The training focused on building muscle mass must be orientated on maximum engaging our muscles as well as in the ascending and descending movement. We can achieve this effect by speeding lifting phase and slowing down descending phase.
Muscles are life parts of our body and very lively react on any type of stimulation and have very good ability to adapt to even hardest conditions they work in. This process of adaptation it’s a key to progress and growth.
For example, if with the specific exercise we use a more weight then thus far our muscles will adapt after a short period of time to this weight, so in practice, we increase strength or mass, or both of this muscle. The same reaction we can achieve by not increasing the weight but increasing or decreasing a speed of the movement in the lifting phase or decreasing a speed in the descending phase.
When we increasing tempo of movement we need to remember the point is to make muscle work harder not making exercise easier.
We need to remember that during every exercise it’s always easier lower down the weight than lifting up. It’s a result of gravity which we need to fight back during lifting phase, while during the descending move we only need to limit a gravitation force.
The other teaching says that slowing down lifting phase of movement will be beneficial either because this also forces our muscles to harder work. As an example best to do a set of squats and increase lifting phase time to 3 seconds. After set like his with maximum force put in we will feel our quadriceps much more tired as there is no dynamic aspect of force to make exercise easier.
The answer to the question today can be only one. As well as one or other reps can very strongly stimulate the muscles and be highly beneficial in our workouts as long they skilfully perform.