Motor control

Simple tasks, such as reaching for a cup of coffee, are actually surprisingly complex, requiring the successful coordination of sensory input (seeing the cup of coffee, sensing one's own movement towards it, feeling one's fingers touch it, sensing its weight when moving it. etc.) and motor output (moving the eyes, extending one's arm, grasping the cup and lifting it, adjusting one's muscle tone to compensate for the added weight, etc.). Motor control are information processing related activities carried out by the central nervous system that organize the musculoskeletal system to create coordinated movements and skilled actions. Thus the study of motor control involves studying perception and cognition, feedback processes, and biomechanics, to name a few.

Motor control is also the name of a thriving field within Neuroscience that analyzes how people, animals and their nervous system controls movement.

Aspects of motor control

Motor control can be thought to concern two types of movements: volitional and reflexive.

Beyond anatomical divisions, motor coordination studies often seek to explore one of the following questions:

• What physics and mathematical modeling of the limb movement may be involved?
• How complicated and coordinated is the limb movement? How are movements of several joints coordinated?

Fortunately for researchers, multi-limb movements can often be modeled by simple mathematical models. A single limb can be broken down into components such as muscles, tendons, bones, and nerves. The physics are then derived with the aid of modern computers. The study of multi-limb movement is then only slightly more complicated. The development of elementary models of intelligence, along with a gambit of built-in reflexive reactions, is suited to the modeling of this system.

Theoretical frameworks of motor control

• Coordination Dynamics framework emphasizes the dynamical and time-continuous interplay between brain, body, and environment as a holistic system.
• Equilibrium point approaches emphasize that biomechanics and in particular the elastic properties of muscles and reflexes in the spinal cord can render many movement problems easy.
• Reinforcement learning based approaches emphasize the learning of movement from motor errors.
• Optimal control and estimation frameworks (see Bayesian brain) start from the computational problems that need to be solved and ask which solutions would be optimal. Many internal model studies fall into this framework.

Motor Control in athletes

Motor Units

Daily tasks, for instance walking to the bathroom, talking one of your friends or eating the dinner all require multiple muscles that innervate body parts to move properly in order to complete specific tasks. Motor units that consist tens, hundreds or even thousands of motor nerves branches are connected to the muscles. In our body, Rectus femoris contains approximately 1 million muscles fibers which are controlled by around 1000 of motor nerves. Within one motor units which can categorized to type I (slow twitch) or Type II fibers (fast twitch), the composition type of the muscle fiber will be consistent (homogeneous); whereas within one muscle, there will be several different combination of two types of motor units (heterogeneous).

There are three primary types of muscle fibers: Type I, Type IIa and Type IIb. As described above, Type I muscle fibers are known as slow twitch oxidative, Type IIa are fast twitch oxidative and Type IIb are fast twitch glycolytic. These three different types of fibers are specialized to have unique funtionalities. Type I fibers are described as high endurance but low Force/Power/Speed production, Type IIb as low endurance but high Force/Power/Speed production and Type IIa fibers are characterized in between the two.

Nervous system organization

Motor units are multiple muscle fibers that are bundle together and when an athlete want to move their body to achieve a certain task, the brain then send out a instantaneously impulse signal that reach the specific motor unit through the [spinal cord]. After receiving the signal from the brain, the motor unit contract muscle fibers within the group for movement of the body. There are no partially firing in the motor unit which means that once the signal is detected, the muscles contract 100%. However, there are different intensity of activities involve in either daily tasks or athletes competitions that require just the right Force/Power/Speed provide from the muscle. Since the motor unit contracts its fiber 100% once stimulated, types of motor unit that generate variety of Force/Power/Speed are significant.

Fiber Type -- Contraction Speed -- Time to Peak Power -- Fatigue

I (slow twitch)-------slow--------------100 milliseconds--------slowly

IIA (fast twitch)-----fast-----------------50 milliseconds--------fast

IIB (fast twitch)-----very fast-----------25 milliseconds--------fast

Mechanism and Structure of motor unit

Low intensity task such as picking up a trash, the brain recruits motor units that group less number of muscle fibers which the muscle fibers are constructed by Type I(Slow twitch) meaning that even contracting at 100%, muscles will no create high Forces/Powers/Speeds performance. If Type II fibers are stimulated instead for low intensity task, a simple moment of picking up a trash will result in fast and powerful movement, knocking one's own face.

• Low threshold motor units vs high threshold motor units

For the Low intensity task, the smaller motor units that have significantly less number of muscle fibers in the group are being activated to. Those smaller motor unites are known as low threshold motor units. In the low threshold motor units, they consist of type I fibers that contract much slower and thus provide less force for daily basic movement such as typing on the keyboard. As the intensity of the task increase which we need more forces applied to be able to complete the task, more and more motor units particularly Type II muscle fibers are involved in the situation. These fast twitch motor units are known as high threshold motor units. The major difference between a low threshold motor units (or slow twitch motor unit) and a high threshold motor units (or fast twitch motor unit) is that the motor unit with high threshold controls more muscle fiber or these cells (fibers) are larger in compare to the low threshold motor unit. On the other hand, the main difference between the slow twitch muscle fiber (Type I) and fast twitch muscle fiber (Type II) has the same theory of the size deviations.

• The order of recruitment of motor unit

During an activity of lifting heavy objects such as working out with a dumbbell, not only does low threshold motor units, but also the high threshold motor units are recruited to compensate forces required in addition to just holding a fork, in which the energy created by the low threshold motor units is sufficient to complete the job. When giving a job, the body first recruits the slow-twitch motor units following by recruit more an more fast-twitch motor units as forces required to complete the movements increase. Thus, when the body has to carry an extremely massive object, it would recruited all the available motor units to contract for the particular muscle that has been used.

• Fiber versus nerves

The type of fiber (type I vs Type II) is controlled by the nervous system. The brain is the central information center that sent out the signal to the nerves, that the nerves control and connect the motor units. For two different motor units present, the body adopt it with two different nerves to control them. Fast twitch motor units are controlled by fast twitch nerves while Slow twitch motor units are controlled by slow twitch nerves.

In the laboratory, a nerve from a motor unit that is connected to a slow twitch muscle fiber was replace with a nerve that are designate for a fast twitch fiber. Amazingly, the slow twitch fiber behaved identically as a fast twitch fiber. In contrast, if the process was reverse, the fast twitch fiber performed as a slow twitch fiber as well. However, the nerves can not possibly transform from fast motor nerves into slow motor nerves and vise versa.

In many sports movements, the duration of certain action usually are within 200 milliseconds, and from the above charts, time to peak power of the individual muscle fibers of each type (I,IIA,IIB) have sufficient time to reach peak power production. This brings out a question of what is the superiority of having more Type II fibers?

This can be documented when you analyze a large group of athletes for vertical jump performance and their execution for a vertical jump. Athletes with more fast-twitch fibers (Type II) change direction quicker during their movement such as left to right direction and they tend to use less knee bend. These results can be confirmed by muscle biopsy and even by special force-plate analysis. This does not mean that athletes with lower fast-twitch fiber cannot jump higher, but they tend to do it a little slower and with a deeper knee bend.

Although having a high percentage of Type II fibers give a person more quicker movement, there is little doubt that the nervous system and the brain is more important on affecting the performance.

• Muscle recruitment

The majority of the time, the real limit to athletes performance is the number of motor units their nervous system can recruit in the short period of time and the amount of forces (size of the muscle fibers) provided of those motor units. The performance is rarely affect by the type of muscle fiber (slow twitch or fast) that constructed to motor units. The nervous system determines the degree of motor unit activated in sport like activities.

It normally takes 0.4 - 0.6 seconds for the nervous system to activate available muscle motor units to contract, same length of time demonstrating maximum strength or force. However, vertical jump activity only takes 0.2 seconds to perform. Therefore the factor of determining the performance is within 0.2 seconds, how many available muscle motor units can be recruited for contraction, yet how much fast twitch fibers in the body. as the result, an athlete lacking fast twitch fibers has better control of nervous system that recruited all the fast-twitch fibers in the body, the athlete tend to have superior performance in comparison to the athlete with a less control of nervous system while having greater number of fast twitch fibers.

From above, people can considerably increase their strength without increase the size of their muscle, because the body becomes more efficient at muscle recruitment and firing synchronization.

Neural and Cognitive Processes

• Forward Models
• Skill development and motor learning
• Sports-Specific decision making

Research in Athletes

• Example from the Paper
  • Statistical results of significant difference
  • Different sports
  • Bar Graph and other figure of results

Suggested Reading

Shadmehr, R. (2004). The Computational Neurobiology of Reaching and Pointing: A Foundation for Motor Learning. MIT Press.

See also

• Motor learning
• Sensory integration
• Motor skill
• Motor system
• Motor coordination
• Two-alternative forced choice