Physiology

FISIOLOGIE 244

Factors that may influence the force and speed skeletal muscle contraction

1. The number of motor units

A motor neuron and the number of muscle fibers that it supplies are called a motor unit.

An increase in the number of more motor units stimulated results in faster and more forceful skeletal muscle contraction.

Better explained: The following (A) is an example of a motor unit, one somatic motor/efferent neuron that innervates three skeletal muscle fibers. Once the motor neuron is stimulated it will activate three skeletal muscle fibers that will generate a certain amount of force.

A

A

Motor neuron axon divert to three muscle fibres

Motor neuron axon divert to three muscle fibres

Motor neuron

Motor neuron

Following (B) is another motor unit but comprises four skeletal muscle fibers. Stimulating this motor neuron, will for obvious reasons (four bigger than three), generate more force than the one above!

B

B

B

B

A

A

Stimulating both motor units A & B at the same time will of course generate much more force in combination with each other!

Stimulating A & B simultaneously

Stimulating A & B simultaneously

Thus, say the biceps, has to lift a load of 1 kg, the brain anticipates such a weight and stimulate just enough motor units in the biceps (muscle group) to lift the weight. If more than the required motor units are stimulated the weight (1 kg) is lifted too fast and forcefully that it may be forced out of your grip. We have all experienced this reaction where you thought something is of a certain weight, say 20 kg, but once you pick it up it is much less, say only one kg! As we all know the result can be catastrophic.

To summarize; increasing the number of motor unites stimulated in a muscle increase the force and speed generated in the muscle and vice versa decreasing the number of motor units decrease the force and speed generated!

2. The frequency of action potentials

An increase in the frequency of action...