Effective Strength and Motor Learning

(Extracts from “ The Physiology of Dry Land Training” Prof W.Heusner –World Clinic Yearbook)

  A muscle is composed of fibres; these fibres are divided into groups and controlled by many motor units, each having a certain number of fibres to control.

Nerves, some of which are excitatory and cause the muscle to contract, in turn activate the motor units and others are inhibitory which prevent contraction, or random muscle action.

All of the muscle fibres within a given muscle motor unit contract or relax simultaneously. This is known as the “All or Nothing” law. It is not possible for some fibres of a muscle unit to relax while others contract; this of course does not apply to the whole muscle.

It would be correct to conclude, that exercise designed to increase the size and strength of muscle fibres would result in fewer motor units being needed to perform a given task. These motor units would then enjoy a better work/rest ratio, and in turn enable the muscle to contract at a high rate for a longer time.

But possibly, especially for a lot of swimmers, the most important increment in strength gained through exercise, is the neuromuscular adaptation.

In the preceding description, inhibitory nerves were mentioned. Inhibition is really a protective mechanism: many our muscles have very much more contractile tissue than is normally used at any one time. Some muscles and groups of muscles are potentially capable of exerting so much force that assuming simultaneous activation of the entire muscle or groups of muscles; we would end up with really serious skeletal damage.

However, specifically performed exercise which progressively stress and cause changes in the muscle, bone and connective tissue making them much stronger, has the effect of overriding part of the neural inhibition which prevents the motor unit from contracting and so releases a much greater amount of muscle force.

Furthermore, this disinhibition gets enormous changes in strength and power without increasing the muscle bulk, because the increased force is due to an increased motor unit activity.

A good understanding of motor learning is important for swimming instruction, stroke technique and training.

Within each motor nerve there are many motor neurons or little alpha fibres. Each one branches out and innervates a whole bunch of muscle fibres. One neurofibre and the muscle fibres that it innervates is called a motor unit. (Mentioned previously)

It is the smallest functional unit that we have in the neuromuscular system.

When a new skill is attempted, the body has to recruit a bunch of motor units, and initially that recruitment may be rather haphazard and quite inefficient. What you get is more units than will do the trick for you efficiently. Some would operate but be inefficient, and some would actually be antagonistic.

This happens on a trial and error basis, but as we continue to work on a new motor skill we gradually refine that recruitment of motor units to a point where we isolate the effective ones. We recruit more effective ones; then we get rid of the ineffective ones and the antagonistic ones, and organize the recruitment on a time related basis.

We now have a nice summed response, which gives us the smooth, flowing kind of actions that you are used to seeing in the skilled athlete.

From the forgoing, it is easy to imagine the problems involved in making a change to wrong movement that has been programmed into the neuromuscular system.