What makes rowing popular with elite athletes and CrossFitters is exactly what many in the general fitness population dislike about it: your weaknesses cannot be hidden on the rowing machine. It is a human polygraph of physical and mental performance. Stroke for stroke, you are provided with feedback that both reveals any weak spots and very visibly demonstrates the relationship between performance and proper technique.  —Angela Hart, "Rowing Technique," CFJ

"Double-unders require the athlete to propel the body upward and lift the feet up to several inches from the ground while making small, fast circular movements with the wrists in order to execute multiple turns of the rope with each jump. They develop dynamic balance, speed, quickness, agility, coordination, concentration, and cardiorespiratory efficiency." —Buddy Lee, "Jump Rope Basics, Part 2" (adapted)