So let's now take a look at how the muscle force is controlled. You know from experience that you have a remarkable ability to control the amount of force you can produce. The same muscles can use a light force to hold a pencil. And a higher force to pinch a fold of skin together, so it doesn't hurt, and an even higher force so that the skin fold pinch will hurt. Now as I mentioned earlier, the brain uses three methods to increase the amount of force or muscle, or a group of muscles can produce. A higher number of motor units can be activated, and this is called recruitment, as you recall. Motor units are recruited according to their size, from small motor units to fast motor units. The firing rate of the motor units can be altered. And as you recall, this is referred to as rate coding. And the motor unit activity can be synchronized, so more motor units can stimulate the muscle fibers at the same time. It's also helpful to know a bit about each of these methods. So let's briefly examine each. And we're going to begin with recruitment. Now in this diagram, motor units are arranged according to their size. The purple area at the bottom of the chart represents the slow motor units and the yellow represents the FOG motor units and the blue represents the FG motor unit. Now the FG motor units, remember are the fastest and the most powerful motor units. Now motor units are recruited into action beginning with the small slow oxidative motor units to the large fast motor units. This line represents the percentage of maximum force produced from very low to very high. When a muscle is called upon to generate small forces, only a few of these smaller motor units is stimulated. Standing for example. What I'm doing right now, requires very little muscle force. Only about 25% of the slow motor units in the motor neuron pool are recruited to maintain a standing position. Walking requires a slightly more activity of the motor unit, they require between 25 and 50% of the motor neuron pool depending on the walking speed. Notice that during walking, FOG fibers are brought into play. The faster the walk, the higher the percentage of the motor neuron pool is recruited to produce the increase in force that's needed. Slow jogging extends from fast walking and requires around 50% of the motor neuron pool to produce the force required to run. Even when the greater force is required when running. The larger, it's easy to do this. Because the larger FG motor units come into play. And more of them are recruited. A fast run requires about 60% of the motor neuron pool, most of which the FG motor units. Now jumping demands about 80% of the motor neuron pool. Much more force is required to jump. In essence, small motor neurons with low firing thresholds are recruited first to do low level activity. And when larger forces are needed, not only are more motor units recruited, but the larger and faster motor units are increasingly recruited. The FG motor unit has the highest threshold and are recruited last. A full activation of the FG motor units is really difficult to achieve. Athletes who engage in strength and power training improve their ability to recruit and activate their fast or FG motor units. The most metabolically efficient type one motor units are recruited first, the slow motor units. The large high threshold motor units tire easily because they rely on non-aerobic ATP production strategies. And they're recruited only after the smaller more metabolically efficient motor units have been recruited. On this chart here, you see the visual illustration of how recruitment of the larger motor units correlates with the force production. Now let's take a look at rate coding. This refers to the adjustments made to the firing rate of a motor unit. Rate coding and motor unit recruitment actually work together to produce a full range of force needed for sports skills. Rate coding of a motor unit ranges from a single twitch to a very high speed of signal stimulus that produces the maximum force possible from a muscle. And this is refer to as tetanus. And occurs when the athlete is producing a maximum asymmetric force. That's the muscle contraction where there is no actual movement occurring. And when a muscle fiber is activated with a single electrical stimulus, it contracts quickly, and then relaxes back to it's resting state. This hardly ever happens in sports movements. But this response is called a twitch. Occurs when the muscle fiber is stimulated at a very, very high rate, so it doesn't have the chance to return back to it's resting conditions between stimulus. Summation is a really interesting phenomena. It is a necessary requirements for fine control of athletic skills. If an electrical stimulus is delivered immediately after the first, it will produce a second twitch that partially piggybacks on the first one. And the strength of the muscle contraction summates. And as the firing rate increases, the summation grows stronger, up to a limit. And when the success of stimulation reaches a plateau, it is at its maximum state of contraction. The muscle is in a state called tetanus. And this occurs at maximal isometric contraction. And you'll measure this in the weight room with special isometric testing. Now synchronization is another pattern of motor unit activation. Muscle strength can increase without noticeable hypertrophy. And proved motor unit synchronization explains strength gains that are not accompanied by muscle hypertrophy. Both recruitment and increased firing of already active motor units are involved in strength gains. An interesting research study comparing synchronizing of centering individuals, skilled musicians who were not strength trained, and athletes hurt that were strength trained. In motor unit synchronization was lowest in the sedentary and highest in the strength train subjects. Here is the force produced by each group of subjects. The musicians are a really interesting case. Their muscles are well trained to play the violin, and they were actually able to produce more force than the sedentary. The athletes however that were trained for strength had the highest motor unit synchronization. And from this research it was theorized that elite power and strength trained athletes not only recruited a high percentage of motor units, they could also activate the motor units more synchronously during maximal voluntary efforts. And this contributed to their ability to produce high muscle forces. The brain normally protects muscle by inhibiting high synchronization of large numbers of motor units. With training, there is a reduction apparently in this inhibition. Now psychological factors are also important. Under extreme conditions, such as the life or death situation, people can develop extraordinary strength. And this suggests that the central nervous system, in extraordinary situations, can increase the rate of motor unit firing. The decrease in central nervous inhibition of motor unit firing also occurs or both can happen. So both of these situations can occur with improved synchronization due to the psychological factors that have an impact on the increase in motor unit firing, or the decrease in the inhibition of the motor units.
