Intentionally slow lifting is believed by some to be the best way to develop strength. This belief is based on the force-velocity relationship of muscle contractions. Here is why this approach is not well based in fact.
The velocity of a muscle contraction plays a key role in how much force can be produced. This is called the force-velocity relationship.
Here's how it works.
Muscles produce the right amount of force to match the weight load being lifted. Contraction speed is faster for light loads, and slower for heavier loads.
For concentric contractions (the upward movement in the curl below), the greatest force is produced at slower velocities of contraction and less force is produced at higher velocities.
Peak isometric force occurs when there is no movement. Loading greater than peak isometric force causes muscles to lengthen (eccentric contraction, the downward movement).
Muscle force increases as velocity increases during eccentric contractions. Force increases as the lengthening velocity increases because the muscle fibers are contracting as they are lengthening, so greater force is produced during eccentric contractions.
Some athletes base their lifting philosophy on the notion that lifting weights slowly is the best way to maximize strength, using the force-velocity relationship as the evidence. The problem with this thinking lies in the effort level given and how this impacts the speed of motion of the weight.
According to the American College of Sports Medicine (2012), when maximal effort is given by the athlete against a sub-maximal load, the net velocity of movement is fast. However, this is not the case during maximal loading when the net velocity is slow.
The underlying assumption of the force-velocity relationship is that muscles are concentrically contracting maximally at a given velocity. During sub-maximal loading, intentionally slow velocities occur when the athletes attempts to limit the speed of contraction.
When the athlete lifts less than maximum loads slowly, the neuromuscular response is limited: less fibers are recruited and the firing rate of muscle fibers is slower. So, it makes sense that muscular endurance or hypertrophy may be stimulated, but maximal strength and power are compromised.
Research has shown that the amount of weight lifted, the force, and the nervous system's responses are lower when a weight is intentionally lifted at a slow speed.
In addition, based on Newton's Second Law of Motion, Force = Mass x Acceleration. With mass constant, greater force produces higher acceleration. The required force to lift a slow moving weight is only slightly more than the weight itself, so velocity is just a bit more than the isometric level.
The bottom line is that there is no scientific justification for lifting slowly with sub-maximal loads to improve strength using force-velocity relationship as the basis for the argument.
This concept is not new. In the 1970s I visited Arthur Jones and Ellington Darden at the Nautilus Headquarters. My fellow strength and power athletes and I listened to lectures, were provided with literature, and tried out these new "isokinetic" machines on which athletes could allegedly gain maximum strength by lifting slowly. The idea was that more muscle fibers were recruited, so we'd get stronger faster. The Olympic weightlifters did not buy into it. I was willing to give slow lifting a try, as Arthur Jones suggested.
You can guess what happened. After six weeks I got good at slow lifting, consistent with the Specificity Principle. My maximum single lifts dropped quite substantially and my power was compromised--didn't help discus and shot performances at all. I went back to my original regimens and regained my previous status within a few weeks.
That's not to say that slow lifting does not improve muscular endurance, save time, expend energy for weight loss, and offer other benefits for those whose goals do not involve maximum strength gains.
Check out this article from the Washington Post from 2001 on Super Slow Lifting. It's from 2001, but outlines some benefits as well as controversies that still exist on the topic.