"Things done well and with care exempt themselves from fear."
Pain is a poor metric by which to measure the severity of an injury. This is because the intensity of the pain someone experiences doesn't strongly correlate to the extent to which structural damage might be observable in tissues, bones, or nerves. The reason for this is that while damage is something that can be measured, with milimeters of torn muscle, the extent to which the two parts of a broken bone are displaced, or the area of a bruise, pain is something that can't accurately be measured. We can try - you can point to a point on a line that corresponds to your level of discomfort, you can describe it with words like sharp, dull, shooting, boring, burning, or lancing, you can determine its patterns of behaviour in its aggravation and its ease, but you can't ever truly measure pain.
The reason behind all of this is that the pain we perceive in our consciousness is the result of the processing of physical sensations within the body through emotional and personal matrices. The stereotypical statement is that pain is an alarm system, and like any alarm system it can be triggered by the thing it is actually trying to detect or something that just happens to set it off. For example, a motion sensor alarm in a store can be set off by a thief trying to break in, or by a possum that crawls in through a hole in the roof space. The alarm is going off because it's detecting something, but the thing that it detects might be something worth immediately responding to or not. The more sensitive the alarm, the more likely it is to be tripped off by something noteworthy or something inconsequential.
It's a tricky thing to nail down because pain is tricky, and as has already been said, the extent to which something feels painful is rarely directly correlated with the extent to which something is injured. This is why managing pain and rehabilitating a painful body part are two different steps in the management plan - it's entirely possible to engage in rehabilitation despite discomfort, and it's quite possible to feel completely normal but to not be able to exercise because of the risk. The example here is that if you have a freshly broken leg and then you get dosed with Morphine, you won't feel any pain, but that doesn't mean your leg is better. It's still busted even if you don't feel it.
All of this leads to an interesting question: if we can't rely on pain as an indicator, how do we determine if the discomfort we’re feeling is from a measurable injury or from irritation? To answer this question, we need to consider both of the alternatives.
An injury is measurable damage to the body, usually caused by an external force. Injuries are typically caused by accidents, falls, blows to the part that has been injured, and by other causes as well. The important thing is that the injury has been caused by an external force: there's a lot of nuance in this definition. That external force can be the result of landing awkwardly on a tilted surface and tearing one of the ligaments in the ankle or the knee. In this case, the external force isn't the ground or the step, but the action of gravity and momentum combined with rotational and directional movement that places a force on a structure for which it is unable to compensate. Injuries can come on quickly or slowly: an injury that happens fast could be breaking a bone during a fall or an accident, an injury that comes on slowly is like the gradual wear-down of someone's rotator cuff and shoulder over the span of their life, which then may present as pain following a fast-onset injury or just with time. In this case, the external force is the load borne by the arm and the shoulder, many times a day, many days of the week, many weeks in a life.
Injuries can happen as a consequence of living, of work, and of recreation. Anyone who's lifted weights has exposed their muscles and bones to controlled external forces by the application of resistance through weights conducted in repetitive movements. The combined effect of these movements causes controlled injury to the body, and in the repair of that injury the body becomes stronger through adaptation. Not all injuries are bad, fast acting, slow acting, and not all of them are severe enough to shut someone down, but they are all the result of external forces acting on the body that have caused measurable change.
Irritation is what happens when structures are over-used, used awkwardly, or used inappropriately, and discomfort arises. This might seem close to the definition to injury, and that's for a good reason: structures that are irritated can become structures that are injured if that irritation is not managed. Injury is irritation that has progressed to the point of causing measurable damage and changed function. This can happen over time with repetition, or quickly due to the nature of the inciting physical incident. Catching a heavy glass pane overhead is likely to cause injury. Lifting a heavy glass pane six times an hour for eight hours a day for six days a week for forty weeks a year for ten years is likely to cause irritation, and that injury becomes likely to progress to injury if the worker keeps lifting that glass pane. In ranked severity, irritation is generally less serious than injury - irritation is what happens when you've been sitting in a chair for eight hours before doing an evening of sweeping that leads to a sore back tomorrow. Irritation is what happens when you spend the weekend reaching overhead to clean out your cupboards, and then have difficulty reaching behind your back to do up your bra the next day. Irritation is what happens when you give your mate Dave a hand moving house on Australia Day, and then you can’t quite sit comfortably tomorrow morning when you’re driving to work.
Irritation most often arises as a consequence of living and working. It’s the accumulation of muscular and metabolic strain that eventually presents as discomfort due to the irritation limiting movement, causing pain, causing discomfort, and slowing us down. Most of the problems dealt with in the clinic are the result of irritation of a muscle, a joint, or a ligament - not an injury.
Irritation is not injury.
So how do we tell the difference between the two?
The fields of pain science, neuroscience, medicine and allied health have poured blood and treasure into improving the accuracy of determining the clinical severity of an injury. Similarly, psychologists, counsellors and clinicians are educated regarding the influence of personal, emotional, social and cognitive factors that can ease or worsen the presentation of pain in an individual. We even know that the décor of a clinic can act to curate individual expectations, and that one of the most important factors in the experience of receiving treatment is the rapport between the client and the clinician. We know that pain is not a reliable indicator of clinical severity. What we need to use is the behaviour of symptoms in and out of the clinic as reported by the patient.
Attempting to track the behaviour of symptoms is a flawed process in and of itself because if symptoms themselves are a poor indicator of the presence or absence of injury, then the patient’s reporting adds another layer of complexity and potential obfuscation into the mix. Attempting to assess improvements in the range of motion of limbs, of strength, of flexibility and balance all carry inherent flaws: the patient is an unreliable reporter, the clinician is an unreliable detector, and the effect of nonphysical and cognitive factors on performance is impossible to ignore. While medical imaging can give us a snapshot of what’s going on with a part at the time of assessment, the result and the report are imperfect because they don’t capture behaviour over linear time. Even while something may be measurably damaged, it might not feel damaged, and that’s a complicating factor.
It’s the job of the clinician to investigate symptoms, their behaviour, the expectations and experience of the patient and use all of that information to assess for the presence or absence of injuries, to determine the treatment course, monitor the reaction of the patient to the prescribed management as well as the progression or regression of symptoms, and curate the process to its conclusion. The work of differentiation is sometimes moot because the problem comes on and goes with such speed that establishing a pattern is impossible. The bottom line is that injury assessment and management is really murky, and sometimes it’s more like throwing the sink at the wall and seeing what sticks instead of an act of technical problem solving. It can be hard to describe symptoms, progression and regression, but it’s always good to get it checked out.