If, in strength training, you want to get the most out of your body, you should understand the recruitment mechanism. This principle explains how and in which order your muscle fibres are activated – and why this is so crucial for strength and muscle building.
Henneman's size principle – the order of activation
Your muscle fibres are organised into motor units – small teams consisting of a nerve (motor neuron) and all the muscle fibres that this nerve controls. A muscle consists of many of these teams. Which and how many of them become active during training follows a clear biological law: the size principle.
This states that your nervous system first recruits small motor units, which mostly contain slow, enduring muscle fibres. Large motor units, which predominantly activate fast and powerful type II muscle fibres, are only added when the load increases – regardless of whether through more weight, higher speed or longer tension duration. As soon as the large units are working, the small ones are automatically active too. This means: you cannot target the large, powerful fibres in isolation without the smaller ones joining in.
The three main factors that influence recruitment
So that your body gradually activates more motor units, three factors play a key role:
- Percentage of voluntary strength The higher the strength effort in relation to your maximum possible strength level, the more large motor units are activated in addition to the small ones. With very high loads, virtually all voluntarily activatable muscle fibres are thus recruited.
- Movement speed Fast, explosive movements prompt your nervous system to recruit large motor units earlier – even with a moderate load. This is one of the reasons why sprint or jump training recruits particularly many fast muscle fibres. But in this case too, in accordance with Henneman's size principle, the small, slower motor units are recruited first.
- Increasing tension duration Even if you train with a moderate weight, a long time under tension (Time under Tension) forces the muscle to gradually activate ever larger units. So even slow repetitions to the point of fatigue can lead to full recruitment.
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Why it is important to recruit all muscle fibres
For hypertrophy – that is, muscle growth – it is especially the fast-contracting type II fibres that are decisive, as they have a greater potential for cross-sectional increase. However, these fibres are only activated when, as already described, the load or the speed is correspondingly high or muscle fatigue sets in. Anyone who never reaches the large motor units in training wastes a large part of their growth potential and at the same time limits the possible strength development.
Conclusion: understand the recruitment mechanism – maximise progress
The recruitment mechanism is therefore the basis of how muscles work and grow. Only if you manage to activate both small and large motor units do you use your full strength and muscle potential. The three influencing factors – strength, speed and tension duration – are your adjustment levers here – regardless of whether you want to become stronger, more muscular or more capable.
It is precisely slow, controlled movements over the full range of motion that offer a decisive advantage here: they lengthen the time under tension, promote the full recruitment of all muscle fibres and at the same time reduce the risk of injury – a combination that makes you stronger, healthier and more capable in the long term.
