The muscle confusion principle
Strength - A study conducted by researchers at the Tempe campus of Arizona State University compared two groups of trained subjects, one using an undulating periodization program—a fancy term for muscle confusion—and the other with a linear periodization program. In the undulating program, the acute variables were changed with every workout, whereas in the linear program, they were changed every few weeks. After 12 weeks, people following the undulating periodization program increased their bench press and leg press strength by 100 percent more than those following the linear program.
Researchers from Brazil saw even more drastic differences when they had trained subjects follow one of three programs: an undulating periodization program, a linear periodization program, or a consistent non-periodized program of 8-10 reps per set. The training program consisted of a two-day split with 3-4 total training days per week.
After 12 weeks, the group following the muscle confusion program increased its bench press by about 60 pounds, almost 200 percent more than both the linear periodization program and the non-periodized program. The undulating program also increased the subjects' strength on the leg press by a staggering 275 pounds, 400 percent more than the non-periodized program and more than 300 percent more than the linear program.
Hypertrophy - This principle can apply to muscle growth as well as strength. Researchers from the Federal University of Rio De Janeiro had a group of untrained men follow a linear periodization program of two sets of 12-15 reps per exercise for four weeks, then three sets of 8-10 reps per exercises for four weeks, and finally four sets of 3-5 reps per exercise for the final four weeks.
Meanwhile, a group used those same set and rep ranges but cycled them each time they trained. The results: The group following the undulating plan increased its triceps size by about five percent, while the linear group saw no such increase. The undulating group also increased its biceps size by 10 percent—twice that of the linear group.
Science proves once again that us gym rats aren't so dumb after all.