  We study vertical jumping in a simple robot comprising an actuated
mass-spring arrangement. The actuator frequency and phase are systematically
varied to find optimal performance. Optimal jumps occur above and below (but
not at) the robot's resonant frequency $f_0$. Two distinct jumping modes
emerge: a simple jump which is optimal above $f_0$ is achievable with a squat
maneuver, and a peculiar stutter jump which is optimal below $f_0$ is generated
with a counter-movement. A simple dynamical model reveals how optimal lift-off
results from non-resonant transient dynamics.
