A demonstration is performed during a lecture on muscle

physiology in which a student is asked to fully extend his

right arm with the palm up. Two large textbooks are placed

on his palm, one at a time. Which of the following facilitates

the maximum amount of tension that allows the student to

keep his arm extended in place under the increasing weight of

the books?

(A) Amount of Ca2 + released from the sarcoplasmic

reticulum

(B) Amount of muscle phosphocreatine

(C) Amplitude of the action potential

(D) Number of motor units recruited

(E) Rate of cross-bridge recycling

The answer is D

Number of motor units recruited

Explanation:

The force a muscle generates is dependent on the length of the muscle and its velocity which is shortened.

A motor unit is the grouping of muscle fibers supplying nerves by the neuron and consists of the motor neuron.

The number of muscle fibers within a motor unit varies, and is a function of the muscle's ability for accurate and refined motion.

Precision can be determined to be inversely proportional to the size of the motor unit. As a result, small motor units can exercise greater precision of movement compared to larger motor units. For example, thigh muscles which are responsible for large powerful movements, can have a thousand fibers in each unit, while eye muscles which requires small precise movements, might only have about ten.

Groups of motor units are supplied with nerves to coordinate contraction of a whole muscle and generate appropriate movement; all of the motor units within a muscle are referred to as a motor pool.

As a result, the number of motor units with are recruited will facilitate the maximum amount of tension that allows the student keep his arm extended in place under the increasing weight of the books.