conscious control of skeletal muscles. I am going to assume that the step up movement happens with the right leg, hence the motor cortex on the left side of the brain controls the muscles in case (right leg). When the movement decision occurs, the cerebellum takes charge and coordinates the movement ordered by the cortex. The thalamus transmits information from the cerebellum to the motor area of the cerebral cortex. At this point axons from the cerebellum and the cerebral cortex form synapses with the red nuclei in the midbrain of the brain stem to coordinate the muscular movement. The impulse passes through the cerebral peduncles of the midbrain, where axons of motor neurons further conduct the nerve impulse to the pons, then to the medulla oblongata’s white matter where the motor tracts extend into the spinal cord. The pathway of the impulse passes through the motor tract synapses with the large alpha motor neurons, located at the appropriate levels of the spinal cord, in the anterior horns of the spinal grey matter, which send impulses through the spinal nerves (we are now in the PNS) of the ventral root, where axons of somatic motor neurons conduct nerve impulses of contraction to the iliacus, fascia latae, and psoas major, which contract to flex the thigh/femur at the hip joint. Other muscles involved in stepping up a step are: sartorius (flexes the hip and the knee), the hamstrings (extend the hip and flex the knee), tibialis anterior (executes the dorsiflexion of the foot), gastrocnemius (executes plantar flexion of the foot and flexion of the knee), and gluteus maximus (extends and steadies the femur).
Tortora, G.J., Derrickson B. (2010). Introduction to the Human Body: The Essentials of Anatomy and Physiology (8th Ed.). Hoboken, NJ: John Wiley & Sons, Inc.