Understanding how humans adapt gait mechanics for a wide variety of locomotor tasks is important for inspiring the design of robotic, prosthetic and wearable assistive devices. We aimed to elicit the mechanical adjustments made to leg joint functions that are required to generate accelerative walking and running, using metrics with direct relevance to device design. Twelve healthy male participants completed constant speed (CS) walking and running and emulated acceleration (ACC) trials on an instrumented treadmill. External force and motion capture data were combined in an inverse dynamics analysis. Ankle, knee and hip joint mechanics were described and compared using angles, moments, powers and normalized functional indexes that described each joint as relatively more: spring, motor, damper or strut-like. To accelerate using a walking gait, the ankle joint was switched from predominantly spring-like to motor-like, while the hip joint was maintained as a motor, with an increase in hip motor-like function. Accelerating while running involved no change in the primary function of any leg joint, but involved high levels of spring and motor-like function at the hip and ankle joints. Mechanical adjustments for ACC walking were achieved primarily via altered limb positioning, but ACC running needed greater joint moments.