Muscle activation
Muscles are composed of a number of motor units (MU). A MU consists of a group of muscle fibres and the alpha motor neuron (nerve) that innervates those fibres. All the muscle fibres in a MU are of the same type, and are spread throughout the muscle. A MU may contain as few as 10 fibres, and up to 1700 fibres. The average is 300-500 fibres. There is a difference in the thickness of the nerve that innervates various MU of differing muscle types. The slow-twitch (I) fibres have the thinnest nerves, the fast-twitch oxidative (IIA) fibres have medium size nerves and the fast-twitch (IIB) fibres have thick nerve fibres. The same goes for the size of the MU's, the smallest ones are composed of I fibres, the next biggest are composed of IIA fibres and the largest MU's are composed of IIB fibres.
So what is the significance of all this? Well, there is a relationship between the size of the MU's and the order that they are recruited in. This is called the Henneman Principle. The smallest MU's are recruited first, and are further recruited in order according to size. So, in low-force muscle contractions, the I fibres are recruited first. As the force or the duration of the contraction increases, then the IIA and then the IIB fibres are recruited into action. When a very powerful contraction is required, all of the fibre types will be recruited as quickly and as fully as possible.
The I fibres are always recruited first, regardless of how strong the muscle contraction is. The IIA and IIB fibres are then recruited according to the force of the muscle contraction, the duration of the contraction and to assist with any fatigue that occurs. As the muscle relaxes, the MU's are derecruited. They are switched off in the reverse order to that in which they were recruited: the IIB switch off first, then the IIA and lastly the I fibres.
Eccentric contractions
Things change a bit when it comes to discussing eccentric contractions. When performing an eccentric muscle contraction, the IIB fibres are more likely to be recruited first, rather than last, then IIA and then the I fibres. This is more noticeable when performing fast, eccentric contractions. In these types of contractions, the fast relaxation times of the IIA and IIB fibres are most suited to the task.
Muscle Contraction
Muscles develop tension when activated. This may cause then to shorten, or to simply become tight. The tension that is generated in a muscle is the result of the activation of many motor units, firing asynchronously and repeatedly. The amount of tension generated in a muscle is altered by varying either the number of motor units that are activated and/or the frequency at which the motor units are activated. Tension is also dependant on the number of cross-bridges (linkages) that a muscle fibre can form. This is maximal in about the middle length of the muscle, and tapers off and the muscle moves away from this mid-length.
As load increases on a muscle, more fast-twitch fibres are recruited. These tend to fatigue rapidly, and when they do so, the muscle cannot develop adequate tension to continue with the movement. At this point, muscle fatigue has occurred. Note that muscle fibre fatigue precedes fatigue of the entire muscle.
There are 2 firing patterns for MU's.
1. MU's that innervate the same type of muscle fibres are recruited at different times so that some MU's are resting (recovering) while others are firing. This pattern functions at low loads, and allows for continuous, low-grade contractions to occur. This pattern is not able to work at high loads as all available motor units will be recruited and fire at the same time to generate enough tension to overcome the load. This synchronous firing of motor units provides for poorly controlled, jerky movements.
2. As mentioned above, MU's that are more resistant to fatigue are recruited before fibres that are more rapidly fatigued.
Neuromuscular Fatigue
There are many contributors to muscle fatigue. Fatigue can be defined as a decline in the capacity of the muscle to develop tension. Many factors interact to cause fatigue, and include:
the central nervous system
the peripheral nervous system
the neuromuscular junction
muscle fibre function
Fatigue is associated with:
a decrease in EMG activity (decrease in neural activity)
glycogen depletion, changes in acidity and neural fatigue
unknown factors both metabolic and neurological
Time Under Tension
Most weight training utilises weights that cause the predominant utilisation of the anaerobic glycolysis pathway (weights that cause fatigue within 30-60 seconds of beginning the set, about 75-85% of your 1RM). From the perspective of tension and time, since muscles grow in response to tension and the time that they are required to produce tension, anything that prolongs the time that they are contracting hard will also increase the growth stimulus.