What is the LEAST amount of training we need our athletes to do to get the BEST results out of them?
What is the role of the central nervous system in strength development?
In a series of posts, we will attempt to answer these questions.
In this post, we will cover some basic muscle physiology.
There are 2 principles that govern the ability of an athlete to develop maximal strength…
1) Henman’s Size Principle
The recruitment threshold of each motor unit is governed directly by the size of its axon. Thus, all motor units are recruited in an ascending order according to their relevant size.
Larger motor units having higher recruitment thresholds produce greater amounts of force, whereas smaller motor units have lower recruitment thresholds and produce less force. Due to their smaller threshold potential smaller motor units are recruited first (Baechle & Earle, 2008; Kraemer & Looney, 2012).
It is for this reasons that well- planned sequential (Hypertrophy –> strength –> power –> complex) resistance training phases are implemented, as it is only the activated motor units that undergo anabolism (“building”). Whereas catabolism (“breakdown”) is found to occur in non-activated motor units, due to exposure catabolic substances such as, inflammatory cytokines, free radicals, cortisol, etc. (Kraemer & Looney, 2012).
The Selective Recruitment Principle
The All-OR-NONE law, i.e. number of motor units recruited, as well as the number of motor units in a muscle, plays a role in power production. However, an exception to the rule is that well-trained athletes may be able to inhibit lower threshold units and in their place, activate the higher threshold motor units, thus increasing rate of force development and power output.
Check out the paper in Strength & Conditioning Journal, “Underlying mechanisms and physiology of muscular power” by Kramer et al (2012) for more detail.
Further to the adaptions above, the neuromuscular junction (NMJ) also undergoes the following changes:
- Increase in NMJ area
- Increase in length of nerve terminal branching
- Increased motor end plate perimeter
- Greater dispersion of muscle receptors
In practice, implementing various advanced strength-training methods increase motor unit recruitment. For untrained individuals though, the ability to recruit fast twitch fibers/high threshold motor units may be limited. Motor unit recruitment for untrained individuals is sparked by the motor cortex activity and through neural efficiency.