Neural Modulation of Skeletal Muscle Force

by Greg Crowther

Skeletal muscles greatly vary the amount of force they exert, based on how much force is needed in a given situation. There are two basic mechanisms for adjusting the level of force: varying motor unit recruitment, and varying firing frequencies of recruited motor units.

Sometimes you need just a little bit of force;
Sometimes you need a lot.
Sometimes you need just a little bit of force;
Sometimes you need all you've got!

One! Two! Here are two things you can do!
Three! Four! Raise the firing frequency! (More, more, more!
Five! Six! And add more motor units to the mix!
Seven! Eight! Add the weak ones early and the strong ones late!

Sometimes you need just a little bit of force;
Sometimes you need a lot.
Sometimes you need just a little bit of force;
Sometimes you need all you've got!

• sheet music (with melody play-back)

Songs like this one can be used during class meetings and/or in homework assignments. Either way, the song will be most impactful if students DO something with it, as opposed to just listening.

An initial, simple follow-up activity could be to answer the study questions below. A more extensive interaction with the song might entail (A) learning to sing it, using an audio file and/or sheet music as a guide, and/or (B) illustrating it with pictures, bodily poses, and/or bodily movements. The latter activity could begin with students identifying the most important or most challenging content of the song, and deciding how to illustrate that particular content.

(1) What is a motor unit?

(2) What is a firing frequency?

(3) When recruiting additional motor units as force increases, what is the advantage of adding "the weak ones early and the strong ones late"?

(4) How does a muscle reach a state of tetanus?