It refers to the amount of force a muscle can produce with a single maximal effort.

This statement is describing the concept of "muscular strength". Here are the steps to understand it:

1. "It refers to the amount of force": This part of the statement is talking about a measurement of force. In the context of muscles, force usually refers to the power or energy exerted by the muscles.

2. "a muscle can produce": This part specifies that the force being measured is produced by a muscle. This means we're talking about some aspect of physical fitness or body mechanics.

3. "with a single maximal effort": This part of the statement tells us that the force being measured is not just any force, but the maximum force that the muscle can produce in one effort. This means we're talking about the greatest amount of force a muscle can exert at once.

Putting it all together, the statement is defining muscular strength. Muscular strength is a measure of the maximum amount of force that a muscle can exert in a single effort.