What Has Happened to Your Muscles

What You Will Learn

• How normal muscle works and what the nerve supply does
• What happens to the muscle when the nerve supply is lost
• Why 'standard' electrical stimulation devices cannot make a denervated muscle contract
• Why is there still reason for optimism?

1. Why This Chapter Matters

Sometimes people try electrical stimulation on a limb following a nerve or spinal cord injury and find that nothing happens. No matter how high the intensity is turned up, the muscle simply will not contract. This can be frustrating, and sometimes people are told that nothing more can be done.

When we then use a specialist device, one capable of producing the type of electrical signal that a denervated muscle actually requires, they are often surprised and relieved to see a contraction for the first time.

That moment of surprise reveals an important gap in understanding. The muscle did not fail to respond because it was beyond help. It failed because the wrong type of electrical signal was being used. Think of it like speaking the wrong language: you were saying the right things, but in a language your muscles could not understand.

A denervated muscle is not simply a weak muscle. It is, in a very real sense, a different tissue with altered structure and electrical properties. Understanding these differences is the foundation for everything that follows in this book.

2. How Normal Muscle Works

The motor unit: the building block of movement

All voluntary movement begins with what is called a motor unit. A motor unit consists of a single nerve cell in the spinal cord, its long fibre (called an axon) that runs through a peripheral nerve, and all the muscle fibres it innervates (connects to). When that nerve cell "fires," every muscle fibre it controls contracts at the same time. This is the fundamental building block of movement.

Motor unit size varies enormously depending on how precise the movement needs to be. In the tiny muscles that control eye movement, a single nerve cell may connect to only five or ten muscle fibres, allowing incredibly fine control. In the quadriceps (the large thigh muscles, which are particularly relevant to spinal cord injury), one nerve cell may control a thousand or more fibres. The trade-off is straightforward: small motor units allow precision; large motor units produce force.

When you want to move, your nervous system recruits motor units in an orderly sequence. Small motor units, which contain fatigue-resistant fibres, are activated first. As more force is needed, larger motor units containing more powerful (but more quickly tiring) fibres are added. This graded recruitment allows your nervous system to produce everything from the delicate grip needed to hold a pen to the force required for a standing transfer.

The relay station between nerve and muscle

For a nerve impulse to produce a muscle contraction, the electrical signal must cross from the nerve to the muscle. This happens at a specialised connection point called the neuromuscular junction, or NMJ. Think of it as a relay station: the nerve sends its command, the NMJ translates it faithfully, and the muscle responds.

Under normal conditions, every nerve impulse produces a muscle contraction. The relay station is remarkably reliable and almost never fails to pass on the signal.

This reliability is crucial for understanding denervation. When the NMJ is destroyed, as it is when the nerve supply is lost, that relay station is gone. The muscle fibre is no longer connected to the nervous system. To activate it, you must stimulate the muscle fibre directly, and as we shall see, this requires a fundamentally different electrical approach.

From electrical signal to muscle contraction

Once the signal passes through the NMJ, an electrical wave travels along the surface of the muscle fibre and deep into its interior through a network of tiny tubes. This triggers the release of calcium from internal stores, which in turn activates the contractile machinery, the molecular mechanism that produces force and movement. When the calcium is pumped back into its stores, the muscle relaxes.

The clinical relevance of this sequence is twofold. First, when the nerve is intact, a tiny external electrical pulse (less than one millisecond in duration, at a modest current) can trigger this entire process by activating the nerve. The nerve then does all the work. This is the basis of standard neuromuscular electrical stimulation (NMES), and it is why NMES works so well on muscles with an intact nerve supply.

Second, and this is the critical point, when the nerve is lost, this process breaks down at multiple levels. Not just at the relay station, but in the internal tube system, the calcium stores, and eventually the contractile proteins themselves. The tissue does not merely lose its controller. Over time, it loses the internal machinery that enables contraction.

3. What Happens When the Nerve Is Lost

Before examining the specific changes, it is worth understanding a fundamental point. The nerve does more than send electrical signals for contraction. It provides a continuous supply of chemical signals that maintain the muscle fibre's structure and health. When this support is withdrawn, denervation becomes not merely a loss of control but a failure of the entire system that sustains muscle tissue. This is why the consequences go so far beyond simple weakness from not using the muscle.

Causes of denervation

Muscle can lose its nerve supply through several mechanisms. For the purposes of this book, two causes are most important.

The first is peripheral nerve injury, from trauma, compression, or surgical damage. Here, the nerve cell body in the spinal cord typically survives; only the connecting fibre is disrupted along its course. Because the cell body is intact, the nerve can potentially regrow, and the question becomes whether it can reach the muscle before irreversible deterioration occurs. Chapter 2 explores this "race against time" in detail.

The second is a spinal cord injury affecting the lower motor neurons. When the injury is at the lower end of the spinal cord (the conus medullaris, roughly at the T12 to L1 vertebral level) or the nerve roots below it (the cauda equina), the nerve cell bodies themselves are damaged or destroyed. There is no intact cell body from which regrowth can begin, and the denervation is permanent. Many spinal cord injuries produce a mixture of both types of damage, and distinguishing between the two is essential for choosing the right stimulation approach.

Other causes include conditions such as Guillain-Barré syndrome, the late effects of poliomyelitis, diabetes, and certain surgical procedures that can damage nerves to individual muscles or muscle groups.

The timeline of change

The consequences of denervation unfold over a broadly predictable timeline, though the exact pace varies between individuals and muscles.

In the first weeks to months, the muscle begins to shrink. Protein breakdown pathways become active, and the muscle starts to lose mass. In this early phase, the muscle still retains much of its internal organisation, but it is already getting smaller. You may notice spontaneous twitching in the affected area, though this is often too subtle to see with the naked eye.

Over months to one or two years, the shrinkage becomes visible and measurable. Individual muscle fibres can lose 30 to 50 per cent of their size within the first year. The mix of fibre types in the muscle begins to change. Normal muscle contains a blend of slow, fatigue-resistant fibres (for endurance) and fast, powerful fibres (for bursts of force). Without the nerve's activity to maintain this balance, the muscle shifts toward fast, easily tired fibres. This means the muscle loses its endurance characteristics.

Perhaps most importantly for the possibility of electrical stimulation, the internal machinery that connects the electrical signal to calcium release (and therefore to contraction) becomes progressively disorganised. Research by Kern and colleagues, using muscle biopsies, showed that long-term denervation damages not only the contractile proteins but also the internal coupling system. Even if you could deliver an appropriate signal to the muscle surface, the internal machinery needed to translate that signal into force is degrading.

After two to six years, the muscle undergoes a process called fibro-fatty degeneration. Muscle fibres are progressively replaced by connective tissue (collagen) and fat. In long-term denervated muscle, biopsy studies have found connective tissue occupying over 60 per cent of the tissue area, with fat accounting for a further 12 to 13 per cent. The remaining muscle fibres become severely shrunken and sparse.

A denervated muscle at this stage feels different to the touch. Where healthy quadriceps muscle is firm and resilient, a long-denervated thigh feels soft and doughy, noticeably reduced in size. Many people notice this change themselves, and the altered appearance of wasted limbs is something people commonly remark upon.

Eventually, if nothing is done, the tissue reaches a point at which contraction is no longer possible, regardless of the stimulation applied.

But - your muscles are still trying to repair themselves

Against this backdrop of progressive degeneration, one finding offers genuine grounds for optimism. Your muscles contain resident stem cells called satellite cells. These cells normally sit quietly on the surface of muscle fibres, ready to be activated when repair is needed.

Research has shown something remarkable: even in muscle that has been denervated for three years or more, these satellite cells are not just surviving but are actively generating new muscle fibres. The biological machinery for recovery is present and working, even years after the nerve supply was lost.

This regenerative activity does diminish over time, and without support, newly formed fibres undergo the same cycle of shrinkage and cannot reach full maturity. But the key point is this: your muscles have not given up. They are continually attempting to repair themselves. Electrical stimulation works with this ongoing process, supporting it rather than trying to restart it from nothing. This is one of the key reasons why electrical stimulation of denervated muscle can produce meaningful results.

4. The Key Differences: Normal vs Denervated Muscle

The preceding sections describe two states of the same tissue: one functioning normally under nerve control, the other progressively changing without it. The table below summarises the key differences that matter most.

Table 1: Normal vs Denervated Muscle

The change in electrical properties is perhaps the most important difference for treatment. The measure that captures this, called chronaxie, shifts dramatically. In normal muscle, a very brief electrical pulse (less than one millisecond) can trigger a contraction through the nerve. In denervated muscle, the pulse needs to be tens or even hundreds of milliseconds long, because you are stimulating the muscle fibre directly rather than using the nerve as an amplifier. This means that standard devices, which are designed to send short pulses through the nerve, simply cannot make a denervated muscle respond. The pulse is too brief for the muscle fibre to react to, no matter how strong the current.

5. Why This Matters: Setting the Stage

Understanding these differences leads to a clear conclusion: a denervated muscle is not unreachable, but you must "speak its electrical language."

Every aspect of the stimulation approach must change: the length of the electrical pulse, the strength of the current, the frequency of pulses, and the size of the electrodes. These are not arbitrary adjustments; they follow logically from the tissue changes described in this chapter. Chapter 4 explains how electrical stimulation works and why these different settings are needed.

The window of opportunity

The timeline of denervation described above carries an important practical message: the changes are progressive but not instantaneous. There is a window of opportunity, particularly in the first one to two years after denervation, when muscle fibres remain structurally intact enough to respond to direct electrical stimulation and satellite cell activity remains strong enough to support recovery.

The evidence from a landmark study by Kern and colleagues (2010) demonstrated that home-based electrical stimulation produced measurable improvements even in patients who had been denervated for up to nine years, with biopsy-confirmed structural restoration of muscle tissue, not merely superficial swelling. The best outcomes, though, were consistently seen in those who started treatment earlier.

In our experience, people who begin a stimulation programme within the first year tend to achieve faster, more pronounced gains in muscle size and quality. Some who were initially assessed as having complete denervation have, over time, progressed to settings more typical of innervated muscle, suggesting that some nerve function was recovering. Others with genuinely permanent denervation have still achieved meaningful improvements in muscle size, tissue health, and limb appearance. Gains of three to five centimetres in thigh circumference within six months are not uncommon, and for many people, this visible change is the first tangible evidence that progress is possible.

The critical point is this: early intervention produces better outcomes, but it is rarely too late to begin. The biological machinery (satellite cells, surviving muscle fibres, and the capacity for structural remodelling) persists longer than earlier thinking suggested.

Looking ahead

This chapter has established the foundations. You now understand what normal skeletal muscle is, how it works, and what happens when the nerve supply is lost. You understand why the electrical properties of denervated muscle differ so dramatically from those of normal muscle, and why standard devices cannot bridge that gap.

In the chapters that follow, we build on this understanding. Chapter 2 examines the mechanisms of nerve injury and recovery. Chapter 4 explains how electrical stimulation works and the principles behind choosing the right settings. And Part 3 translates the science into practical treatment guidance.

Science is the foundation. Now let's build on it.

Chapter Summary

Normal skeletal muscle is a precisely organised tissue. Motor units provide controlled recruitment, the neuromuscular junction relays signals reliably, and the internal coupling system converts electrical impulses into force. A brief external pulse of less than one millisecond can trigger this entire process through the nerve.

When the nerve supply is lost, the consequences extend far beyond weakness. The relay station breaks down, the contractile proteins degrade, the calcium-handling machinery progressively fails, and the muscle's fibre composition shifts toward fast, easily tired types. Over months to years, muscle fibres are replaced by collagen and fat. The electrical properties change so dramatically that standard stimulation devices cannot produce a contraction at any intensity.

Yet the picture is not hopeless. Satellite cells persist and actively generate new muscle fibres even years after denervation. This biological machinery provides a foundation for recovery, provided stimulation begins before irreversible degeneration. Early intervention yields the best outcomes, but meaningful gains remain possible even after prolonged denervation. The key is using the right type of electrical stimulation: longer pulses, higher currents, lower frequencies, and larger electrodes.