Vastus lateralis fascicle length changes during stair ascent and descent.

Study Design: Experimental descriptive laboratory study.

Objectives: To describe the change in fascicle length of the human vastus lateralis (VL) muscle during the stance phase of stair ascent and descent.

Background: Muscle fascicle length changes during lower limb functional activities, such as walking and jumping, do not always coincide with joint angle changes.