To recruit the greatest amount of motor units possible, you must generate as much force as possible at any given moment of a set. Some people will be quick to mention that since force production is the key to motor unit recruitment, that we should always lift maximal weights (in the 90-100% range). This is not the case and it shows a lack of understanding of the definition of force. In biomechanics (and physics) force is defined at:
Force (F) equals mass (m) times acceleration (a). So an increase in the generation of force can be accomplished either by increasing the acceleration with a certain load, or by using more weight. Maximum recruitment is generated when the intended force production is its greatest. For that reason, we should try to reach maximum acceleration with any given weight and any given fatigue level. Obviously when the weight used is very heavy, or when we are tired at the end of a set, the actual movement of the bar to be lifted will be slow. However the actual intent to accelerate as much as possible has the same training effect on the nervous system (including MU recruitment, high firing rate, and rapid rate of force development) as if the bar was actually moving fast.
This is what led to the compensatory acceleration technique (CAT). CAT means that you compensate for a non-maximal weight by accelerating it as much as you can; a nonmaximal weight lifted without the intent to create as much acceleration as possible will not lead to the recruitment of the high threshold motor units until you reach a level of fatigue that requires your nervous system to finally tap into these strong fibers. So if you were using a moderate load, lifted without CAT you would only have the last 2-3 reps of a set that would actually recruit the HTMUs. And according to Dr. Vladimir Zatsiorsky, a motor unit that is not fatigued is not being trained. As a result, if you are not able to thoroughly fatigue those HTMUs with the last 2-3 reps of a set (if muscle failure occurs due to an accumulation of metabolites for example) that set was wasted, at least when it pertains to stimulating maximal muscle growth.
The fact that you can eventually tap into your HTMU pool as fatigue sets in has led to the saying, "those last few reps are the key" (the most effective for growth). While with regular bodybuilding training this is probably true, using CAT will make each single repetition effective at recruiting the HTMUs. So the HTMUs will get stimulated with 812 reps per set, instead of only 2-3. As a result, they are more likely to fatigue, and stimulate to growth.
1. High-threshold motor units have the greatest growth potential.
2. HTMUs are brought into play when the demand for force production is high.
3. HTMUs can also be brought into play when the fatigue in the intermediate threshold fibers leads to an insufficient force production. Then the HTMUs must be recruited.
4. You can "compensate" for the lack of fatigue in the first reps of a set by always trying to create as much acceleration as possible on each rep of every set.
5. The intent to (trying to) accelerate is responsible for the recruitment of the HTMUs. Even if the bar doesn't move rapidly, if you are really trying to push it as hard as you can, it will have the same effect as actually lifting with great speed.
6. The key is to reach maximum acceleration at a given weight and fatigue level.
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