The Myth of Safe Numbness

We tend to think of tingling and numbness in rope bondage as having two distinct causes -- loss of circulation ("not dangerous") and nerve compression ("dangerous"). Many experienced bottoms believe they can tell the difference between these sensations. However, this is really just a convenient shorthand for thinking about a much more complicated set of phenomena and in actual fact there is no distinct line between these two categories.

Let's start by examining what is commonly meant by each of these terms in the bondage community:

Nerve compression means mechanical pressure that is put on a nerve at the site of (potential) injury, which may be a crushing force applied by the rope, something else digging into the body, or a positioning of the body that either constricts an area the nerve passes through or stretches the nerve itself. This is generally assumed to be the source of nerve injuries in bondage.

Loss of circulation implies an effect on nerves due to reduced oxygenation of the surrounding tissue rather than direct pressure on the nerve. This is generally assumed to not be a significant danger on the basis that sufficient restriction of circulation to cause tissue damage would be difficult to achieve and an obvious problem.

Problems With This Model

Loss of Circulation is Not Actually Common

Transient paresthesia is the medical term for a condition of tingling/numbness due to loss of regular nerve function that rapidly resolves when the proximate cause is eliminated. It includes the common experience of an arm or foot going to sleep in daily life due to being in an awkward position. However, this experience is not due to loss of circulation according to any reputable source I have found; rather, the common cause of transient paresthesia is direct pressure on the nerve (NIH, WedMD).

Lack of oxygen supply to the nerve (ischemia) is a common factor in chronic paresthesia caused by conditions such as diabetes or Raynaud's that chronically impair peripheral circulation. However, absent such chronic conditions there is no reason to believe transient paresthesias in rope are due to circulation rather than mechanical stress.

It's Hard to Restrict Circulation Without Mechanical Pressure

You might argue that while transient paresthesia in daily life is not generally caused by restricted circulation, rope bondage could restrict circulation in ways not otherwise likely to occur. There are two obvious mechanisms whereby rope bondage could restrict circulation: putting the body in stress positions, or by directly tourniquetting an extremity. Since stress positions are essentially similar to the daily causes of limbs going to sleep, tourniquetting effects seem the more plausible culprit for novel ischemic effects. Luckily, this is something that's been studied extensively in the medical literature. One of the clearest summaries I've found appears in this paper:

The 2 main mechanisms of pneumatic tourniquet-induced nerve injury are ischemia and direct mechanical effects (Hodgson 1994). Ochoa et al. (1972) found that compressive neurapraxia rather than ischemic neuropathy or muscle damage was the underlying cause of tourniquet paralysis.

There is some interesting additional reading here and here. In general, references agree that there is a complicated relationship between mechanical pressure and ischemia in tourniquetting. Regardless, the key point is that any significant reduction in circulation is likely accompanied by mechanical nerve compression, and that the latter is likely to be a larger factor in interfering with nerve function.

Conclusions

The thing to take away from all this is that if you get tingling and numbness in rope, probably a nerve is being compressed. That isn't necessarily going to cause injury -- we sustain and recover from minor nerve compression all the time in daily life -- but the difference between a compression that is harmless and one that causes serious injury is only a matter of degree. Something that you've always recovered from immediately in the past could cause lasting injury next time if you do it slightly longer or harder, or if your body is just in a different state.

So What About Generalized vs Focused Sensations?

The anecdotal experience from many bottoms is that tingling and numbness that has a slow onset and is dispersed over a large area seems less likely to result in injury than that which occurs immediately and is highly localized. While both of these scenarios likely result from nerve compression, that doesn't mean there is no actual difference. For instance, it may be that more generalized sensations mean that pressure on nerves is distributed over a larger area vs. a single point. Slow onset might be associated with lower levels of pressure.

The unfortunate fact is we just don't know how meaningful these signals are in terms of avoiding injury. Some injuries are not associated with any sensation at the time they occur, so there's no silver bullet.

Stages of Damage to Nerves

There are two distinct types of injury to nerves likely to occur in rope bondage, and understanding the difference can help shed light on differing experiences with recovery from injuries.

Neurapraxia is a condition where a nerve's ability to properly conduct electrical impulses is impaired by damage to the myelin nerve sheath. This may be caused by either mechanical stress or ischemia (in other words, any sustained paresthesia may become neurapraxia). Because the nerve fiber (axon) itself is not significantly damaged, it may retain partial function, and will gradually regain normal function as the myelin is repaired; complete recovery within 6-8 weeks is typical, often with significant improvement in the first 2-3 weeks, or even sooner for less severe cases. This is the most common form of nerve injury in rope.

Axonotmesis is when the axon is damaged to the extent that it must regenerate from the point of injury outward. This regeneration is very slow (generally quoted as 1mm/day under good conditions), and so this type of injury takes many months to recover from, and you don't see the same early improvements as with neurapraxia (because until the new axon reaches its destination, it's not doing any good). Recovery is generally good, but not always complete. In rope bondage, this type of injury appears to be less common than neurapraxia, but with similar causes/circumstances.

There are some cool pictures here.

Wrist Drop and the Myth of the Correct Box Tie

The vast majority of debilitating rope bondage injuries are caused by a single tie, the box tie or takate-kote (TK), so it merits a bit of specific discussion. Some authorities whose entire style/career rely upon this tie have tried to promulgate the notion that injuries from TKs are entirely the result of incompetent practitioners, and that if you tie a TK "correctly" it is perfectly (or at least reasonably) safe. However, there are several major flaws with this idea.

Nobody Can Agree What Is Correct

If there were in fact a correct way to tie a TK, you would expect acknowledged experts on Japanese bondage to generally use similar approaches; but in fact there is tremendous diversity in all aspects of TK construction and wrap placement. Depending on whose style you observe, wraps may be high or low on the arm, with a large or small gap between wraps, totally parallel or with a sharp angle. Upper or lower kannuki may be present or absent in any combination. Suspensions may be loaded from the stem or the top wrap or both wraps.

Of course you may argue that each of these options has a time and place with a certain body and a certain suspension. However, there are no generally agreed-upon guidelines for selecting such combinations. Perhaps you are intended to come to intuit those choices after years of experience -- leading to a conundrum around what injuries you're going to cause on the way to gaining said experience.

The Data Indicates No Correct Choice

I performed a survey of TK-related injuries, analysis of which indicates that there's no single approach to placing the wraps of a TK that can be known in advance to be safe for any given individual.

Something That Must Always Be Perfect Is Not Safe

Even if you ignore all the evidence to the contrary and take as given that a sufficiently skilled practitioner can somehow choose the exact right TK for the situation at hand, then you have to ask -- is that person also infallible? If you assume that all the celebrated practitioners who nevertheless have caused injuries were insufficiently trained, and the lack of agreement between experts is due to the great subtlety of the art, then the "correct" TK must truly be a work of extreme skill and precision. Under such conditions, presumably even a small error could make the difference between a "safe" and unsafe TK. And who doesn't ever make a small error?

In any safety-critical field, you'll find numerous mechanisms in place to guard against the inevitable mistakes that human experts make. Pilots have checklists. Climbers inspect each others' knots. To call a system safe, you need to have a margin for error. In the theory of the "correct" TK, it's not clear where that margin would be.

Conclusions

The TK is not a safe tie. Is it less safe than other ties that put pressure on the upper arms? That's hard to say; it certainly causes the most injuries, but it is also by far the most popular.

Does that mean you shouldn't use this type of tie? That depends on your risk tolerances. However, in considering any potentially dangerous play informed consent relies on being informed. If you are misleading your partner (or yourself) into believing this is not a dangerous type of play, then your consent process is broken.