Skylar wants to improve her fitness level for a chance to be on the rowing team. She has been rowing 2–3 times per week at 9 m. p.h. at her local gym for 30 minutes, with her heart rate increasing to between 155–170 beats per minute. She is using the frequency principle to improve her fitness level when she increases her: days rowing to 4–5 per week heart rate to 165–180 beats per minute rowing speed to 11–12 m. p.h. time rowing to 40–45 minutes per session