Some researchers have recently investigated the effects of joint stiffness on both robot safety and performance in human-friendly robots using pneumatic artificial muscles (PAMs). However, due to the limited control bandwidth of pneumatic muscles, dynamic stiffness characteristics and their effects on safety as well as performance should be taken into account in the frequency domain. This paper introduces the concept of instantaneous stiffness and validates its model with the Stanford Safety Robot (S2ρ). The potential effects of dynamic stiffness on safety and performance are explored through experimental comparison of peak impact accelerations and sinusoidal position tracking, respectively. Simulation and experimental results with the Stanford Safety Robot show that the stiffness demonstrates limited effects on the impact acceleration given the same impact velocity and controller gain, whereas it significantly affects control performance due to pressure-induced non-linearities. A strategy for stiffness optimization for robot safety and performance is discussed as a design guideline of human-friendly robot design.