher angle of pennation. These observations, reproducible by a simple mathematical and physical model, are consistent with the idea that muscle bulging during contraction is modulated by force through the elastic behavior of the extracellular matrix. This architectural spring may store elastic energy to contribute to power output during animal movement.