A new study in the International Journal of Applied Exercise Physiology had 12 healthy men, all experienced strength trainers, train their biceps by doing curls twice a week for 12 weeks. Each workout consisted of three sets of eight reps. The men trained to failure.
Half of the men lifted fast. They took one second each to do the lifting and lowering parts of the movement. The other group did their reps more slowly. They took one second for the upward movement, but three seconds to do the lowering movement.
The researchers used Musculoskeletal Ultrasound scans to measure the circumference and density of the participants' biceps, before and after the twelve weeks. In addition, they measured changes in strength by comparing the amount of weight with which the men were just able to complete one rep of the curl (their 1RM, or One Rep Max) before and after.
At the end of the 12 weeks, both groups increased their strength. However, the participants in the slow-speed group demonstrated five times greater strength gains than those in the other group.
In terms of hypertrophy/muscle mass, the slow-speed group gained three times more size than the high-speed group.
"We conclude that slow speed training is more effective to improve hypertrophy in well-trained adults", the researchers wrote.
This isn't an isolated study. A tremendous body of research supports their findings: Time under tension (often shortened to TUT), particularly emphasizing the lowering, or negative portion of each rep, is a powerful muscle-building tool.