La forza può essere definita come: “Strength can be defined as: " The ability to overcome a resistance or to oppose it with a muscular effort ".“.
(Zaciorrskij)
This ability is susceptible to 3 factors:
- cross section of the muscle
- intermuscular coordination
- motor unit recruitment
A lot has already been said about the first 2 factors, the increase in muscle volume should increase strength (I use the conditional because, as we will see, it is not always the case) and the coordination of different muscles which corresponds to a correct technique, certainly allows to produce a force majeure. But the third factor is in my opinion the most important: Motor unit recruitment. Motor unit recruitment .
The central nervous system (CNS) starts an impulse which, through a motor neuron, through its axon, reaches the motor plate of the fiber causing it to contract.
A motor neuron with its fibers innervated by it is called a Motor Unit (MU).
Each motor neuron can innervate a different number of fibers but always of the same type.
There are 3 types of fibers:
- Type 1 or ST: red fibers because they are rich in myoglobin, weak, slow and very resistant. They have a low activation threshold.
- Type 2A or FFR: Intermediate fibers, relatively strong and quite tough. Average activation threshold.
- Type 2B or FF: white fibers, very strong and fast but not very resistant. They have a high activation threshold. The recruitment of the MUs takes place according to 2 criteria, the spatial one spatial which concerns the number of units involved, and the temporal one which concerns both the recruitment times and the frequency of discharge of the nerve impulse. The greater the number of MUs involved and the higher the firing frequency, the greater the force produced.
According to Henneman's principle Henneman, MU recruitment occurs according to the order of magnitude of the motor neuron and the excitability threshold.
It follows that type 1 fibers will be the first to contract followed by 2A and only after reaching very high thresholds with loads close to maximal, will 2B fibers be recruited.
In order to have a maximal contraction of the fibers, the discharge frequencies must be around 30 Hz for type 1 fibers and up to 100 Hz for type 2B fibers.
When the discharges follow one another at these frequencies we speak of muscolar tetany and it allows us to reach force potentials clearly superior to a normal contraction.
AUTHOR
Tony Lo Grasso
released March 20, 2016
English 
Italian