Zitat:
Motor units have activation thresholds - any stimulation from the nervous system below that threshold will not cause them to activate. Motor units are recruited according to the size principle, which more or less tells us that ST fibers are recruited before FT, and the FT fibers with the highest output are the last to be used. The highest-threshold MU`s require large amounts of tension to recruit.
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As a set progresses, the smaller MU`s begin to fatigue. In order to keep up with the demands imposed upon the muscle, the nervous system begins changing things. Based on feedback from proprioceptors in the muscle, the nervous system increases the frequency of impulses (rate coding), and begins to recruit MU`s synchronously. Synchronous recruitment means that the nervous system will recruit every potential MU at once, instead of the usual asynchronous fashion (33). Failure occurs when the pool of MU`s recruited to lift the weight fatigue and the nervous system increases its output to activate other MU`s to take up the slack. This is more or less the same thing that occurs when achieving a maximal attempt. Due to the principle of specificity, the body will adapt to these firing patterns. This process of failure is both a good thing and a bad thing.
Bad news first. When consistently exposed to high-frequency impulses, the CNS has the peculiar trait of inhibiting its own output (12, 14, 17, 18, 19, 20, 21, 28, 29). This is theorized to be a protective mechanism, since consistent exposure to a high-intensity stimulus will be damaging to a neuron much the same way it would be to a muscle fiber, or any other tissue for that matter.
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Beyond the central fatigue that can develop in the brain, damage of a sort can also happen in the peripheral nervous system, at the neuromuscular junction and in the excitation-contraction coupling (ECC) system. The neuromuscular junction is where the motor neuron connects to the muscle. Excessive buildup of potassium ions (K+), which are normally required to transmit the nerve impulse from the neuron to the muscle, can cause a limited form of damage to the neuromuscular junction, inhibiting the transmission of impulses (4, 6, 7, 13, 22, 23). Intracellular K+ build-up occurs as a result of disruption to the muscle`s membrane during mechanical action, as well as during high-frequency neural firing, in a sort of feedback mechanism.
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That`s a lot of bad news. Fortunately it has some good karma to balance it out. As noted by Dr. Zatsiorsky, there are three main ways to increase muscular tension (33). The two that are relevant here are the maximal-effort method and repeated-effort method. Maximal effort involves working with limit or near-limit loads, while repeated effort entails lifting a non-maximal load to muscular failure.
It´s pretty widely known that training with maximal weights causes extensive neural adaptation, resulting in almost immediate strength gains. This can also occur when the repeated-effort method is used. According to Zatsiorsky, the final "failure rep" causes strength gains as well, due to recruitment and fatigue of higher-threshold MU`s, and a wider array of total MU`s, through the processes described above (28, 33). So taking your sets to the limit is a good way to increase muscular strength as well as targeting a wide range of muscle fibers. As you could imagine, this is beneficial for folks interested in either strength or muscular size.
Not only that, but the neuron itself adapts to the stresses. It`s been shown that motor neurons exposed to high-frequency impulses end up with more developed neuromuscular junctions, apparently capable of handling high-intensity impulses better than those not exposed to similar stress (4, 6, 7). This is not unlike the way a muscle grows in response to the damage inflicted upon it. These adaptations to the junction seem to be fairly permanent (4,9).
Finally, all this metabolic and neural damage may well lead to additional hypertrophy via biochemical signaling. There`s been some recent work showing that a variety of satellite cells that tend to cluster around the NMJ, and K+ concentrations set off a biochemical cascade that may serve to activate them (34). There`s a whole mess of processes inside the fibers themselves that can be activated by the myriad chemical signals that result from both metabolic fatigue and eccentric trauma, and even the activity of nerve impulses themselves, so there`s definitely a case to be made for taking things to the limit.