This is a side view of a barbell curl. The circle that changes color is representative of the side view of the plates on the barbell. The movement starts, it takes 2 seconds to complete the concentric phase, there is a pause, then there is another 2 seconds for the eccentric phase, pause again, repeat as needed for likely 6-12 repetitions.

Notice from the beginning of the movement there is total control. When performing repetitions in this manner, you are doing a few things:

1) You are stressing the muscles the entire time. You have eliminated velocity, which is an external force helping you complete the movement. It is the same as someone's hands helping you lift the weight, which means you have to do less to move the weight. That's not good when your goal IS trying to stimulate the muscle.

2) You are training safely. Take out the herky-jerky execution of the movement and you eliminate (lessen at least) the chances of pulling muscles, hurting tendons, etc.

This is the ideal repetition speed for those interested in improving their appearance by increasing hypertrophy. Training for strength and/or power involved faster repetitions at times.

Many people may disagree with me when I recommend a speed that seems slow. They may say that slowing down their speeds to this speed will cause them to have to use lower weights. That's true. Know why? It's because they were doing the repetitions wrong in the first place. Going faster does make it easier and you can use heavier weights. But the level of stimulation isn't as great although the weight may be heavier. Stimulation is what we are after here, not maximally heavy weights at the expense of form and progress.

Extrapolate this movement speed to all movements of course.

Be in control all the time. That's the underlying theme.