Q&A: Repetition Speed, Recruitment and Stimulation

This Q&A is a response to several questions I’ve recently received. Since they’re all related I’m going to answer them together.

Since intentionally moving slowly during exercise involves less than a maximum effort wouldn’t it reduce motor unit recruitment and the effectiveness of the exercise?

No. Motor unit recruitment is determined by the amount of force the muscles are required to produce in order of size starting with the smallest motor units with the fewest fibers and ending with the largest with the most fibers. Regardless of the speed of movement if the weight is heavy enough (adequate to cause momentary muscular failure within a minute or two) you will recruit all of the motor units in the muscles being worked within a few reps. After all of the motor units have been recruited and are fatiguing further increases in force production are achieved by increasing rate-coding (the frequency of the signal to the motor units to contract).

Even if you are intentionally moving slowly at first, if the weight is heavy enough you will eventually have to contract as intensely as possible to lift it. Effort is relative to your momentary strength. As fatigue reduces your strength during an exercise the weight requires a larger percentage of your mometary ability which is why it feels progressively heavier. For example, a weight that only requires is 80% of your available strength at the beginning of an exercise will require an 89% effort at the moment fatigue has reduced your strength by 10% (80/90 = 0.89 or 89%) and when fatigue has reduced your strength by 20% you are contracting with 100% of your momentary strength just to hold the weight (80/80 = 1 or 100%). By this point in the exercise every motor unit in the working muscles has already been involved for a while.

My personal trainer told me I have to lift explosively to recruit the high-threshold type IIB motor units which have the greatest potential for hypertrophy and that slow reps only recruit the slow-twitch, type I fibers.

Your trainer doesn’t know what he’s talking about.

The terms “fast” and “slow” in fast and slow twitch muscle fibers refer to the twitch speed – how quickly the fibers can reach maximum tension – not the speed of movement they produce. Fast twitch fibers reach maximum tension between around 50 and 80 milliseconds, slow around 100 to 200 milliseconds. On average this is a difference of only 85 ms, less than one hundredth of a second. The biggest difference between fast and slow twitch fibers is the relative fatigue and recovery rates; fast twitch fibers fatigue more rapidly and recover more slowly, slow twitch fibers fatigue more slowly and recover more rapidly.

Contrary to popular misinterpretation of their names, your muscles’ slow twitch fibers are capable of producing very fast movements and your fast twitch fibers will be recruited during even the slowest movements if the resistance is high enough.

For example, an isometric contraction is about as slow as it gets during exercise – you don’t move at all. However, when performing timed static contractions or heavy static holds you will recruit even the high threshold fast twitch motor units because the muscles need to recruit all of their motor units to produce the required amount of force.

Moving fast is neither necessary for recruiting the fast twitch motor units nor more effective for that purpose. The only thing moving faster tends to do is compromise form and increase the risk of injury. The idea one must “train fast to be fast” or that “slow training makes you slow” is nonsense.

My coach said I have to lift explosively to improve power. How am I supposed to increase my power for (football, martial arts, golf, etc.) if I don’t train explosively?

Your coach doesn’t know what he’s talking about. I’m guessing he also told you power cleans were the best way to improve your ability to explode off the line, punch hard, increase the force of your golf swing, and become rich and good looking, which would make him doubly ignorant.

If you want to become more powerful in general you just have to train to become stronger, and you can do that training fast or slow. Power is the amount of work you can perform over a period of time. Work is force x displacement. A stronger muscle can produce more force, meaning you can either move a greater mass over the same distance in the same period of time, move the same mass over more distance in the same period of time, or move the same mass over the same distance in less time.

For example, if the most you can lift in an exercise one time is 200 pounds you won’t be able to lift it very quickly. If you increase your strength so that you can lift 250 pounds in that exercise, you will be able to lift 200 pounds more quickly.

If you want to become more powerful in a specific movement you must also learn and practice the skill of performing that movement in a way that makes efficient use of the strength of the involved muscles. Skill improvement is very specific; to become more skilled at a movement you must practice that movement, not something that somewhat resembles it, not a similar movement while holding a weight or with something heavier than you would normally use, but the exact movement.

Many well-meaning but misinformed coaches and trainers will tell you doing power cleans will result in a transfer of explosiveness to just about any other activity. They’re wrong. The only thing power cleans are good for is improving your ability to perform power cleans. They are relatively ineffective for improving the strength of the involved muscles when compared to other exercises, have a greater risk of injury, and will do nothing at all to improve your skill in any other movement. If you are a competitive Olympic lifter you must perform cleans as part of your training for the clean and jerk, but there is no good reason for anyone else to do them.