Training Dictionary
Size Principal
Definition: The orderly recruitment of motor units is referred to as the size principal.
The force of contraction can be increased by recruiting additional motor units and by increasing the rate of firing. The size principal states, motor units are recruited according to their size. 1
Smaller motor units, controlling slow twitch muscles are recruited first. When load increases to a moderate tension, small motor units are contracted along with the moderate size motor units, which include a mix of slow twitch and fast twitch muscles (likely containing a great number of fast twitch oxidative muscles). As load increases even more, the small motor units are contracted along with the moderate motor units along with large motor units (likely containing predominately fast twitch glycolytic muscles). 2
A high level view of muscles types engaged during different exercise loads::
| Level of Load | Slow Twitch Muscles Engaged |
Fast Twitch (oxidative) Muscles Engaged |
Fast Twitch (glycolytic) Muscles Engaged |
|---|---|---|---|
| Light Load | slightly to fully | 0% | 0% |
| Moderate Load | fully | slightly to fully | 0% |
| Heavy Load | fully | fully | slightly to fully |
During Exercise
Each motor unit contracts in a coordinated pattern and in a sequential order, recruiting slow twitch fibers first and progressively recruiting fast twitch (oxidative) and fast twitch (glyolytic) respectively
Important Details
Learning this is fundemental in understanding how to futher push your training further using compartmentalized training
Sources:
1 Nicholls, Martin, Wallace, & Fuchs, 2001. From Neuron to Brain. 4th Edition. Sinauer Associates, Sunderland, MA 449 p.
2American College of Sports Medicine,2006. ACSM's Resource Manual for Guidelines for Exercise Testing and Prescription. 5th Edition. Lippincott Williams & Wilkens, Philadelphia: 60-61 pp.
