Spindle Physiology and the Phenomenology of the Hum

Created 2026-03-28

Tags scienceproprioceptionmuscle-spindlephenomenologyhumresearch

Overview

This document compiles the detailed spindle physiology needed to ground the Baseworks hypothesis about muscular mechanosensation — specifically, the “background hum”: a localized conscious sensation in muscles that is proportional to effort and contraction intensity but does not fully disappear at rest or minimal activation.

It draws on Vallbo (1974), Macefield (2018), and Luu et al. (2011), and connects the physiology to the phenomenological question of what a person might actually be perceiving. See also science §1.7 and §3.2 for the Baseworks-framing summary.

1. The Naming Problem: “Fiber” Means Three Different Things

Before any of this makes sense, the terminology needs to be untangled.

Intrafusal fiber = the muscle fiber inside the spindle capsule. The mechanical element — the thing that can be stretched or contracted. Muscle, not nerve.

Fusimotor fiber = the axon of a gamma (γ) motor neuron. An efferent nerve fiber carrying a command down from the spinal cord to innervate the intrafusal fibers. Nerve, not muscle. (Fusi = spindle-shaped, Latin fusus; motor = movement command.)

Afferent fiber (Group Ia or II) = the sensory axon carrying signals up from the spindle to the spinal cord and brain. This is what we perceive, ultimately.

So every spindle involves all three: intrafusal fibers (muscle, inside the capsule), fusimotor fibers (efferent nerve in), and afferent fibers (sensory nerve out).

2. The Three Intrafusal Fiber Types

Each spindle contains a mixture of three intrafusal fiber types, distinguished by internal structure and motor innervation:

Intrafusal fiber	Fusimotor innervation	Structural feature
Nuclear bag 1 (dynamic bag)	γ-dynamic (γd) neurons	Nuclei clustered in a central bag; large-diameter; elastic
Nuclear bag 2 (static bag)	γ-static (γs) neurons	Also bag-shaped but stiffer; intermediate properties
Nuclear chain fibers	γ-static (γs) neurons	Nuclei in a chain; small-diameter; stiff; multiple per spindle

The distinction matters because dynamic and static fusimotor neurons have different central connectivity and different roles in modulating what the spindle reports:

γd → bag 1: Sharpens the spindle’s sensitivity to velocity of stretch — the dynamic, phasic component
γs → bag 2 + chain: Maintains spindle sensitivity during sustained stretch or contraction — prevents the spindle from going silent as the muscle settles

3. The Afferent Fibers: What Gets Sent to the Brain

Two types of sensory axons leave the spindle:

Afferent	Ending type	Located on	What it detects	Discharge character
Group Ia (primary)	Annulospiral (spiral wrap)	All three intrafusal types; functionally dominated by bag 1	Velocity of stretch (dynamic); some static length	Irregular; strong phasic response to stretch onset; off-discharge at contraction termination
Group II (secondary)	Flower-spray (diffuse terminal branches)	Chain fibers + bag 2	Static muscle length — “tautness” of the spindle at that moment	Regular, tonic; does not cease during passive shortening

Group Ia is primarily a change detector: it fires hard when stretch begins, tracks velocity, and quiets down when things stabilize. Many Group Ia afferents are silent at a given resting joint position (Vallbo 1974 found <10% of primaries active at rest in finger flexors; mean resting sensitivity 0.18 impulses/second/degree of joint movement).

Group II is primarily a state reporter: it encodes the current degree of stretch as a continuous tonic rate. It fires regularly and persistently. It doesn’t stop when the muscle passively shortens — it just slows. Spinal cord injury patients show the same resting Group II discharge as intact subjects, confirming this signal is not driven by any descending (fusimotor) command (Macefield 2018).

4. What Fusimotor Drive Does (and Does Not Do) at Rest

At rest in humans, resting fusimotor outflow is negligible. This is empirically established (Vallbo 1974, Macefield 2018). The tonic spindle discharge that exists at rest is passive — driven purely by the mechanical stretch imposed by resting muscle length, not by any gamma motor neuron firing.

This is different from the decerebrate cat, which has significant resting gamma drive. Humans do not.

During voluntary contraction, fusimotor neurons (both γs and γd) are co-recruited with alpha motor neurons — this is alpha-gamma coactivation. The effect: intrafusal fibers shorten in parallel with extrafusal fibers, keeping the spindle under tension even as the whole muscle shortens. Without this, the spindle would go slack during contraction and become uninformative. With it, spindle discharge scales with contraction strength — Group Ia and II both increase their firing proportionally to the effort level (Vallbo et al. 1981, cited in Macefield 2018).

So:

At rest: spindle discharge exists, but it is passive and fusimotor-independent
During contraction: spindle discharge increases, and the increase is fusimotor-dependent (via alpha-gamma coactivation)

5. Luu et al. (2011): What the Paper Actually Shows

Luu’s experiment uses weight-matching tasks: one thumb lifts 500g; the subject matches perceived weight with the other hand. He manipulates spindle sensitivity via high-force fatigue (which desensitizes spindle primary afferents) and complete neuromuscular block with rocuronium (which eventually paralyzes intrafusal fibers, lagging behind extrafusal paralysis).

Key findings:

After high-force fatigue: weights feel lighter — opposite to what the central efference-copy theory predicts
After complete paralysis + recovery: weights also feel lighter (because intrafusal fibers recover more slowly than extrafusal, leaving spindles relatively unloaded)
Deafferented subjects (no large-fiber afferents): weights feel twice as heavy after fatiguing to half-strength — consistent with pure central signal

Conclusion: Peripheral afferent inflow from muscle spindles — not central efference copy — dominates force and weight perception in healthy subjects. The spindle afference that matters here is reafference: the ascending signal that results from the fusimotor loop being driven by alpha-gamma coactivation during voluntary contraction.

What Luu does not study: Resting sensation, the background hum, conscious awareness at low or zero contraction levels. The entire paradigm is active lifting tasks. The paper’s contribution to the hum question is indirect: it establishes that spindle afference reaches conscious perception and scales with force — but it does not address the resting or minimal-contraction component.

6. The Hum as a Percept: Two Components, Two Mechanisms

The “hum” — the localized background sensation proportional to effort that doesn’t fully disappear at rest — has two phenomenologically distinct components that map onto different afferent types and different mechanisms:

Component 1: The resting baseline (persists at rest and minimal activation)

Best candidate: Group II secondary endings on nuclear chain fibers.

These fire tonically, proportionally to muscle length, even with no fusimotor drive. A muscle at resting length has its spindles under passive stretch; Group II endings translate this into a continuous low-level signal. The signal is stable, regular, and graded — exactly what a background sensation would require.

At a given resting posture, perhaps 50–200 spindles per muscle are each registering their local mechanical environment via Group II discharge. The aggregate is a distributed read of the whole muscle’s current state.

Component 2: The contraction-proportional intensification

Best candidates: Group Ia + Group II (both), via alpha-gamma coactivation. Group Ib (Golgi tendon organs) for the active-force component.

During voluntary contraction, spindle discharge scales with effort via fusimotor co-activation. Luu et al. (2011) shows that this spindle reafference — not the central command signal — is the dominant source of perceived force in healthy subjects. GTOs contribute a parallel force-proportional signal and are active at very low contraction levels (firing in discrete steps corresponding to individual motor unit recruitment — Vallbo 1974).

7. Your Intuition About Group II and “Spindle Tautness” — This Is Accurate

“Group II fibers are sensing the state of the spindle — how taut the spindle is.”

Yes. Group II secondary endings, via their flower-spray contacts on chain fibers, are reading the mechanical tension state of the intrafusal fibers at that moment. Chain fibers are stiff and behave like a simple spring under stretch. When the surrounding extrafusal muscle is at a given length, the spindle is under a corresponding degree of passive stretch — and the chain fibers transmit this to the Group II endings as a tonic discharge rate. More length = more tension on the chain = higher Group II firing rate.

“Maybe the hum is literally the spindle hum? A choir of individual spindle environments?”

This is not only poetic — it is mechanistically defensible. Each spindle is a capsule that samples the local mechanical environment of its small region of muscle. A typical limb muscle has 50–200 spindles distributed through the belly. At rest, each one is firing at a rate determined by the local stretch at that location. The aggregate afferent signal from all of them — conducted up to the cortex via Group II fibers — represents a spatially distributed read of the whole muscle’s current state.

The reason this doesn’t show up in standard neuroscience descriptions is that textbooks focus on what spindles are good for — stretch reflexes, position sense — not on what continuous discharge from a population of them might feel like when consciously attended to. The percept is real; the phenomenology has just never been studied.

There is no place in the existing literature for the discussion of this hum. It falls between established categories: too slow and tonic to be the stretch reflex story, too localized and muscular to be skin-based exteroception, and dismissed as “unconscious” by the proprioception literature that never asked whether people with trained attention could access it. The gap is not because the signal doesn’t exist — it exists and has been measured. The gap is because no one thought to ask what it might feel like.

8. Implications for the Hypothesis Paper

For a literature-review-style hypothesis paper on the phenomenology of the hum as a conscious localized sensation in relaxed/toned/voluntarily contracted muscles, the argument structure would be:

Establish the phenomenon (survey data, phenomenological description, lack of scientific literature)
Rule out the obvious candidates — skin mechanoreceptors (wrong location), joint receptors (only active at extremes), Group III/IV free nerve endings (noxious quality, wrong phenomenology), GTOs (silent in passive muscle)
Establish Group II tonic discharge as the resting-state substrate — tonic, regular, length-proportional, fusimotor-independent (Macefield 2018 SCI data), distributed across the muscle belly
Establish spindle reafference via alpha-gamma coactivation as the contraction-proportional substrate — Luu et al. (2011), Macefield (2018) isometric holding data
Address the perceptual access question — area 3a receives muscle afference; large individual variation in conscious access; training-dependent development (Baseworks practitioners, dancers); the blind subject data point
Frame the gap — this is not a gap in the signal; it is a gap in the phenomenological inquiry

Key References

Vallbo AB. Afferent discharge from human muscle spindles in non-contracting muscles. Acta Physiol Scand. 1974;90:303–318.
Macefield VG. Functional properties of slowly adapting mechanoreceptors and muscle spindles in humans. In: The Senses: A Comprehensive Reference. 2nd ed. 2018.
Luu BL, Day BL, Cole JD, Fitzpatrick RC. The fusimotor and reafferent origin of the sense of force and weight. J Physiol. 2011;589(13):3135–3147.

science — §1.7, §3.2
mystery-of-proprioceptive-awareness-v1 — the original blog article this research is being used to update