Sensory Motor Integration
Signals from the CNS directly affect muscular activity specifically force production. It should be noted here that the production and increase of strength is a basic tenet of physical training for the exercise training and sport performance communities. Strength is considered to be the ability of a muscle or group of muscles to exert maximum force at a specified (or any) velocity (Knuttgen and Kraemer, 1987). Strength is not primarily a function of muscle size, but is substantially affected by nervous system stimulation. This is the foundation of all strength training. Please note here that strength’s definition coincides with the formula for power: Power is equal to force times velocity.
Physiology states that structure is determined by function, where increase in muscular size (hypertrophy) is an adaptive response to neuromuscular stimulation of a given minimal intensity. Therefore, nervous stimulation produces two basic adaptive and interrelated effects:
- Functional muscular action (the functional effect)
- Muscle hypertrophy (the structural effect)
Verkhoshansky and Siff (2009) stated that the fundamental principle of strength training is that all strength training is initiated by neuromuscular stimulation. While hypertrophy is a major long-term result of various neuromuscular stimulations, not all physical training will result in this type of adaptation against some specified resistance. Based on this information, two types of resistance training may be recognized:
- Functional resistance training
- Structural resistance training
Please note that this distinction draws the conclusion that there is no such thing as purely structural training since all training is essentially functional (in some respect) which, under specified conditions, may reduce structural changes. Additionally, maximum strength also depends a good deal on optimal hypertrophy.
Numerous training protocols are aimed principally at increasing tissue hypertrophy (tissue refers to muscle and all associated connective tissue, e.g. bone, ligaments, tendons, fascia) as well as tissue flexibility (structural training which refers to aesthetic improvements, i.e. bodybuilding types of programs). However, functional strength training is associated with countless distinctive performance goals to include enhancements in various strength categories (static-strength, speed-strength, strength-endurance, reactive abilities). Fundamentally, the former produces increases in diameter and/or strength in individual muscles fibers, where the later creates or augments the speed or rate of contraction of numerous muscles of the various systems for a specified performance effect. Functional training (training stimuli to enhance nervous system stimulation for improved performance) involves the following:
- Inter-muscular coordination (between muscle groups)
- Synchronization – specific muscle groups working all together (firing) for a specific motor pattern producing specified joint movements.
- Sequencing – specific muscle groups working (firing) in a specified sequence for a specific motor pattern producing specified joint movements.
- Disinhibition – certain muscles turned-on/off while a certain movement is performed
- Intra-muscular coordination (coordination of the various fibers within the same muscle group). This entails one or more of the following means:
- Number Encoding – muscle tension is controlled by activating or deactivating certain number of fibers
- Rate (frequency) Encoding – muscle tension is controlled by the firing rate of active fibers
- Pattern Encoding – muscle tension controlled by synchronization or sequencing of firing of the different types of muscle fibers (e.g. fast or slow twitch)
- Facilitatory and Inhibitory reflexive process – modification of the neural pathways acting at various levels in the nervous system which optimizes the development of strength by either improving intra or inter-muscular coordination or by promoting adaptive changes in the various reflex systems of the body (e.g. muscle spindles and golgi-tendon organ). Muscle reflexes are a preprogrammed response to various stimuli.
- Motor Learning – process of programming the brain/CNS to carry out specific tasks
- Engrams – specific learned motor patterns stored in the brain (memorized motor patterns stored in both the sensory and motor portions of the brain (pre-motor cortex).
- Slow movements are stored in the sensory portion
- Fast/rapid movements are stored in the motor portion.
The key elements for stimulating motor responses are:
Each muscle fiber is innervated by a single neuron, but one neuron may innervate several thousand muscle fibers.
All muscle fibers within a motor unit are of the same fiber type.
Motor units are recruited in an orderly manner. Thus, specific units are called on each time a specific activity is performed; the more force needed, the more units recruited.
Motor units with smaller neurons (ST units) are recruited before those with larger neurons (FT units).
Motor unit recruitment depends on the force/resistance of the exercise. With light intensity exercise the Type I (slow twitch) motor units are recruited. When the load is increased, the Type IIa (fast twitch) will be recruited with the help of the Type I fibers. When the load becomes even greater, the Type IIb/x will be recruited with the help of the Type IIa and Type I motor units. Therefore Type I motor units are always firing no matter what the intensity.
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