Biological pennate muscles, denoted by muscle fibers arranged obliquely relative to the line of action, have shown the ability to passively regulate the effective transmission ratio coupling fiber contraction to overall muscle contraction. In this paper, a model for a bio-inspired variable-stiffness pennate actuator is developed. The pennate topology observed in natural musculature is leveraged to create an actuator capable of varying stiffness based on its mutable configuration. Variable Stiffness Actuators (VSA’s) are useful for roboticists and engineers because they enable features atypical of traditional, stiff kinematic linkages, such as energy storage or increased human-interactive safety. Typically, VSA’s are constructed of rigid materials, such as motors and springs. However, by utilizing non-rigid actuators in a pennate configuration, a pliable, soft VSA can be conceived. Previous studies have experimentally utilized McKibben artificial muscles and Twisted Coil Polymer wires in lieu of muscle fibers to recreate the pennate muscle architecture. This paper expands on previous pennate actuator studies by providing a general modeling framework, allowing roboticists to make informed design decisions and understand associated tradeoffs when recreating the remarkable properties of pennate musculature. Theoretical case studies are performed to better understand the design tradeoffs. The Variable Stiffness Pennate Actuator is a promising actuator configuration that can readily integrate with other bio-inspired actuator technologies, such as orderly recruitment.