Creating tension in the muscle is one of the main factors that build muscle. In the gym, you can do this in two ways. The first is to lift heavier and heavier weights, what we call progressive overload. The second is TUT, or time under tension. This means that you maintain the tension in the muscle against a load for a certain amount of time.
Metabolic stress is another factor that might promote gains. The accumulation of metabolites like lactate does not in and of itself seem to create muscle growth, but it probably works together with other processes to create a more anabolic environment and increase muscle mass.
You can play around with TUT to create more metabolic stress in the muscle you are training. You can slow things down and perform the set over a longer period of time, which makes it harder to do as many reps. Or you can pump the reps out faster and do more reps with the same weight.
In a new study, young men performed 8 sets of biceps curls over the course of 2 training sessions, separated by one week.
During one of the training sessions, they did 20 reps with a 2-second eccentric phase and a 1-second concentric phase. That’s a set of fast reps. During the second session, they slowed the movements down, taking 4 seconds to lower the weight, and 2 seconds to curl it up for a total of 10 reps. The sessions were matched for time under tension.
The researchers measured blood lactate levels before the subjects started curling, as well as 2, 15, and 30 minutes post-exercise.
Not surprisingly, training with a high time under tension created large amounts of metabolic stress in the form of blood lactate, regardless of the training protocol.
More surprisingly, however, was the fact that the high rep protocol, with faster movements, was significantly more effective than creating a high time under tension by performing the reps slowly.
This suggests that, if you want to create high levels of metabolic stress when you lift, slowing your reps down is not the way to go.
PLOS One, 15 January 2020. Comparison of blood lactate and perceived exertion responses in two matched time-under-tension protocols.