What does a muscle spindle measure? Ask any textbook and it will say length.
That answer is about half right.
In 2006 I taught a three-day seminar in Cyprus. On the last day I invited the practitioners to bring me their most difficult patients. One was a former Olympic athlete, still in great shape, with muscles in places ordinary people don't have places. He'd had spinal surgery four times.
When I tested him, his power was immense. But as he moved into different positions, his strength deserted him.
A muscle that strong shouldn't fail just because you change the angle. Unless the problem isn't the muscle at all, but the signal coming back from it.
Muscle Spindles: The Sensor Inside Every Muscle
A muscle spindle is a tiny muscle inside the muscle. It has its own fine fibres, called intrafusal fibres, wrapped in a sensory nerve ending. When the spindle is stretched or loaded, that nerve fires back to the spinal cord. There it drives the anterior motor neuron, the nerve cell that tells the muscle how hard to contract.
That loop is the myotatic reflex. It runs continuously, in every muscle, all day.
The textbook version says the spindle reports length, like a tape measure.
Force and Yank: What Muscle Spindles Really Measure
In 2020, Lena Ting's group built a detailed model of how the spindle works from the inside, down to the cross-bridges in its fibres. They found that the spindle's firing tracks force and yank. Yank is simply how fast the force is changing.
The spindle is less like a tape measure and more like your hand on a steering wheel, feeling the pull and how quickly it builds.
That matters. A sudden load that arrives fast produces a very different signal from the same load arriving slowly, even when the muscle length is identical.
Gamma Motor Neurons and Resting Muscle Tone
The spindle also has its own motor supply, the gamma motor neurons. They tighten the spindle's fibres and set how sensitive it is. Turn the gamma drive up and the spindle fires more at rest, raising the baseline signal that sets resting tone.
This is why I keep saying that tone is a signal, not a property of the tissue. You can't stretch a signal longer or roll it flatter.
Resting muscle tone exists in everyone, whether or not they go to the gym. It cannot be changed by exercise, stretching or massage.
The Spindle Paradox: Firing During Contraction
When a muscle contracts, it shortens. The spindle inside it should go slack and fall quiet.
Often it doesn't. Sometimes it fires more.
The model explains why. When the main muscle drive (alpha) and the spindle's own drive (gamma) are switched on together, the spindle's fibres keep contracting. The force within them changes, and the sensor keeps reporting.
The model also showed that spindle firing depends on history: what the muscle did just before shapes what it reports now. So the signal reaching the anterior motor neuron during any movement depends on the task, the speed, the position and what came before. It is not just a reading of where the limb is.
Why Protective Reflex Testing Changes Position
If the signal depends on the task, the test has to as well.
That's why Protective Reflex Testing (PRT) checks the muscle's response to an external force it didn't plan for, in more than one position. A muscle can hold perfectly at one angle and give way at another. If you only test it in one position, you can miss the problem completely.
My Olympian was exactly that. Strong in one position, weak in the next. All I did was remove his neck chain, and the weaknesses were gone.
Only then did he tell me his back injuries had started at 18, the same year his grandfather gave him the chain. He had never taken it off. It was a Nail in the Foot he'd worn for decades: an input his nervous system was trying to get away from, and his muscles paid for it whenever the task changed.
What the Research Does, and Doesn't, Show
This is a computer model, built on and checked against recordings from animal spindles.
What it does is retire the idea of the spindle as a simple length meter. The sensor driving your muscle tone is measuring force, how fast that force changes, and its own recent history, under a volume knob the nervous system can turn up or down. It's also only one source of proprioceptive input, out of thousands - all of which contribute to the central integrative state of the motor neuron.
Which brings us back to the whole reason for Afferentology. When we find muscle weakness, it's not enough to prescribe exercises, we have to ask why and do it often enough, until we find the source of the bad input.
To your strength!