Questions: Motor Unit Recruitment and Force Scaling

Question 1

A person gradually increases their grip force from very light to maximal. At the cellular level, how is this increasing force produced?

• A. Individual muscle fibers contract more forcefully as the nervous system increases their stimulation intensity
• B. Additional motor units are progressively recruited in order from smallest to largest, each adding its force increment
• C. Fast-twitch fibers are activated first, then slow-twitch fibers are added as endurance is required
• D. The firing frequency of active motor units decreases to allow newly recruited units to contribute

Answer: (B)

This directly tests the core misconception. Individual muscle fibers obey the all-or-none principle — they cannot contract 'harder' in response to stronger nervous stimulation; each fiber is either contracting maximally or not at all. Graded force therefore requires recruiting additional motor units, not varying individual fiber output. The Henneman size principle dictates the order: smallest motor units (slow-twitch, fatigue-resistant) first, progressing to larger (fast-twitch, fatigable) units as force demand increases. Option C reverses the actual order — slow-twitch units are recruited before fast-twitch.

Question 2

Why do the small intrinsic hand muscles have many more motor units with low innervation ratios compared to the large quadriceps, which have fewer units with high innervation ratios?

• A. Hand muscles contain a higher proportion of fast-twitch fibers that require separate neural control
• B. A low innervation ratio produces smaller force increments per unit recruited, enabling finer gradations of force and more precise motor control
• C. Hand muscles require less total force output and therefore need fewer muscle fibers overall
• D. The size principle operates in reverse in hand muscles, with large units recruited first

Answer: (B)

The innervation ratio (fibers per motor neuron) determines the resolution of force control. When each motor unit contains only a few fibers, each recruitment step adds a tiny increment of force — allowing very fine-grained graded control. This precision is essential for tasks like writing, threading needles, or playing piano. The quadriceps, by contrast, has motor units with over 1,000 fibers each — each recruitment step adds substantial force, providing power but coarser gradations. The size principle still applies (small units first), but the force steps are larger.

Question 3

According to the Henneman size principle, the motor units active during slow, sustained walking are a different set than those initially recruited at the start of a maximal sprint.

• T. True
• F. False

Answer: True

The size principle is orderly and universal: slow-twitch (small, fatigue-resistant) motor units are recruited first for any voluntary contraction. For slow walking, only the smallest units are needed and stay active throughout. A sprint requires maximal force, recruiting the full sequence through to the largest, fast-twitch units. The walking units are also active during the sprint (they were recruited first), but additional large units are added on top. This sequencing automatically matches fiber type to task — fatigue-resistant fibers handle sustained low-intensity work, while fatigable fibers are reserved for brief high-intensity bursts.

Question 4

Once a motor unit is recruited, the main way to further increase force is to recruit additional motor units — changing the firing frequency of already-active units has no effect on force output.

• T. True
• F. False

Answer: False

Rate coding is a real and important second mechanism for scaling force. Once a motor unit is recruited, increasing the frequency of its motor neuron's action potentials causes temporal summation of the muscle fiber twitches — at high enough frequencies, individual twitches fuse into a smooth, sustained tetanic contraction that produces significantly more force than individual twitches. The nervous system uses both mechanisms simultaneously: recruitment adds new units, and rate coding increases the contribution of already-active units. Both are essential for the full range of force gradation.

Question 5

Why does the Henneman size principle produce an automatic match between muscle fiber type and task demand, without requiring conscious deliberation about which fibers to activate?

Model answer: Because recruitment order is determined by motor neuron size, which is a fixed biophysical property. Small motor neurons have lower input resistance and depolarize more easily in response to synaptic input, so they reach firing threshold first. These small neurons happen to innervate slow-twitch (type I) fibers that are fatigue-resistant and suited for sustained low-force tasks. Larger neurons, with higher thresholds, innervate fast-twitch (type II) fibers capable of large force outputs but prone to fatigue. The size principle means that whenever synaptic drive increases (as force demand grows), units are automatically recruited in the order: slow-twitch first, fast-twitch later — precisely matching the energy-efficient fiber type to low demands and reserving powerful but costly fibers for high demands.

This automatic matching is elegant because it maximizes efficiency without conscious metabolic accounting. You do not need to decide 'I should use my slow-twitch fibers for this light task to conserve the fast-twitch ones' — the biophysics of motor neuron size makes that decision for you. The result is that fatigue-resistant fibers handle the vast majority of daily activity, and the powerful fast-twitch fibers are protected for situations that actually require them.