Your nervous system is responsible for modulating the amount of force produced by your muscles. It does so via:
1. The number of motor units recruited: The more motor units are brought into play, the more force you can generate. More force = more motor units (this is a key principle). Under most circumstances, muscle recruitment follows the size principle: the smaller motor units (low threshold) as the demand for force production increases intermediate MU will come into play and if the demand for force production is extremely high, the larger (high-threshold) MU will be recruited. So it should be clear that to recruit the high-threshold motor units, the demand for force production must be very high and to accomplish this it is primordial for each repetition, you make a concentrated maximal voluntary effort.
2. The firing rate of the recruited motor units: We just saw that the nervous system can increase force production by recruiting more motor units. Force production can also be modified by varying the rate at which muscle fibers are fired. The more frequently a fiber twitches the more force it can generate. When a muscle fiber twitches at around 5060 impulses per second, it reaches a titanic contraction which can be up to 10-15 times more forceful than an low firing frequency.
3. The rate of force development: When a desired movement requires force to be developed at a very rapid rate, the nervous system can adjust its mode of recruitment by having the motor-units fire at a very high rate. Training that utilize explosive or ballistic concentric actions (or having the intent to accelerate as much as possible) can increase the capacity of the nervous system to utilize muscle fibers at a very high firing rate. We will address this specific point in more depth in the "principle 3" chapter.
The coordination factor: To be efficient at producing force in a movement, your nervous system must be good at coordinating the action of the various motor units within a target muscle. It must also coordinate the action of all the muscles involved in the movement. For that reason, the greatest strength gains from weight training exercises are seen in the movement patterns being trained, with some carryover to other related movements.
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The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.