Activation of motor units
The type of motor unit recruited for a given activity is determined by its physiological characteristics. For an activity such as distance running, slow-twitch motor units are engaged to take advantage of their endurance capacity and resistance to fatigue. If additional force is needed, as in a sprint finish, the fast-twitch motor units are called into play to increase the pace (although exercise at this intensity cannot be maintained for very long). If the activity requires near-maximal performance, such as when performing the clean, most of the motor units are called into play, with fast-twitch units (especially Type IIb fibres) making the more significant contribution to the effort. Complete activation of the available motor units is probably not possible in untrained individuals.
The activation of motor units is influenced by a concept called the size principle – based on the relationship between motor unit twitch force and recruitment threshold. Motor units are recruited in order, according to their recruitment thresholds and firing rates. Because most muscles contain a range of motor units with both Type I and Type II fibres, force production can span from very low levels to maximum effort. Also, motor units can span a range of muscle fibre sizes (Type I and Type II) which also allow different force productions due to difference in fibre size (e.g. small Type II fibres vs. large Type II fibres in other motor units).
Typically, to get to a high-threshold motor unit, all of the motor units below it are sequentially recruited. Therefore, with heavy resistance training all muscle fibres get bigger, because for the most part they are all recruited to produce more and more force with heavier weights. In addition, motor units high in the recruitment order are used mainly for high speed or power production. Maximal force production requires not only the recruitment of all motor units, including the high threshold motor units, but also the recruitment of these motor units at a high enough firing rate to produce maximal force.
‘Untrained’ individuals may not be able to voluntarily recruit the highest-threshold motor units or maximally activate their muscles. Part of the training adaptation appears to be the ability to recruit all motor units when needed to perform a task. This may explain the rapid initial gains in force development seen in the first few weeks of commencing training.