Bridges are a very basic yet effective way to activate your glutes, hamstrings, and abdominal muscles. They are a foundational core movement that puts your glutes under constant tension while lengthening (opening up) the hip flexors to improve and facilitate hip extension. 

This video demonstrates a straight leg bridge variation, along with a single-leg progression, that allows the hamstrings to contract without shortening the muscles.  Using a Swiss Ball adds instability to a basic bridge, stimulating and conditioning your deep core muscles improving spinal stability.