Question

A graded whole muscle contraction is produced in vivo primarily by variations in a. the strength of the fiber's contraction. b. the number of fibers that are contracting. c. both of these. d. neither of these.

Short answer

The correct answer is b. the number of fibers that are contracting. Graded whole muscle contraction in vivo is primarily produced by variations in the number of fibers that are contracting through the process of motor unit recruitment.

Step-by-step solution

Option a: The strength of the fiber's contraction

The strength of muscle fiber contraction, or the force it can generate, can change based on the frequency of stimulation and the length of muscle fibers. However, this alone does not primarily produce graded whole muscle contraction in vivo.

Option b: The number of fibers that are contracting

Graded muscle contraction is mainly controlled by the number of muscle fibers that are contracting within a muscle, as the greater the force generated by the whole muscle, the more fibers need to contract. This process is known as motor unit recruitment, and it is the foundation for graded muscle contraction.

Option c: Both, the strength of the fiber's contraction and the number of fibers that are contracting

Both factors play a role in muscle contraction overall, but graded muscle contraction in vivo is primarily controlled through the recruitment of motor units (the number of fibers that are contracting) rather than changes in the strength of individual fiber's contraction.

Option d: Neither of these

As we discussed earlier, graded muscle contraction is controlled mainly through motor unit recruitment (i.e., the number of fibers that are contracting).

Conclusion

The correct answer is: b. the number of fibers that are contracting. The primary factor that produces graded whole muscle contraction in vivo is the number of fibers that are contracting, which occurs through the process of motor unit recruitment.